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Published by Josh Wood, 2019-10-18 14:45:14

AM compiled papers

AM compiled papers

Volume 104, 2019 Earth in Five Reactions Collection

American Mineralogist, Volume 104, pages 465–467, 2019

Introduction

Deep carbon cycle through five reactionsk

Jie Li1,*,†, Simon A.T. Redfern2,3, and Donato Giovannelli4,5,6

1Department of Earth and Environmental Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, Michigan 48109, U.S.A.
ORCID 0000-0003-4761-722X

2Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, U.K. ORCID 0000-0001-9513-0147
3Center for High Pressure Science and Technology Advanced Research (HPSTAR), Shanghai 201203, China

4National Research Council of Italy, Institute of Marine Science CNR-ISMAR, la.go Fiera della Pesca, Ancona, Italy
5Department of Marine and Coastal Science, Rutgers University, 71 Dudley Road, New Brunswick, New Jersey 08901, U.S.A.

6Earth-Life Science Institute, 2-12-1-IE-1 Ookayama, Tokyo, Japan

Abstract

What are the key reactions driving the global carbon cycle in Earth, the only known habitable
planet in the Solar System? And how do chemical reactions govern the transformation and movement
of carbon? The special collection “Earth in Five Reactions: A Deep Carbon Perspective” features
review articles synthesizing knowledge and findings on the role of carbon-related reactions in Earth’s
dynamics and evolution. These integrative studies identify gaps in our current understanding and
establish new frontiers to motivate and guide future research in deep carbon science. The collection
also includes original experimental and theoretical investigations of carbon-bearing phases and the
impact of chemical and polymorphic reactions on Earth’s deep carbon cycle.

Keywords: Deep Carbon Observatory; habitable planet; Earth in Five Reactions: A Deep Carbon
Perspective

Background liaison to the DCO, watched this exchange and asked: “How
about Earth in five reactions?”
The Earth in Five Reactions (E5R) project was conceived in
the fall of 2015 at the University of Rhode Island, U.S.A., where The idea emerged as a promising framework for synthesis:
the Deep Carbon Observatory (DCO) held a synthesis-planning Chemical reactions are widespread and play important roles in
meeting. DCO is a 10-year project supported by the Alfred P. Earth’s carbon cycle. Viewing Earth processes through the lens
Sloan Foundation with the overarching goal of understanding of reactions would highlight the chemical aspect of DCO sci-
the quantities, movements, forms, and origins of Earth’s deep ence and could stimulate dialogues across disciplines. Like math
carbon. Members of the international DCO Science Network and music, chemical reactions are the same in the United States,
are addressing this goal through investigations focused on four China, Italy, or France. The concept works internationally, even
distinct and interconnected thematic disciplines—Deep Life if people understand little or no English, and therefore could be
(DL), Deep Energy (DE), Reservoirs and Fluxes (RF), and Ex- widely reported or easily translated.
treme Physics and Chemistry (EPC). Since its launch in 2009,
the DCO has initiated and supported scientific campaigns to Why five? The number was inspired by the familiar “five
investigate deep carbon, leading to numerous findings reported types of chemical reactions” in high-school chemistry textbooks.
in more than 1400 scholarly publications to date, and created an Mathematicians and physicists have had success with celebrat-
international network of more than 1200 deep carbon scientists ing “Five Equations that Changed the World.” We considered
(www.deepcarbon.net). selecting five reactions in each DCO community, in addition to
the five that encompass all communities. However, for the idea
“Serpentinization is the most important reaction in the uni- to work effectively, we chose to limit the number to five, much
verse!” This bold statement made by a workshop participant like the number of medals in each Olympic sport is limited to
provoked Jie Li, an EPC representative who had studied chemical three. In reality, “five” is not a magic or required number as the
and polymorphic reactions for decades but thought little about outcome, but rather a fun way to stimulate the DCO community
serpentinization, to challenge the assertion. Li argued that redox to build its shared experiences. It was exciting to find out what
and melting reactions dictate global-scale differentiation and the outcome would be!
therefore are far more important than serpentinization. This fun-
damental question about the key drivers in deep carbon science The E5R project aimed to identify the five most important
sparked a lively and spirited debate and revealed a general lack reactions governing the transformation and movement of
of consensus. Jesse Ausubel, the Sloan Foundation’s primary carbon in Earth, and then use these reactions as the central
themes for synthesizing and disseminating the findings of
* E-mail: [email protected] the Deep Carbon Observatory. This thematic structure also
† Special collection papers can be found online at http://www.minsocam.org/MSA/ provides a new and integrative perspective for understanding
AmMin/special-collections.html. and advancing deep carbon science as a new, multidisciplinary
kOpen access: Article available to all readers online. This article has an MSAlicense. scientific discipline.

0003-004X/19/0002–465$05.00/DOI: https://doi.org/10.2138/am-2019-6833 465

466 LI ET AL.: INTRODUCTION TO EARTH IN FIVE REACTIONS SPECIAL COLLECTION

Figure 1. E5R logo (left) and special collection theme figure (right) showing where the reactions likely occur during Earth’s deep carbon cycle.

Selecting the top five carbon-related reactions occur under broad ranges of pressure and temperature (for example,
redox reactions in solids and liquids). Unique reactions that can be
We introduced the “Earth in Five Reactions” initiative by used as indicators, tracers, or diagnostic tools for carbon cycling are
launching a survey just before the Third DCO International Science other possible targets of interest.
meeting in St. Andrews, Scotland, in the spring of 2017. The poll
was distributed at the meeting and through newsletters of relevant The distribution of responses to the criteria question were not
organizations, providing the opportunity for all members of the DCO particularly clear-cut, with prevalence, timing, location, nature, and
science network and others to weigh in. By the end of the year, we impact all having an approximately similar number of votes. The nar-
received 120 submissions with dozens of proposed reactions. Repre- rative comments were revealing as well, ranging from one individual
sentatives from all four DCO communities and researchers at various stating that most significant was “importance in terms of the fluxes
academic levels ranging from emeritus professors to undergraduate of carbon they process and their impacts on the habitable planet”
students completed the survey. More than half of the respondents not to another who felt criteria should be based on how the reactions
only answered the multiple-choice questions but provided additional “change the oxidation state of carbon-reduced, neutral, oxidized, with
comments. We also received about 20 very detailed answers with the product of the reaction having very different transport proper-
elaborate essays, illustrations, and references. ties.” Most telling was a third commenter, who stated “I feel very
strongly about this,” which was reassuring given the effort that the
The first survey question was: What criteria should be used for team had put in to enable the whole exercise. A further provocative
selecting a handful of reactions out of myriad chemical processes response suggested that “one way is to ask what if the Earth could
involving carbon in different host phases, variable valence states, be made again, but with only five reactions, which five involving
under a wide range of pressure and temperature conditions, and over carbon would make it look most like it does today?” Clearly, there
a vast span of spatial and temporal scales?Areaction may be consid- are many routes to discussing the “importance” or interest in any
ered important because it is essential to sustaining life on Earth (e.g., particular reaction, or indeed what is meant by reaction—whether
photosynthesis that converts carbon dioxide and water into sugar and the term should be restricted to chemical reactions or whether process
releases oxygen). A top-ranking reaction may involve a component or physical reactions might also be included.
that is minor in quantity but is of special economic and geological
interest (e.g., diamond formation). Proposed as a potential solution Survey respondents were then asked to pick their favorite reac-
to the global warming problem, carbonation of mantle peridotite may tions. The outcomes of the suggested reactions were diverse. The
be viewed as potentially important. On a more fundamental level, importance of photosynthesis to the development of life on Earth,
crystallization of Earth’s molten core to concentrate carbon in the and the importance of life to the respondents, is an understandable
solid inner sphere could stand out because it may bear on the driving priority. Other reactions mentioned at this stage include precipita-
power of the Earth’s magnetic field. If all carbon at Earth’s surface tion of calcite and dolomite in the sea followed by mineralization to
was initially dissolved in the mantle, as previously hypothesized, form limestone and dolomite (to sequester CO2), silicate weathering
then the transformation of diamond to graphite could be an important to carbonate, asthenosphere melting (to allow plate tectonics), dis-
reaction. Without this polymorphic transition, the activation energy solution of CO2 gas into water, respiration (reverse photosynthesis,
barrier to reacting diamond with anything may be so great as to lock to generate sugars), redox reactions of CO to C or CO2, redox melt-
up a large fraction of carbon in the form of diamond. Some critical ing, the Sabatier reaction (the passage from inorganic to organic
reactions may have stretched over an extended timescale (e.g., inner geochemistry), the burning of fossil fuels, and the polymerization
core formation), whereas others may be widespread spatially and increase in C-bearing minerals inside the deep Earth.

On the basis of the polling responses, we defined five broad

American Mineralogist, vol. 104, 2019

LI ET AL.: INTRODUCTION TO EARTH IN FIVE REACTIONS SPECIAL COLLECTION 467

categories of reactions for further consideration. In March 2018, Table 1. The eight reaction classes on the ballot
we convened a two-day workshop to select the top five carbon-
related reactions on Earth and develop a plan for sharing advances Reaction class Representative reactions
in deep carbon science with the scientific community and broader
audiences using the E5R framework. The workshop was held at Hydrogenation dehydrogenation CO2 + 4H2 = CH4 + 2H2O
the Carnegie Institution for Science in Washington, D.C., U.S.A. Carboxylation decarboxylation 6CO2 + 6H2O = C6H12O6 + 6O2
About 50 participants from seven countries on three continents Carbonation decarbonation CO2 + CaSiO3 = CaCO3 + SiO2
represented the DCO community. The group was selected to Carbon dioxide dissolution outgassing CO2(aq) = CO2(g)
reflect the totality of the DCO in terms of interests and scientific Hydration dehydration H2O + CO2 = H2CO3
expertise and achieved balance in terms of academic level, gender, CO2 + 18Mg2SiO4 + 6Fe2SiO4 + 26H2O = CH4 + 12Mg3S2O5(OH)4 + 4Fe3O4
and geographic distribution. Education and media experts, along Redox freezing melting MgCO3 + 2Fe = 3(Fe2/3Mg1/3)O + C
with several members of DCO’s Executive Committee, Secretariat, Metal silicate partitioning C(alloy) + 2FeO(silicate) = CO2(silicate)
and Synthesis Group 2019, rounded out the attendees. + 2Fe(alloy)
Fe-C solidification melting FeCx(l) = FeCy(l) + Fe7C3, where y < x
The participants were charged with choosing five discrete
chemical reactions from among hundreds that make Earth the only and through geological time, we may gain insights into the connec-
known habitable planet. They began by considering the survey tions among the deep carbon cycle, the “great oxidation event,” and
results and pondered what carbon-related reactions make Earth the origins of life on Earth.
unique. The group discussed the role of the deep carbon cycle in
plate tectonics and the geodynamo, the development of an oxygen- At the workshop, the top five reactions received comparable
rich atmosphere, how microbial life has persisted throughout numbers of votes, suggesting that the richness of DCO findings
Earth’s history giving rise to a diverse biosphere, various ways cannot be straightforwardly captured by a small number of reactions
water has influenced Earth’s evolution, and the origin of diamonds. and that there is a healthy diversity of equally important processes.
All attendees presented their perspectives and shared their ideas The three deep Earth reactions, including two diamond-forming
on how we could use chemical reactions as a framework to under- reactions, did not make the final five. These deep Earth reactions
stand and advance deep carbon science. With keynote speakers, are undoubtedly important because at least 90% of Earth’s carbon
short-talk presenters, and panelists primed to argue for or against is likely stored in the deep mantle and core. The voting results thus
their chosen reactions, debates were passionate and sometimes suggest a lack of awareness and appreciation for this deep carbon,
intellectually divisive. By the middle of day two, however, the even among DCO researchers. It implies that understanding extreme
group converged on a set of reactions central to defining Earth. carbon remains at the frontier of future research and will require
more effort to bring public awareness.
Five reactions were selected through anonymous voting. Prior
to voting, participants agreed that a pair of forward and reverse Following the discussion of the E5R survey and selection of
reactions counted as one reaction, and that similar and closely reactions, a group of scientists was motivated to review and integrate
related reactions would be grouped into a reaction class. With this recent findings through the lens of the selected reactions. This special
understanding in mind, eight reaction classes made to the ballot collection will feature review articles using the selected reactions
(Table 1). Hydrogenation, carboxylation, carbonation, carbon to integrate DCO science findings and illuminate the forms and
dioxide dissolution, and hydration emerged as winners (Fig. 1). flows of carbon on Earth. The collection as a whole provides a big
picture view of DCO discoveries in the preceding decade, what its
Understanding carbon cycle through reactions four Science Communities have learned about the role of carbon
in planetary function, and how the identified five reactions play an
The quest to identify the five most important reactions in deep integral role in carbon storage and pathways in Earth. The collection
carbon science has demonstrated that chemical reactions can provide will also include additional contributions of original research on
a unique and effective framework for synthesizing deep carbon other carbon-related polymorphic and chemical reactions.
research. Looking at a particular reaction such as serpentinization
has stimulated dialogue across DCO communities, leading to a The E5R synthesis project distilled the planet’s essence into a
deeper appreciation of its role in Earth’s volatile cycles. Mafic and set of key carbon-related reactions that make Earth special and then
ultramafic rocks react with water to form serpentinite. The geologi- used the reactions to encapsulate much of deep carbon science. It
cal process of serpentinization significantly affects the reservoirs has led to new insights to motivate and guide future research. We
and fluxes of carbon at subduction zones. In the presence of iron, hope that the special collection will help establish new frontiers for
serpentinization may produce hydrogen and form methane, thus scientific exploration and investigation to address the fundamental
profoundly influencing deep life on Earth, and maybe even life’s question of Earth’s habitability.
origins. Chemical reactions also can be used as “threads” to con-
nect disparate findings into coherent and meaningful pictures. For Funding
example, redox reactions are prevalent in geological and biological
processes and often involve carbon-bearing species with variable The “Earth in five reactions” project is supported by the Alfred Sloan Foundation
valence states. They are of interest to all communities within DCO: through grant G-2016-7157 to the University of Michigan. The principal investigators
Redox reactions have been found to influence volcanism, diamond of the project are Jie Li and Simon Redfern.
formation, the abiogenic production of hydrocarbons and are central
to life’s metabolism. By comparing the mechanisms, conditions, and Acknowledgments
energetics of these reactions and studying how they vary spatially
The project received guidance from the Synthesis Group (a.k.a. SG2019) Chair
Marie Edmonds and crucial support from SG2019 manager Darlene Trew Crist, Katie
Pratt, and Josh Wood, survey respondents, and workshop participants. The authors thank
Keith Putirka for supporting the special collection.

Manuscript received October 3, 2018
Manuscript accepted October 13, 2018
Manuscript handled by Keith Putirka

American Mineralogist, vol. 104, 2019

American Mineralogist, Volume 104, pages 468–470, 2019

Earth in five reactions: Grappling with meaning and value in science k

Robert M. Hazen1,*

1Geophysical Laboratory, Carnegie Institution for Science, 5251 Broad Branch Road NW, Washington, D.C. 20015, U.S.A. Orcid 0000-0003-4163-8644

Abstract
The Earth in Five Reactions Workshop posed two significant challenges: (1) the formulation of a
conceptual definition of “reaction” and (2) the identification and ranking of the “most important reac-
tions” in the context of planetary evolution. Attempted answers to those challenges, collated in this
collection of articles, reflect both the opportunities and hurdles when scientists deal with questions
of meaning and value.
Keywords: Epistemology, value in science, planetary evolution; Earth in Five Reactions: A Deep
Carbon Perspective

Introduction a general conceptual definition involving a transformation by
rearrangement of atoms in one or more materials, but several
The objective of the “Earth in Five Reactions” project was types of uncertainty in meaning complicated the discussions.
to identify the five “most important reactions” that influence
planetary history (Li et al. 2019). A diverse team of 50 scientists One aspect of this uncertainty related to the degree to which
tackled this task during a workshop at the Carnegie Institution a reaction can be idealized. For some participants with a more
for Science in Washington, D.C., March 22–23, 2018. The event chemical background, a reaction is a specific rearrangement of
was conceived both as a forum to promote discussions among atoms and their electrons, such as the oxidation of iron:
scientists with diverse backgrounds in geology, chemistry, biology,
and space science, and to start conversations that might provide 4Fe + 3O2 ↔ 2Fe2O3. (1)
opportunities to engage a broader community of non-professional
science enthusiasts. To many chemists, this form of reaction equation represents
real atoms of iron reacting with real molecules of oxygen. A
On the surface, the task seemed straightforward. Each partici- similar aqueous reaction, “hydrogenation,” represents a useful
pating scientist was asked to formulate an opinion regarding the model in the context of natural Earth systems:
“most important reaction” that has influenced Earth’s origin and
evolution and then advocate that position to the larger group. At 2FeO + H2O ↔ Fe2O3 + H2. (2)
the Workshop’s conclusion the proposed reactions were tabulated,
everyone voted, and the top five reactions “won.” Each of the top Others argued for a more general definition of reaction
five reactions, as well as several “runners-up” promoted by minori- that recognized classes of related chemical reorganizations,
ties of passionate advocates, were given the chance to contribute for example, “serpentinization,” which can be represented
articles to this special section of American Mineralogist. by a number of different reactions of Mg- and Fe2+-bearing
basalt minerals via aqueous alteration (Schrenk et al. 2013).
In reality, this task proved exceptionally challenging for two On the one hand, serpentinization can be defined in terms of
reasons that only emerged through lively, and sometimes confused, the reaction of anhydrous magnesian olivine and water to form
conversations. The first challenge related to meaning: general the hydrous minerals serpentine and brucite (also known as
agreement was lacking on what constitutes a “reaction” in the con- a “hydration” reaction):
text of planets and their evolution. The second challenge focused
on value: it was unclear by what metrics we should evaluate the 2Mg2SiO4 + 3H2O ↔ Mg3Si2O5(OH)4 + Mg(OH)2. (3)
“most important” reactions. Both points of discussion—meaning
and value—have a character that provoked intense and enjoyable However, serpentinization’s role in supporting microbial
debates, but neither question is amenable to unambiguous resolu- communities may be more closely linked to the oxidation of
tion by the scientific method. This contribution is an attempt to iron-bearing olivine to produce magnetite, silica, and hydrogen:
characterize the Workshop’s gestalt, and to draw lessons from the
exercise that might inform similar efforts in the future.

What do we mean by “reaction”? 3Fe2SiO4 + 2H2O ↔ 2Fe3O4 + 3SiO2 + 2H2. (4)

The first hurdle facing the Earth in Five Reactions Workshop In another respect, the essence of all of these reactions from
was lack of a collective agreement on the definition, or rather the perspective of biological energy flow is the oxidation of Fe2+
broad range of meanings, of “reaction.” All participants accepted and release of hydrogen—a process that can be idealized as:

* E-mail: [email protected] 3FeO + H2O ↔ Fe3O4 + H2. (5)
k Open access:Article available to all readers online. This article has an MSAlicense.

0003-004X/19/0003–468$05.00/DOI: https://doi.org/10.2138/am-2019-6745 468

HAZEN: GRAPPLING WITH MEANING AND VALUE IN SCIENCE 469

Scientists who considered reactions from the perspective separation of immiscible silicate- and iron-rich liquids from
of biology and the origins of life also emphasized the role of a homogeneous fluid and the subsequent physical process of
serpentinization in producing organic molecules—a process gravitational segregation:
epitomized by the unbalanced schematic reaction:

[Fe,Si,O](fl) ↔ [Si,O](fl) + Fe(fl). (12)

(Fe,Mg)2SiO4 + H2O + CO2 ↔ Mg3Si2O5(OH)4 + Fe3O4 + CH4. (6)

Therefore, while many of us agreed that “serpentinization” is In a similar vein, Papineau et al. (2017) propose that a class
one of the most important chemical reactions on wet terrestrial of chemically oscillating reactions, by which phase separation
worlds, multiple facets of serpentinization exist. Consequently, leads to complex concentric patterning and self-organization in
the representation of that reaction by an equation remains some- many natural and synthetic chemical environments, represents a
what ambiguous. Bioscientists provided another perspective on central organizing principle in both living and nonliving systems.
the definition of “reaction,” epitomized by the globally important
process of oxygenic photosynthesis, but more generally described A part of the debate centered on whether changes in state and
as “carboxylation,” idealized as: phase transformations should be included as “reactions.” For
example, some participants suggested that the transformation
of carbon dioxide in an aqueous fluid to a gas phase should be
numbered among Earth’s most important reactions:

6H2O + 6CO2 ↔ C6H12O6 + 6O2. (7)

CO2(aq) ↔ CO2(g). (13)

However, this equation for the oxygenic photosynthetic Finally, at one point the discussion led to consideration of
reaction is a simplified representation of an intricate reaction stellar nucleosynthesis, by which a range of new chemical ele-
cascade involving 10 or more individual enzyme-induced steps, ments form through cascades of nuclear reactions. Such reactions
driven by the energy gathered in two different photon-capturing are confined to stellar processes and are beyond the scope of
complexes, called Photosystem I and Photosystem II (e.g., Cox Earth and other terrestrial bodies, but they were instrumental in
2017). In this instance, the simplified reaction of Equation 7 is the formation of all planets and moons. Similarly, nuclear reac-
a proxy for several complex reaction networks, each a sequence tion associated with radioactive decay, though fundamentally
of biochemical steps. important to planetary heat production, were not considered by
the Workshop participants.
The biological case is also striking in that some scientists
argued that the reverse reaction, “respiration” as employed by By the end of the Workshop, all participants developed a
many animals (including us), is equally important: more nuanced understanding of the breadth and depth of the
question, “What is a reaction?” Ultimately, the majority agreed
C6H12O6 + 6O2 ↔ 6H2O + 6CO2. (8) that “reaction” refers to a constellation of processes, all of which
can be expressed by an equation, some more fictive than others,
Indeed, all of the proposed reactions may be written with but all involving the rearrangement of atoms and their electrons
arrows pointing in either direction; consequently, some par- and all serving as representations of planetary events that shaped
ticipants asked whether bi-directional arrows should be used. the evolution of Earth.
Of special note in this regard are carbonation/decarbonation
reactions, which are critical to Earth’s deep carbon cycle (e.g., What are Earth’s “most important” reactions?
Dasgupta 2013; Kelemen and Manning 2015). These reactions
can be written in idealized form as: The second and arguably more difficult challenge to the Earth
in Five Reactions Workshop was the ranking of reactions as most
CaSiO3 + CO2 ↔ CaCO3 + SiO2. (10) important. “Most important” implies value, but scientists are
not typically schooled in assigning a value to natural objects or
The reaction from left to right plays an important role in sili- phenomena. Indeed, the epistemology of science, rooted as it is
cate weathering and carbon sequestration, whereas the reaction in independently reproducible and verifiable observations, would
from right to left occurs both in nature (charnockitization) and seem antithetical to assigning relative values to natural processes.
in human industry (e.g., a net, long-term effect of the curing of And so Workshop attendees grappled with competing perceptions
Portland cement). Interestingly, hydrocarbon burning, the class of importance.
of oxidation reactions most implicated in Earth’s recent anthro-
pogenic increases in atmospheric carbon dioxide (and arguably A revealing aspect of the Workshop was the initial general
the single most prominent chemical reaction informing the news mood that some agreement might be reached regarding a “correct”
and policy today), was not included in the final Workshop list: answer that could be identified by focused presentations, conver-
sations, and debate. Only gradually did the subjective challenge
2[CnH2n+2] + (3n + 1)O2 ↔ 2nCO2 + (2n + 2)]H2O. (11) of the task of identifying the “most important reactions” dawn on
workshop participants. None of us was trained in the epistemol-
Geophysicists presented yet another discipline-informed ogy of assigning value to natural processes. Faced with the task of
perspective on the nature of a “reaction.” For example, planetary- ranking “reactions,” we were stymied. Nevertheless, we tried and,
scale differentiation and core formation, which is fundamental as the Workshop progressed, participants became bolder (and in a
to planetary evolution, can be represented as the chemical sense more exuberantly playful) in their advocacy of one subjec-
tive “truth” vs. another.

American Mineralogist, vol. 104, 2019

470 HAZEN: GRAPPLING WITH MEANING AND VALUE IN SCIENCE

Some scientists attempted to rank reactions by calculating A key meta-message of the Workshop—one still being pro-
quantitative consequences: which reactions transfer the greatest cessed by many who attended—is the importance of recognizing,
planetary mass, affect the largest planetary volume, or sustain the perhaps embracing, the subjective role of “value” in science.
largest near-surface redox gradients. Others favored reactions that Though we are trained as scientists to be objective in our collec-
most dramatically altered planetary-scale structures, mechanisms tion and analysis of data, and we are not generally schooled in the
that synthesized molecules of life, or processes that created habit- philosophy of ranking, we are nevertheless faced with subjective
able planetary environments. Almost invariably, choices were choices every day of our careers. We make judgment calls about
biased by one’s scientific specialty. Biologists favored biological what topics we should spend our time studying. We provide prose
reactions such as biomolecular synthesis, metabolic pathways, and on the “Broader Impacts” of our research to National Science
photosynthesis, while geophysicists pointed to the global-scale Foundation proposals, while we evaluate and rank the propos-
processes of planetary differentiation, core formation, and the als and manuscripts of other scientists. We write “Implications”
establishment of a magnetodynamo. In addition, significant spon- sections as conclusions to our articles in American Mineralogist.
sorship by the Deep Carbon Observatory, whose 10 yr mission is Each of these activities and many others carries the responsibility
to understand the physical, chemical, and biological roles of carbon of evaluating and ranking ideas and opportunities.
in Earth and other planets (see https://deepcarbon.net; accessed
October 30, 2018), significantly swayed Workshop attendees to In that context, it is inspiring the extent to which one common
pay special notice to carbon-bearing reactions. theme emerged from our consideration of Earth’s most important
reactions. To those scholars who devote their lives to understand-
The centrality of several chemical reactions to the origins and ing Earth, our planetary home is unique, fascinating, and valued
evolution of life rose to the top of many scientists’ lists. Oxygenic beyond all other worlds. The most important reactions are those
photosynthesis (and the reverse reaction, respiration), was a leading that contribute to Earth’s unique geosphere and biosphere. We
candidate. Some planetary scientists’ focus included questions of value reactions that created a habitable world—a protective mag-
habitability and the possibility of life’s origins on other worlds; netosphere, a dynamic hydrosphere, and a benign atmosphere.
hence they advocated the Urey reaction, by which primitive at- We value reactions that led to life’s origins and evolution—the
mospheric molecules reacted to form amino acids and other key prolific synthesis of essential biomolecules, the release of redox
biomolecular building blocks when exposed energetic electric energy through serpentinization, and the self-organization of
discharges, UV radiation, or other ionization events (Miller and chemical systems. And we embrace reactions, notably oxygenic
Urey 1959). photosynthesis, that ultimately led to multi-cellularity, to the ter-
restrial biosphere, and to Earth’s unmatched mineral diversity,
Comparative planetology added a layer of complexity to the as well. In that sense, the joyous task of identifying Earth’s most
question of value. While some participants focused exclusively on important reactions became a celebration of the beautiful home
Earth and reactions specifically in the context of Earth’s evolving that we cherish.
geosphere and biosphere, other scientists considered reactions in
the broader context of any terrestrial planet or moon. Thus, serpen- Acknowledgments
tinization may have “beaten out” oxygenic photosynthesis in the
rankings because the former is likely to be a dominant near-surface I am grateful to Jackie Li, Simon Redfern, Donato Giovannelli, and all of the
process on any wet, rocky world, whereas the latter requires an Earth in Five Reaction Workshop organizers and participants for the opportunity
evolutionary pathway thus far unique to Earth. to contribute this essay. I received valuable comments and suggestions from
Marie Edmonds, Jie Li, Craig Manning, Craig Schiffries, David Walker, and an
Given the diversity of scientific backgrounds, it quickly became anonymous reviewer.
apparent that participants embraced different conclusions regarding
Earth’s most important reactions—conclusions that are reflected This publication is a contribution to the Deep Carbon Observatory. Studies
in the diversity of articles in this special section. Perhaps the most of the deep carbon cycle and the philosophy of science are supported by the Deep
intriguing shift in attitudes to occur during the Workshop was the Carbon Observatory and the Alfred P. Sloan Foundation, the John Templeton Foun-
general realization that ranking of the “most important reactions” dation, the NASA Astrobiology Institute, a private foundation, and the Carnegie
inevitably is subjective; consequently, the participants became free Institution for Science.
to advocate for one position or another based on more qualitative
and subjective arguments than are common in scientific discourse. References cited

Implications Cox, N. (2017) Lehninger Principles of Biochemistry, 7th International Edition.
Freeman.
Participants in the Earth in Five Reactions Workshop came
away with several insights beyond the details of planetary reaction Dasgupta, R. (2013) Ingassing, storage, and outgassing of terrestrial carbon through
mechanisms. One key lesson was the value of interdisciplinary geologic time. Reviews in Mineralogy and Geochemistry, 75, 183–229.
conversations. Each participant left the meeting with a broader
perspective of the natural world, thanks to the open and thoughtful Kelemen, P.B., and Manning, C.E. (2015) Reevaluating carbon fluxes: What goes
interactions among individuals with diverse geo-, bio-, and plan- down, mostly comes up. Proceedings of the National Academy of Sciences,
etary science backgrounds. Planetary evolution involves complex 112, E3997–E4006.
connections among, physical, chemical, and (in the case of Earth)
biological processes. The only way to understand terrestrial worlds Li, J., Redfern, S.A., and Giovannelli, D. (2019) Deep carbon cycle through five
is to document interactions from varied perspectives, at many reactions. American Mineralogist, 104, 465–467.
scales, from crust to core, and over immense spans of time.
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American Mineralogist, vol. 104, 2019

American Mineralogist, Volume 104, pages 1369–1380, 2019

Carbonation and decarbonation reactions: Implications for planetary habitability k

E.M. Stewart1,*,†, Jay J. Ague1, John M. Ferry2, Craig M. Schiffries3, Ren-Biao Tao4,
Terry T. Isson1,5, and Noah J. Planavsky1

1Department of Geology & Geophysics, Yale University, P.O. Box 208109, New Haven, Connecticut 06520-8109, U.S.A.
2Department of Earth and Planetary Sciences, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, U.S.A.

3Geophysical Laboratory, Carnegie Institution for Science, 5251 Broad Branch Road NW, Washington, D.C. 20015, U.S.A.
4School of Earth and Space Sciences, MOE Key Laboratory of Orogenic Belt and Crustal Evolution, Peking University, Beijing 100871, China

5School of Science, University of Waikato, 101-121 Durham Street, Tauranga 3110, New Zealand

Abstract

The geologic carbon cycle plays a fundamental role in controlling Earth’s climate and habitability.
For billions of years, stabilizing feedbacks inherent in the cycle have maintained a surface environ-
ment that could sustain life. Carbonation/decarbonation reactions are the primary mechanisms for
transferring carbon between the solid Earth and the ocean–atmosphere system. These processes can
be broadly represented by the reaction: CaSiO3 (wollastonite) + CO2 (gas) ↔ CaCO3 (calcite) + SiO2 .(quartz) This
class of reactions is therefore critical to Earth’s past and future habitability. Here, we summarize their
significance as part of the Deep Carbon Obsevatory’s “Earth in Five Reactions” project. In the forward
direction, carbonation reactions like the one above describe silicate weathering and carbonate formation
on Earth’s surface. Recent work aims to resolve the balance between silicate weathering in terrestrial
and marine settings both in the modern Earth system and through Earth’s history. Rocks may also
undergo carbonation reactions at high temperatures in the ultramafic mantle wedge of a subduction
zone or during retrograde regional metamorphism. In the reverse direction, the reaction above repre-
sents various prograde metamorphic decarbonation processes that can occur in continental collisions,
rift zones, subduction zones, and in aureoles around magmatic systems. We summarize the fluxes and
uncertainties of major carbonation/decarbonation reactions and review the key feedback mechanisms
that are likely to have stabilized atmospheric CO2 levels. Future work on planetary habitability and
Earth’s past and future climate will rely on an enhanced understanding of the long-term carbon cycle.

Keywords: Decarbonation, carbonation, Urey reaction, carbon flux; Earth in Five Reactions: A
Deep Carbon Perspective

Introduction CaSiO3 (wollastonite) + CO2 (gas) ↔ CaCO3 (calcite) + SiO2 (quartz) (1)

Life has existed on planet Earth for more than three billion and its reverse, decarbonation:
years. In that time there have been profound changes in the
brightness of the Sun, the temperature of the deep Earth, and even CaCO3 (calcite) + SiO2 (quartz) ↔ CaSiO3 (wollastonite) + CO2 (fluid) (2)
the length of a day, yet throughout all of these changes, the envi-
ronment has remained stable enough to support life. The global Consequently, these reactions are critical controls on the long-
carbon cycle is generally agreed to have played a critical role term atmospheric composition, climate, and habitability of Earth,
in maintaining this habitable climate on Earth. Carbon dioxide and they form an essential piece of the Deep Carbon Observa-
(CO2) acts as a greenhouse gas, in effect trapping solar energy and tory’s “Earth in Five Reactions” initiative (introduced by Li et al.
raising the temperature of the planet. Over geologic timescales 2019). Note that these two simple reactions are used to represent
(about one million years or longer), carbon is exchanged between many decarbonation/carbonation reactions involving other cat-
the solid Earth and the atmosphere. The rate of atmospheric CO2 ions (especially Mg2+) and other silicate minerals (see below).
removal increases with temperature, thus the exchange acts as a
global thermostat, stabilizing atmospheric CO2 concentrations Carbonation reactions such as reaction 1 occur when carbon
and therefore moderating Earth’s surface temperature. Carbon in a gas or fluid reacts with silicate minerals to form a solid,
dioxide is exchanged between the solid Earth and the atmosphere commonly a carbonate mineral. Studies of these reactions have
via carbonation reactions such as the archetypal: a long history in petrology and geochemistry. As early as 1894,
the Swedish chemist Arvid G. Högbom suggested that geologic
* E-mail: [email protected] processes could remove CO2 from the atmosphere (Högbom
k Open access: Article available to all readers online. 1894; see review by Berner 1995). In particular, the weathering of
† Special collection papers can be found online at http://www.minsocam.org/MSA/ silicate rocks provides the necessary chemistry to form carbonate
AmMin/special-collections.html. minerals, ultimately transforming gaseous CO2 into solid rock.
This fundamental carbonation reaction was discussed in detail

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1370 STEWART ET AL.: CARBONATION/DECARBONATION REACTIONS AND PLANETARY HABITABILITY

by Nobel Prize-winning chemist Harold Urey more than 50 years MgSiO3 (enstatite) + CO2 (gas) ↔ MgCO3 (magnesite) + SiO2 (quartz). (3)
later. In his 1952 book, The Planets, Urey rearticulated the rela-
tionship between carbonate and silicate rocks, writing reaction 1, From here forward we will focus our discussion on reaction 1, but
known today as one of the “Urey reactions.” Furthermore, he note that reaction 3 functions essentially identically (Urey 1952).
suggested that such reactions have controlled atmospheric CO2
concentrations throughout Earth history. In fact, reactions 1 and 3 as written do not often occur on the
surface of the Earth. For one, the mineral wollastonite actually
Decarbonation (e.g., reaction 2) generally occurs when a rock makes up very little of the Earth’s crust. About 50% of the crust
containing carbonate minerals, such as a siliceous limestone, is is composed of feldspars (e.g., Ronov et al. 1990). These are
metamorphosed at elevated temperatures and pressures. Victor also silicate minerals, but with more complicated chemistry and
Moritz Goldschmidt (1912) was the first to recognize the sig- extensive solid solution. Plagioclase feldspars, for example, are
nificance of metamorphic decarbonation. Goldschmidt noted a solid solution between CaAl2Si2O8 (anorthite) and NaAlSi3O8
that the minerals quartz (SiO2) and calcite (CaCO3) react to form (albite). In nature, weathering of these more complex Ca-bearing
wollastonite (CaSiO3) and CO2 gas when a rock is heated to a silicates may produce phases in addition to calcite and quartz,
high enough temperature; his work was among the earliest to such as aluminous clays. Similarly, reaction 3 references the
use thermodynamic principles to calculate a mineral equilibrium carbonate mineral magnesite, which is also relatively rare on
and to quantitatively constrain the conditions of metamorphism. Earth’s surface in its pure form. Thus reactions 1 and 3 are used
as exemplars of carbonation reactions in general but do not reflect
Norman L. Bowen (1940) demonstrated that, depending on the typical mineralogy involved.
bulk composition, 13 different decarbonation reactions might
occur as a siliceous limestone or dolomite is progressively Additionally, on Earth’s surface, carbonation reactions
heated. In 1956, Harker and Tuttle produced wollastonite in the involve a series of reaction steps as follows (after Siever 1968).
laboratory via reaction 2. Their experiments more tightly con-
strained the P-T (pressure-temperature) conditions of reaction, First, CO2 gas in the atmosphere is dissolved in water (H2O)
yet, remarkably, Goldschmidt’s thermodynamic estimate from to form carbonic acid (H2CO3):
more than 40 years earlier was close to their result.
CO2 (gas) + H O2 (liquid) ↔ H2CO3 .(aqueous) (1A)
Today, researchers continue to study carbonation/decarbon-
ation reactions in essentially all of Earth’s geotectonic settings This carbonic acid can dissociate to form a negatively charged
(Fig. 1) through numerical simulations, measurements of modern bicarbonate anion (HCO3-) and positively charged H+ cation:
CO2 fluxes, and examination of the history preserved in the rock
record. A detailed accounting of the rates, timing, location, and H2CO3 (aqueous) ↔ HCO3-(aqueous) + H .+ (1B)
magnitude of these reactions is essential to understanding CO2 (aqueous)
fluxes and our planet’s past, present, and future habitability.
The acidity (H+ cations) in the water allows a calcium-bearing
Carbonation silicate mineral, here wollastonite (CaSiO3), to dissolve and form
silicic acid (H4SiO4):
The Urey reactions
CaSiO3 (wollastonite) + 2H+ + H O2 (liquid) ↔
The two Urey reactions, one of which is mentioned above, (aqueous)
are the quintessential exemplars of carbonation reactions on
Earth. The other reaction is quite similar to reaction 1 and may Ca2+ + H4SiO4 .(aqueous) (1C)
run in parallel. The only difference is that it involves the Mg2+ (aqueous)
cation instead of Ca2+:
The Ca2+ cation is now free to react with the HCO3- anions to form
the mineral calcite (CaCO3), more CO2, and water:

Collisional Orogen Arc Volcanism
Contact
Regional Continental Mid-Ocean Ridge
Metamorphism Weathering Volcanism Metamorphism

Carbonate Marine Subduction
Precipitation Weathering Metamorphism

Sources of CO2 Carbonation
Sinks of CO2

FiFgiugruere11. Schematic cross section showing the tectonic context of major carbonation/decarbonation processes. Sources of atmospheric CO2

are indicated in red, whereas sinks are in blue.

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STEWART ET AL.: CARBONATION/DECARBONATION REACTIONS AND PLANETARY HABITABILITY 1371

Ca2+ + 2HCO3-(aqueous) ↔ Earth history, carbonation has been the primary way that CO2
(aqueous) is removed from the atmosphere (Urey 1952), and without this
process life on Earth could not exist (e.g., Berner and Caldeira
CaCO3 (calcite) + CO2 (gas) + H2O(liquid). (1D) 1997). For comparison, consider our neighboring planet, Venus,
where the Urey reactions rarely occur. Much less carbon is stored
Finally, the mineral quartz (SiO2) may grow from the silicic acid in the solid rock of Venus, thus the Venusian atmosphere con-
in solution with more water as a by-product: tains massive amounts of CO2, contributing to average surface
temperatures of more than 400 °C (Sagan 1962).
H4SiO4 (aqueous) ↔ SiO2 (quartz) + 2H2O(liquid). (1E)
Silicate weathering
We can sum up these five sub-reactions (doubling reactions
1A and 1B for balance) to make one total reaction: On geologic timescales, silicate weathering is the rate-
limiting step of the Urey reaction. Because CO2 dissolution,
2CO2 (g) + 3H2O(l) + 2H2CO3 (aq) + CaSiO3 (wol) + 2H(+aq) + carbonic acid dissociation, and the other intermediate reactions
Ca2(a+q) + 2HCO3-(aq) + H4SiO4 (aq) ↔ occur relatively quickly, the availability of silicate-bound Ca2+
2H2CO3 (aq) + 2HCO3- (aq) + 2H+(aq) + Ca(2a+q) + (or Mg2+) (reaction 1C) is of critical importance. This can be
thought of in terms of the seawater’s alkalinity, that is, its ability
H4SiO4 (aq) + CaCO3 (cc) + CO2 (g) + 3H2O(l) + SiO2(q) (1F) to neutralize acid. Silicate weathering increases the alkalinity of
the seawater that drives carbonate precipitation. Note that any
and by cancelling species present on both sides of the reaction contribution to total alkalinity drives carbonation, thus weather-
return to the simplified reaction 1. ing of Mg-silicate minerals could ultimately drive the formation
of Ca-carbonates.
This reaction sequence has several important aspects to note.
First, there are some nuances of the full reaction 1F that one can- Traditionally, geologists have considered continental rocks
not observe in the simplified reaction 1. For example, the full to be the primary contribution to global silicate weathering (e.g.,
reaction 1F is only 50% efficient at storing CO2; for every two Walker et al. 1981; Berner et al. 1983). Continental weathering
molecules of CO2 that are dissolved in water, only one molecule depends on a sequence of discrete processes. First, continental
is transformed into calcite while the other molecule is re-released rocks must be exposed at Earth’s surface. Surface rocks are then
as CO2 gas. Second, note also that each of the steps may occur at physically (or mechanically) weathered, that is, broken apart
a different point in space and time. A silicate mineral may weather into smaller pieces. Chemical weathering can then occur on the
and dissolve in a river in the middle of a continent (reaction exposed surfaces (as seen in Fig. 2a), partially dissolving the
1C), but the Ca2+ ion may travel thousands of kilometers before rock and releasing aqueous ions (i.e., reaction 1C). There is a
forming calcite in the ocean (reaction 1D). Since the evolution positive association between physical and chemical weathering–
of marine calcifiers, carbonate mineral precipitation has often mineral dissolution can contribute to denudation while physical
been facilitated by biological processes (e.g., the formation of weathering can expose more reactive surface area and facilitate
a foraminifera skeleton), although abiotic precipitation also chemical weathering. The ions resulting from weathering are
occurs. In either case, the formation of carbonate minerals from transported by rivers and ultimately delivered to the ocean where
solution functions as a key piece of this Urey reaction sequence. they continue along the Urey reaction sequence.

The most important take-away from the Urey reactions is Many key factors can affect the rate of continental weathering.
this: surface carbonation reactions are a major sink for atmo- For example, tectonic collisions that form high mountain belts
spheric CO2. In fact, more than 99% of all carbon in the crust, help to expose more rock at Earth’s surface, which may increase
biosphere, and ocean-atmosphere system is stored in sedimentary
and metasedimentary rocks (e.g., Archer 2010). Throughout

A B carbonated serpentinite + magnetite C

Granite Di+ Qz Bi
Amp+ vein

Zo

{ Weathered 5 cm 5 cm
Rind

1m

FigFuigruere22. Photographs of carbonation/decarbonation processes. (a) Silicate weathering: granite exposed at Earth’s surface undergoes spheroidal

weathering resulting in discrete boulders with thick physical and chemical weathering rinds. Volax, Tinos Island, Greece. (b) High-temperature
carbonation: yellow Ni-bearing calcite has precipitated in an ultramafic rock of the Maltby Lakes Metavolcanics, Connecticut. (c) Infiltration-driven
decarbonation: quartz vein (center; Qz) with diopside (Di) + amphibole (Amp) + zoisite (Zo) selvages cutting biotite-bearing metacarbonate rock
(dark margins; Bi) of the Wepawaug Schist, Connecticut. Prograde reactions, such as Phlogopite + 3 Calcite + 6 Quartz = 3 Diopside + K-feldspar
+ 3 CO2 + H2O, generated significant CO2 (Ague 2003; Stewart and Ague 2018).

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1372 STEWART ET AL.: CARBONATION/DECARBONATION REACTIONS AND PLANETARY HABITABILITY

the potential for weathering (e.g., Raymo and Ruddiman 1992; emplified by reaction 1. When CO2-bearing fluid infiltrates a
Edmond et al. 1995; Dessert et al. 2003). Increased precipitation silicate rock (especially a mafic or ultramafic rock with a high
can lead to more physical erosion, and the additional water can concentration of Mg2+) this fluid may react with the silicate
also drive more chemical weathering (H2O is a necessary reactant minerals, removing CO2 from the fluid phase and forming new
in reaction 1C) (Jenny 1941; Loughnan 1969; Amiotte et al. 1995; carbonate minerals (see Fig. 2b). The particular silicate minerals
White and Blum 1995; Maher and Chamberlain 2014). At higher involved in the reaction will depend upon the rock composition
temperatures any chemical reaction will have a faster reaction and the P-T conditions.
rate (Arrhenius 1915); thus, increasing surface temperatures
may result in faster continental weathering (Walker et al. 1981; One locus of such carbonation is the mantle wedge above a
Berner et al. 1983; Manabe and Stouffer 1993). subducting slab (Falk and Kelemen 2015; Piccoli et al. 2016,
2018; Scambelluri et al. 2016). As oceanic crust is subducted
Marine weathering into the mantle, it releases fluid into the overriding plate. This
fluid may be dominantly H2O, but subduction zone decarbonation
More recently, some researchers have proposed that marine reactions may supply CO2 as well (see section on decarbonation).
weathering processes have an important role to play in global The ultramafic rocks of the mantle are highly reactive with CO2
carbon cycling (e.g., Staudigel et al. 1989; Brady and Gíslason fluids, so when the slab-derived fluid rises into the mantle,
1997; Wallmann et al. 2008; Coogan and Gillis 2013; Coogan and carbonation reactions are fast (Sieber et al. 2018). Because the
Dosso 2015). The concept is the same; silicate minerals undergo degree of carbonation increases with lower temperatures (Sieber
chemical reaction that supplies alkalinity to the oceans and helps et al. 2018), mantle wedge carbonation probably dominates in
form carbonate minerals. Marine weathering can occur within the cooler (but still hot at < ~700 °C) fore-arc region and is less
the marine sediment pile (e.g., Wallmann et al. 2008; Solomon pronounced in the hotter mantle directly below the volcanic arc.
et al. 2014) or in basalts in “off axis” hydrothermal systems
(Coogan and Dosso 2015). Evidence for chemical weathering The fate of this carbonated mantle is not well known. It may
of silicate minerals within the sediment pile comes from deep serve as a location of long-term deep carbon storage (Kelemen
anoxic (oxygen-free) sediments (Wallmann et al. 2008; Solomon and Manning 2015). Alternatively, melting of carbonated mantle
et al. 2014). material may ultimately contribute CO2 to the atmosphere when
the melt erupts from overlying arc volcanoes (Kerrick and Con-
In “off-axis” hydrothermal systems, large amounts of sea- nolly 2001; Gorman et al. 2006; Kelemen and Manning 2015;
water flow through the oceanic crust (off-axis simply refers to Mason et al. 2017).
the fact that these systems are not located directly adjacent to
mid-ocean ridge volcanoes). Along its flow path the water is Similar high-temperature carbonation of ultramafic rocks can
heated to moderate temperatures (tens of degrees Celsius) and also occur in orogenic belts during prograde regional (Evans and
dissolves silicate minerals as in reaction 1C. Once again, this Trommsdorff 1974; Ferry et al. 2005) and contact (Ferry 1995)
dissolution delivers the alkalinity that allows carbonate minerals metamorphism. In addition, fluid infiltration during retrograde
to form (e.g., Staudigel et al. 1989; Brady and Gíslason 1997; metamorphism (i.e., metamorphism as rocks cool down from
Gillis and Coogan 2011). Hydrothermal circulation may also peak T) may drive carbonation reactions. For example, the
affect ultramafic rocks, creating carbonated serpentinites in both mineral wollastonite is stable at high temperatures (see below).
mid-ocean ridge and off-axis systems (Kelemen et al. 2011). The As the temperature falls, the wollastonite will react with any
impact these marine processes have on global carbon cycling may available CO2 to create calcite and quartz (Ferry 2000). Critically,
be just as significant as the effect of continental weathering (e.g., this retrograde carbonation reaction cannot occur in the absence
Wallmann et al. 2008; Coogan and Dosso 2015). of a CO2-bearing fluid (Tian and Ague 2014).

Reverse weathering Decarbonation

Reverse weathering refers to the formation of silicate clay Decarbonation reactions, such as reaction 2, are a major
minerals from solution. It is “reverse” weathering in the sense source of atmospheric CO2 in geologic history. In the example
that it consumes the alkalinity and silica that the forward silicate reaction, a carbonate mineral (calcite) reacts with a silicate
weathering reaction 1C provides (Sillén 1961; Garrels 1965; mineral (quartz) to form the Ca-silicate mineral wollastonite
Mackenzie and Garrels 1966). Reverse weathering is regarded and CO2. Reaction 2 represents myriad decarbonation reactions
as a net positive source of atmospheric CO2. In other words, that all share these features: (1) a carbonate mineral reacts with
reverse weathering allows for efficient recycling of carbon a silicate mineral, (2) a new silicate mineral is formed using
within the ocean-atmosphere system, elevating atmospheric CO2 divalent cations from the carbonate (e.g., Ca2+, Mg2+, ...), and (3)
concentrations. Recent work suggests that changes in the amount CO2 is released. (Rarely, decarbonation reactions may occur in
of reverse weathering may have had profound climatic impacts the absence of silicate minerals when a carbonate mineral breaks
over the course of Earth history (Isson and Planavsky 2018). down into a mineral oxide and CO2.)

High-temperature carbonation in subduction zones and Increasing temperature drives decarbonation. Certain
orogens carbonate–silicate mineral assemblages (such as calcite and
quartz) are only stable together up to a certain temperature at
Carbonation reactions also take place deeper in Earth and a given pressure and fluid composition. When the temperature
at higher pressure-temperature conditions. High-temperature rises beyond that point, they react and release CO2 (Goldschmidt
carbonation occurs via many different reactions that are ex- 1912; Bowen 1940; Harker and Tuttle 1956). Figure 4 is a P-T
phase diagram showing that mineral assemblage is stable at a

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STEWART ET AL.: CARBONATION/DECARBONATION REACTIONS AND PLANETARY HABITABILITY 1373

Proterozoic suture Decarbonation in subduction zones

Phanerozoic suture Subduction zones are a particularly important setting of
CO2 exchange. As oceanic crust sinks into the mantle, it brings
Modern subduction carbon-bearing minerals into the deeper Earth (Plank and
Langmuir 1998). However, metamorphic decarbonation reac-
Figure 3 tions in both the subducting slab and the overriding plate may
consume carbonate minerals and release CO2 back toward the
Figure 3. Global map indicating the locations of modern subduction surface. Thus, subduction may transfer carbon from the crust
(red lines with carets), and ancient sutures formed in the Phanerozoic into the ocean-atmosphere system or into the deep mantle.
(blue) and Proterozoic (green). Sutures that were active during both
eons are dashed blue and green. Modified after Orme (2015) and Burke Another mechanism of CO2 production has recently been
et al. (1977). documented in subduction zones. In the presence of a fluid,
carbonate minerals may undergo congruent carbonate dissolu-
given P-T condition and fluid composition. tion such as:
Therefore, these reactions generally occur when a mixed
CaCO3 (calcite) + 2H+(aq) ↔ H O2 (fluid) + Ca2(a+q) + CO2 (aq) (4)
carbonate-silicate rock undergoes significant increases in tem-
perature and pressure (Goldschmidt 1912; Bowen 1940; Harker (Frezzotti et al. 2011; Ague and Nicolescu 2014). The CO2(aq)
and Tuttle 1956). On Earth, this can take place via prograde in reaction 4 represents aqueous carbon species in general;
metamorphism, that is, metamorphism of a rock driven in part carbonate and bicarbonate ions (Pan and Galli 2016), as
by increasing temperature. Geologists recognize different cat- well as organic carbon species (Sverjensky et al. 2014), may
egories of metamorphism relating to different tectonic environ- be more important at depth. This reaction is different from
ments (Fig. 1). reaction 2 in two important ways: First, it does not require
the presence of a silicate mineral to proceed, and it could,
Contact metamorphism therefore, occur in a pure carbonate rock. Second, it has the
potential to be highly efficient at releasing carbon. For ex-
Contact metamorphism occurs when a magma intrudes into ample, in subducted rocks on the Greek islands of Syros and
solid rocks, and so can be located anywhere volcanism and/or Tinos, carbonate dissolution released 60–90% of the solid
magmatism are active (Delesse 1858). In contact metamorphism, carbon from some rocks, while decarbonation reactions might
rocks are not necessarily tectonically buried; they are simply be expected to release considerably less (Ague and Nicolescu
heated by the adjacent magma. Therefore, pure contact metamor- 2014). Thus, a small proportion of carbonate dissolution could
phism is associated with higher geothermal gradients (e.g., high have a relatively large effect.
temperatures at relatively low pressures) compared to regional
metamorphism. Additionally, it is more restricted in area, oc- Whatever the decarbonation mechanism, CO2 released
curring in haloes (aureoles) around magmatic intrusions. It has by subducting rocks may follow several paths. It could flow
been suggested that degassing in contact aureoles around large through the many kilometers of overriding lithosphere (or per-
igneous provinces drove catastrophic global warming associated haps along the subduction interface) to escape to the atmosphere
with some of Earth’s largest mass extinctions (e.g., Ganino and as part of a diffuse (i.e., spatially widespread) metamorphic flux
Arndt 2009; Burgess et al. 2017), but the relative importance of (e.g., Sakai et al. 1990; Sano and Williams 1996; Campbell et
this metamorphism remains debated (e.g., Nabelek et al. 2014). al. 2002), or it could become trapped in the overlying mantle
wedge by a carbonation reaction (e.g., Piccoli et al. 2016, 2018;
Regional metamorphism Scambelluri et al. 2016; Sieber et al. 2018). Once in the mantle
wedge, that carbon could be stored for millions of years. If car-
Regional-scale metamorphism takes place where two tec- bonated mantle melts, however, this carbon could form part of a
tonic plates converge and ultimately collide (subduction zones magma and ultimately be released to the atmosphere as part of
and continental collisions) or pull apart (e.g., continental rifts) the arc volcanic CO2 flux (Varekamp et al. 1992; Kelemen and
(Kennedy 1948; Miyashiro 1972). Figure 3 shows a map of both Manning 2015; Poli 2015). Thus, decarbonation in a subducted
modern and ancient collisional belts on Earth. As can be seen, slab could contribute CO2 to both a diffuse metamorphic flux
such convergent plate boundaries are common features. During and the associated volcanic flux.
regional-scale metamorphism, rocks are subjected to both high
pressures (~0.2 to more than 2.0 GPa) and high temperatures Decarbonation reactions also occur in the overriding plate
(~250 up to ~1000  °C). Note that wollastonite is a relatively in subduction zones, driven by elevated temperatures from
uncommon mineral in regionally metamorphosed rocks, and magma ascent and magmatic dewatering that would result in
other silicate minerals, such as biotite and plagioclase, are more aqueous fluid infiltration. This decarbonation flux should be
commonly produced (Ferry 1988). more significant in continental arcs with thick carbonate layers
and less prominent when subduction occurs beneath oceanic
crust (i.e., island arcs). Lee et al. (2013) recognized a relation-
ship between total continental arc length and global temperature
in the past. They suggested that contact metamorphism-driven
decarbonation in these continental arcs may be an important
source of atmospheric CO2.

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1374 STEWART ET AL.: CARBONATION/DECARBONATION REACTIONS AND PLANETARY HABITABILITY

0.3 0.25 0.5 1.0 Discussion: Carbon fluxes and planetary
XCO2= 0.001
Pressure (GPa) habitability
Quartz
+ Carbonation/decarbonation reactions play a vital role in car-
bon transfer on Earth. To understand how these reactions relate to
Calcite planetary habitability, we must consider how different processes
interact and balance through the long-term (geologic) carbon
0.2 cycle. A great deal of the work done by geochemists focuses on
constraining the magnitudes of major geologic carbon fluxes.
0.1 These fluxes are broadly divided into sources (inputs into the
Wollastonite ocean-atmosphere system) and sinks (outputs from the ocean-
+ atmosphere system). Thus, decarbonation reactions are sources
CO2 of atmospheric CO2, and carbonation reactions are sinks (Fig. 5).

0 Carbonation reaction fluxes
300 400 500 600 700 800
It is generally agreed that the primary long-term sink of CO2
Temperature (oC) from the ocean-atmosphere system is the precipitation of carbon-
ate rocks using alkalinity derived from silicate weathering (the
FigFuigruere44. Pressure-Temperature diagram showing the stability of the Urey reactions). The continental silicate weathering flux can be
estimated from measurements of river discharge, although on a
assemblage calcite + quartz relative to wollastonite + CO2. Conditions very heterogeneous planet there are many complexities to con-
of this reaction are calculated for equilibrium between minerals and sider. Nevertheless, continental silicate weathering is estimated,
fluid of differing CO2 content (XCO2 is the mole fraction of CO2 in fluid). with reasonable uncertainty, to consume ~11.5 to 23 Tmol CO2
Calculations made using the program Theriak-Domino (de Capitani and yr-1 (e.g., Gaillardet et al. 1999 and references therein).
Petrakakis 2010) with the Holland and Powell (1998) database.
The marine silicate weathering flux is less studied. One
Infiltration-driven decarbonation estimate of CO2 drawdown resulting from chemical weathering
of deep-sea sediments is ~5 to ~20 Tmol CO2 yr-1, comparable
The presence of a water-bearing fluid (either liquid or gas) has in magnitude to the continental flux (Wallmann et al. 2008).
a profound effect on the stability of carbonate minerals. Metamor- Off-axis carbonation of basaltic oceanic crust may provide an
phism in a closed system will not evolve much CO2 until relatively additional sink of ~0.2 to ~3.7 Tmol CO2 yr-1 (e.g., Coogan and
high temperatures are reached (Greenwood 1975). On the other Gillis 2018 and references therein). However, reverse weathering
hand, infiltration of a water-rich fluid into a reactive rock can recycles some CO2 back into the surface environment, effectively
depress the required temperature of a given decarbonation reac- acting as a source of ~0.5 to ~1.25 Tmol CO2 yr-1 (Isson and
tion (e.g., Ferry 1976, 2016; Kerrick 1977; Tracy et al.1983; Ague Planavsky 2018).
2002; Penniston-Dorland and Ferry 2006). This can be observed in
zones of enhanced decarbonation around fluid conduits, as shown Decarbonation reaction fluxes
in Figure 2c. Classic studies in the metamorphic belts of the Ap-
palachian Mountains were among the first to demonstrate that fluid Earth’s major decarbonation reaction fluxes are the result
infiltration was essential for driving reactions and releasing CO2 of metamorphic outgassing reactions in continental collisions,
(e.g., Ferry 1978, 1980; Rumble et al. 1982; Tracy et al. 1983). subduction zones, and contact metamorphic aureoles. Metamor-
phism in continental rifts is less well-studied, but it may also
As a demonstration, we calculate the P-T conditions of re- make a significant contribution.
action 2. Figure 4 is a P-T phase diagram showing the mineral
assemblage that is stable at a given P-T condition and fluid Metamorphic outgassing in continental collisions has pri-
composition. When the XCO2 (the mole fraction of CO2) in an marily been studied at the regional scale. As shown by Stewart
H2O–CO2 fluid is low, the reaction can occur at a much lower and Ague (2018), multiple estimates from ancient and modern
temperature. For example, at 0.2 GPa the reaction occurs at mountain belts converge on an area-normalized flux of ~0.5 ×
~350 °C when XCO2 = 0.001 and ~700 °C when XCO2 = 1.0 (Fig. 4). 106 to ~7 × 106 moles CO2 km-2 yr-1 (Kerrick and Caldeira 1998;
Chiodini et al. 2000; Becker et al. 2008; Skelton 2011). These
This effect has two important implications. First, a rock that is estimates are derived from independent, quite disparate methods,
metamorphosed in the presence of a water-bearing fluid can release ranging from thermodynamic modeling of metacarbonate rocks
over 500% more CO2 than the metamorphism of the same rock in in the Appalachians (Stewart and Ague 2018) to modern direct
a closed system at the same pressure and temperature (Stewart and measurements of CO2 escaping from springs in the Himalayas
Ague 2018). This enhanced decarbonation could ultimately result in and Italian Apennines (Chiodini et al. 2000; Becker et al. 2008).
a greater concentration of CO2 in the atmosphere and a correspond- The agreement between estimates from deeply exhumed rocks
ingly higher global surface temperature. Second, CO2 generated by and measurements at Earth’s surface suggests that most devolatil-
infiltration is automatically released into a regional fluid flow system ized CO2 is ultimately released to the ocean-atmosphere system.
that provides the mechanism for transporting evolved CO2 from the We can multiply this areal flux estimate by the area of active
deep crust to the atmosphere and hydrosphere. continental collision for a rough global collisional metamorphic
flux. The present area, dominated by the Himalayas with the area
~7.5 × 105 km2 (Becker et al. 2008), is estimated on the order

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STEWART ET AL.: CARBONATION/DECARBONATION REACTIONS AND PLANETARY HABITABILITY 1375

Modern Flux to Ocean - Atmosphere System particular, may occur overshort timescales (e.g., Lyubetskaya and
Ague 2010). The total contact metamorphic flux is difficult to
(Tmol yr -1) estimate, but some researchers suggest it has played an important
role in changing climate conditions through Earth history (e.g.,
OTHER FLUXES CARBONATION / DECARBONATION -20 -10 0 10 20 Lee et al. 2013).

Continental Although our discussion of CO2 sources is focused on decar-
Weathering bonation reaction fluxes, we can compare their magnitudes to
other important CO2-generating fluxes. Typical estimates for the
Marine three major volcanic fluxes in the modern era are as follows: ~1.5
Weathering to ~3.1 Tmol CO2 yr-1 for arc volcanism (Marty and Tolstikhin
1998; Hilton et al. 2002; Dasgupta and Hirschmann 2010), ~0.5
Reverse Weathering to ~5.0 Tmol CO2 yr-1 for mid-ocean ridge volcanism (Marty
and Tolstikhin 1998; Dasgupta and Hirschmann 2006, 2010;
Collisional Metamorphism Le Voyer et al. 2019) and ~0.12 to ~3 Tmol CO2 yr-1 from
ocean island volcanoes (Marty and Tolstikhin 1998; Dasgupta
? Subduction Metamorphism and Hirschmann 2010). Volcanogenic CO2 may also reach the
atmosphere via diffuse outgassing (Allard 1992). Organic carbon
Arc Volcanism* weathering is somewhat larger at ~7.5 to ~10 Tmol CO2 yr-1
(Holland 1978; Kump and Arthur 1999), but is largely balanced
Mid Ocean Ridge Volcanism out by the organic carbon burial flux of ~5.3 to ~10 Tmol CO2
yr-1 (Berner 1982; Kump and Arthur 1999). Thus, metamorphic
Ocean Island Volcanism outgassing fluxes are of the same order of magnitude as other
major source fluxes. Metamorphic decarbonation reactions are
Organic C Burial Organic C Oxidation therefore more important to the net global carbon budget than
is often appreciated (Fig. 5).
Sinks Sources
Earth’s habitability and the need for balance
Figure 5
It has been commonly argued that surface temperatures on
Figure 5. Estimates of modern CO2 fluxes to the ocean-atmosphere Earth have been remarkably stable for billions of years. Sedi-
system. Error bars indicate a range of possible values, not necessarily a mentary rocks record the presence of liquid water since at least
normal distribution. * Note that the flux from Arc Volcanism includes ~3.8 billion years ago (Lowe 1980), which requires global surface
some contribution from decarbonation of subducting slabs; in fact, slab temperatures to remain between 0 and 100 °C for a vast amount
decarbonation could account for the vast majority of the arc volcanic flux. of time. Some researchers suggest there is evidence for liquid
Arc magmas may also incorporate partially melted carbonate lithologies water even earlier (e.g., 4.3 billion years ago by Mojzsis et al.
and drive contact metamorphism in adjacent rocks. 2001; 4.4 billion years ago by Wilde et al. 2001).

of ~106 km2. The resultant estimated global flux is ~0.5 to ~7 In an apparent contradiction, the Sun has been increasing in
Tmol CO2 yr-1, but note that this value is not constant through luminosity and, therefore, supplying more heat to Earth over
geologic time (Fig. 3). time. It is estimated that the sun’s luminosity in early Earth his-
tory was only ~70% of the modern intensity (Sagan and Mullen
Estimates of metamorphic degassing fluxes at subduction 1972); thus, one might expect Earth’s temperature to have
zones also cover a considerable range. In their compilation, Kele- changed markedly in response to these changes in incoming solar
men and Manning (2015) estimate ~0.3 to ~4.9 Tmol CO2 yr-1 are energy. As Sagan and Mullen pointed out in 1972, an Earth with
released from the slab via metamorphic reaction and dissolution. today’s atmospheric composition would have been completely
This estimate in itself carries significant uncertainties, largely frozen (below 0 °C) until about 2 billion years ago. This is at
due to uncertainties in the degree and nature of fluid infiltration odds with geologic evidence for a warm climate early in Earth’s
during metamorphism. Closed-system calculations predict that history (e.g., Knauth and Epstein 1976). This logical problem has
the majority of subducted carbon is not released (e.g., the negli- been referred to as the “Faint Young Sun Paradox,” and remains
gible flux estimate from Kerrick and Connolly 2001). Models that the subject of active debate today (see Kasting 2010). However,
allow for fluid infiltration predict more decarbonation (e.g., 0.35 one simple solution to this paradox lies in Earth’s atmosphere.
to 3.12 Tmol CO2 yr-1, Gorman et al. 2006), with intermediate A more carbon-rich atmosphere would result in a more intense
fluxes also suggested (Cook-Kollars et al. 2014). Carbonate dis- greenhouse effect and, perhaps, higher surface temperatures
solution, only recently identified in subduction zones (e.g., Ague even with a weaker sun (Owen et al. 1979; Walker et al. 1981).
and Nicolescu 2014), could significantly increase these estimates.
On the other side of the spectrum, Earth has likewise never
In addition, it is highly uncertain what proportion of the become too hot since the emergence of the earliest life forms.
devolatilized CO2 makes it to the atmosphere (either escaping If CO2 concentrations became extremely high, the temperature
through arc volcanoes or through its own diffuse outgassing) could skyrocket and the oceans could boil. Thus, we find our-
and how much is stored in the overlying lithosphere. Kelemen selves on a type of Goldilocks planet: CO2 concentrations never
and Manning (2015) report a diffuse outgassing flux of ~0.3 get excessively high or excessively low, but, within relatively
to 1.0 Tmol CO2 yr-1, but emphasize that they suspect it might
actually be much larger.

Contact metamorphism, though spatially limited, has the
potential to contribute large quantities of CO2. High temperature–
low pressure conditions can drive decarbonation reactions such
as reaction 2 particularly efficiently. Contact metamorphism, in

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1376 STEWART ET AL.: CARBONATION/DECARBONATION REACTIONS AND PLANETARY HABITABILITY

narrow limits, they remain just right. Atmospheric CO2 (μatm) 106
In their classic analysis, Berner and Caldeira (1997) argue
105 25% excess in
that this cannot be a coincidence. On long timescales, the amount CO2 degassing
of CO2 added to the atmosphere must equal the amount of CO2 10 times pre-industrial
removed. Without this balance, atmospheric CO2 concentrations
would run-away, resulting in an extreme hot house or ice house 104 atmospheric CO2
climates. Berner and Caldeira (1997) demonstrate this with a
simple calculation of the effect of only small (25%) imbalances 1000
between CO2 inputs and outputs (Fig. 6).
100 pre-industrial 25% excess in
Major CO2 fluxes have been changing throughout Earth 10 atmospheric CO2 silicate-rock
history. To maintain balance, then, some stabilizing mechanism weaterhing
or negative feedback must be in place, ensuring that inputs and 1 0.1
outputs eventually reach a steady state. By simply summing 0.01 1 10 100
modern flux estimates (Fig. 5), we find that the predicted net
change of the atmospheric reservoir is between ~ –45 and ~ +11 Time (Millions of Years)
Tmol CO2 yr-1. This estimate does overlap the necessary value of
zero net change, but the large uncertainty is obvious. Nonethe- FigFiugruere66. Predicted atmospheric CO2 concentrations in an Earth
less, we can outline several different end-member Earth states.
With high weathering rate estimates (e.g., with large marine system where CO2 sources and sinks do not balance, modified after
weathering fluxes; Wallmann et al. 2008; Coogan and Gillis Berner and Caldeira (1997). Regardless of starting CO2 concentration,
2018) upper-end-member outgassing rates are required. High a 25% excess in CO2 degassing (red curves) or a 25% excess in silicate
amounts of reverse weathering (e.g., Rahman et al. 2017) could weathering (blue curves) result in a run-away atmospheric composition
also help balance high silicate weathering rates with outgassing within ~1 million years.
estimates. In contrast, in the traditional view—where silicate
weathering occurs predominantly in continental settings—only have proposed that global surface temperatures exert direct
the lowest outgassing fluxes allow the modern Earth to be close control on silicate weathering through a simple temperature-
to a steady state. dependent reaction rate. In laboratory experiments, the rate
of chemical weathering of silicate minerals (e.g., reaction 1C)
The continental silicate weathering feedback. The silicate has been demonstrated to increase with increasing temperature
weathering feedback is the most prominent suggested mechanism (e.g., Lagache 1976; Brady and Carroll 1994), yet in field stud-
for stabilizing the global carbon cycle. It was first proposed by ies results are mixed. Edmond et al. (1995) report no evidence
Walker et al. (1981) and stated that the rate of continental silicate of increased chemical weathering at higher temperatures, while
weathering and resultant carbonate precipitation (whether abiotic Meybeck (1979) finds a significant relationship. This work is
or biologically mediated) speeds up at higher temperatures and complicated by the correlations between, for example, river
higher CO2 concentrations. Recall that silicate weathering is runoff and temperature that exist in nature.
the primary pathway for removing CO2 from the atmosphere,
thus this constitutes negative feedback: rising CO2 concentra- Another possible mechanism relates CO2 to chemical weath-
tions drive increasing global temperatures and increased silicate ering directly: as atmospheric CO2 concentrations increase,
weathering that, in turn, draws more CO2 out of the atmosphere, more CO2 will be dissolved in water to from carbonic acid. This
lowering CO2 concentrations and global temperature. more acidic surface environment could also contribute to faster
chemical weathering of silicate minerals (e.g., Berg and Banwart
This does not imply that CO2 concentrations are essentially 2000). This factor is likely more important prior to the rise of
fixed throughout time. If CO2 input fluxes are permanently land plants and the onset of extensive soil respiration.
doubled, the atmospheric CO2 and temperature will not return
to previous values as a result of this feedback. Rather, the con- Most research today focuses on indirect relationships between
centration of CO2 and global temperature will increase until temperature and silicate weathering. In particular, higher tem-
the CO2 output flux—silicate weathering—again matches the peratures drive increased global precipitation rates and increased
inputs. The system will then reach a new steady state such that river runoff (Holland 1978; Manabe and Stouffer 1993). There-
the concentration of CO2 and the temperature are higher than fore, many models suggest that it is primarily this invigoration
before, but they are stable. This also implies that there will be of the water cycle that enhances silicate weathering and CO2
periods of Earth history when inputs and outputs are temporarily drawdown (Berner and Berner 1997; Maher and Chamberlain
imbalanced. For example, Dutkiewicz et al. (2018) suggest that 2014). Teasing out the influence of temperature, precipitation,
Cenozoic carbonation has outpaced solid earth decarbonation, or other factors can be extremely challenging in such complex
causing a global cooling trend. Recall, also, that the “Faint Young systems, but modern statistical techniques (e.g., machine learn-
Sun Paradox” of Sagan and Mullen (1972) requires that the ing) could be an effective means to probe the factors driving the
atmosphere has systematically lost CO2 over billions of years. As silicate weathering feedback.
a consequence, feedback does not guarantee fixed temperatures,
but it prevents run-away warming or cooling trends. Marine weathering feedback. Because weathering of con-
tinental material has historically been considered as the primary
The nature of the relationship between temperature and sili- contribution to global silicate weathering, it has also been assumed
cate weathering remains the subject of vigorous debate. Some to be the source of the associated negative feedback. However,
recent studies have demonstrated that seafloor weathering and
carbonation may offer an additional, complementary negative
feedback. The general idea is the same: rising CO2 concentrations

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STEWART ET AL.: CARBONATION/DECARBONATION REACTIONS AND PLANETARY HABITABILITY 1377

and global temperatures increase the rate of marine weathering, in subduction zones is hosted in the minerals dolomite and
thereby drawing down more CO2 and stabilizing the system. magnesite (Tao et al. 2018), yet experimental constraints on
Indeed, off-axis hydrothermal alteration of the basaltic crust is their solubilities at these conditions are lacking. Recent theoreti-
enhanced at higher temperatures (Coogan and Dosso 2015). The cal work is addressing the issue (e.g., Sverjensky et al. 2014;
rate of silicate mineral dissolution will be significantly faster when Connolly and Galvez 2018), and should be supplemented by a
the ocean bottom water temperature is elevated. On the other hand, new generation of experimental data.
it has been suggested that chemical weathering of marine anoxic
sediments is largely independent of temperature (Wallmann et al. Exoplanet habitability
2008). Importantly, CO2 that is stored in the oceanic crust may, in
the future, contribute to a metamorphic decarbonation flux when Each of the processes discussed has important implications
it is inevitably subducted (Fig. 1). in the study of distant exoplanets and our search for other
habitable worlds. The presence of a silicate weathering feed-
Reverse weathering feedback. It was recently proposed back, for example, will significantly increase the size of the
that reverse weathering can act as important stabilizing feed- “habitable zone” around a given star—the range of planetary
back for carbon cycling (Isson and Planavsky 2018). In this distances from the star that falls within a habitable temperature
case, increasing atmospheric CO2 makes the ocean more acidic, range (Kasting et al. 1993). The balance between continental
reducing the amount of clay formation, and thus CO2 release, and marine weathering feedbacks is also important. If marine
from reverse weathering. This, in turn, lowers atmospheric CO2, weathering is not a significant negative feedback mechanism,
pushing ocean water pH back toward less acidic values. Today then we would not expect planets that are mostly ocean (“water
this feedback is less effective—clay formation is limited by the worlds”) to have a stable, habitable climate (Abbot et al. 2012).
availability of SiO2 dissolved in the ocean. There is evidence, If, on the other hand, marine weathering is strongly tempera-
however, that this process was much more important early in ture dependent, these worlds would be more likely to sustain
Earth’s history. Prior to the Cambrian Period (~542 million life. The presence of volcanism and/or plate tectonics could
years ago) oceanic silica concentrations were much higher. also have profound effects on an exoplanet’s carbon cycle and
This would allow for more reverse weathering and, perhaps, potential habitability (e.g., Sleep and Zahnle 2001). Studying
enhanced efficiency of a reverse weathering negative feedback these processes on Earth may lead to better predictions of which
(Isson and Planavsky 2018). distant planets might be hospitable to life.

Implications Carbonation reactions and anthropogenic climate change

Future work Since the industrial revolution, Earth’s global carbon cycle
has been subject to a fast and massive perturbation, evidently
Major gaps remain in our understanding of global carbonation/ unequaled in geologic history. Through the burning of fossil fu-
decarbonation reactions. More work is needed on constraining els, deforestation, and other human activity, ~795 Tmol CO2 are
the magnitude of the various carbon fluxes and how they balance added to the ocean-atmosphere system every year (Friedling-
one another throughout time. One area of particular uncertainty stein et al. 2010). This is more than 100 times greater than the
is the fate of subducted carbon. As of today, it is unknown global volcanic CO2 flux or, as Gerlach (2011) notes, equivalent
whether most of the carbonate minerals in the oceanic crust are to about 9500 Kilauea volcanoes. One could, perhaps, take
ultimately delivered to the deep mantle, or if they devolatilize comfort in the knowledge that the natural geologic carbon
during subduction. And for the CO2 that does escape the down- cycle can eventually stabilize global temperatures, but most
going slab, is most of it ultimately released to the atmosphere, will consider a lag time of about 1 million years unacceptable.
or is it stored in the subarc lithosphere (Kelemen and Manning With this in mind, some researchers are attempting to harness
2015)? Observations made in ancient and modern subduction and accelerate the power of silicate weathering to counteract
zones in concert with constraints from experiments and numeri- human-driven climate change in our lifetimes (O’Connor et al.
cal modeling must begin to answer these questions if we are to 2001; Lackner 2003; Park and Fan 2004; Kelemen and Matter
make progress in a global understanding of carbon mobility. 2008; Lal 2008; Wilson et al. 2009; Lechat et al. 2016; Power
et al. 2016; Kelemen et al. 2018).
Another significant uncertainty is the strength of the
silicate weathering feedback. This feedback is not perfectly In many cases, ultramafic rocks are used. These rocks are
efficient—the Earth has swung between the hot house and composed of Mg-rich silicate minerals that are particularly
ice house conditions many times in the past (e.g., Royer et unstable at Earth’s surface, which facilitates dissolution (like
al. 2004). In fact, the strength of the feedback is certainly not reaction 1C) and subsequent carbonation. In some cases, these
fixed. We see evidence for periods of reduced and enhanced rocks are merely ground up and exposed at Earth’s surface to
feedback efficiency in the geologic record (e.g., Caves et al. undergo natural reaction with the atmosphere (e.g., Lechat et
2016). Nevertheless, the ability to constrain the magnitude of al. 2016), while other studies consider more active processes,
the effect of silicate weathering in ancient and modern Earth such as pumping a CO2-rich fluid through the rocks (this is
systems will allow us to make more accurate calculations of more similar to off-axis hydrothermal alteration; e.g., Park and
past and future climate. Both theoretical (e.g., Winnick and Fan 2004; Matter et al. 2016). In either case, understanding
Maher 2018) and observational approaches could provide natural geologic carbonation reactions have the potential to
valuable new insights. inform future work on carbon sequestration and contribute to
the continued habitability of planet Earth.
There is limited work on the behavior of carbonate minerals
at high pressures. For example, recent work suggests carbon

American Mineralogist, vol. 104, 2019

1378 STEWART ET AL.: CARBONATION/DECARBONATION REACTIONS AND PLANETARY HABITABILITY

Acknowledgments Science Letters, 415, 38–46.
Coogan, L.A., and Gillis, K.M. (2013) Evidence that low-temperature oceanic
We thank the Deep Carbon Observatory and specifically the Reservoirs and
Fluxes Community and the attendees of the 2018 “The Earth in Five Reactions” hydrothermal systems play an important role in the silicate-carbonate weath-
workshop for their support of this work. We are grateful for insightful discussions ering cycle and long-term climate regulation. Geochemistry, Geophysics,
with G.E. Bebout, R.A. Berner, O. Beyssac, A.V. Brovarone, C.P. Chamberlain, Geosystems, 14(6), 1771–1786.
D.A.D. Evans, M.E. Galvez, B. Marty, F. Piccoli, D. Rumble, D.M. Rye, M. Tian, ——— (2018) Low-temperature alteration of the seafloor: Impacts on ocean
and J.L.M. van Haren. This work also benefits from constructive reviews by A.D.L. chemistry. Annual Review of Earth and Planetary Sciences, 46, 21–45.
Skelton and an anonymous reviewer. Cook-Kollars, J., Bebout, G.E., Collins, N.C., Angiboust, S., and Agard, P. (2014)
Subduction zone metamorphic pathway for deep carbon cycling: I. Evidence
Funding from HP/UHP metasedimentary rocks, Italian Alps. Chemical Geology, 386,
31–48.
Funding provided by the National Science Foundation (EAR-1650329 to J.J.A.) Dasgupta, R., and Hirschmann, M.M. (2006) Melting in the Earth’s deep upper
and Yale University is gratefully acknowledged. mantle caused by carbon dioxide. Nature, 440, 659.
——— (2010) The deep carbon cycle and melting in Earth’s interior. Earth and
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American Mineralogist, vol. 104, 2019

American Mineralogist, Volume 104, pages 671–678, 2019

Melting curve minimum of barium carbonate BaCO3 near 5 GPa

Junjie Dong1,*,§, Jie Li1,†, Feng Zhu1,‡, Zeyu Li1, and Rami Farawi1

1Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109, U.S.A.

Abstract
The melting point of barium carbonate (BaCO3) was determined at pressures up to 11 GPa using
the ionic conductivity and platinum (Pt) sphere methods in a multi-anvil press. The melting point
decreases with pressure from 2149 ± 50 K at 3 GPa to a fitted local minimum of 1849 K at 5.5 GPa,
and then it rises with pressure to 2453 ± 50 K at 11 GPa. The fitted melting curve of BaCO3 based on
the ionic conductivity measurements is consistent with the Pt sphere measurements that were carried
out independently at selected pressures. The negative slope of the BaCO3 melting curve between 3 and
5.5 GPa indicates that the liquid is denser than the solid within this pressure range. Synchrotron X‑ray
diffraction (XRD) measurements in a laser-heated diamond-anvil cell (LH-DAC) showed that BaCO3
transformed from the aragonite structure (Pmcn) to the post-aragonite structure (Pmmn) at 6.3 GPa
and 1026 K as well as 8 GPa and 1100 K and the post-aragonite structure remained metastable upon
quenching and only reverted back to the witherite structure upon pressure release. The local minimum
near 5 GPa is attributed to the triple point where the melting curve of BaCO3 meets a phase transition
to the denser post-aragonite structure (Pmmn). Local minima in the melting curves of alkaline earth
carbonates would lead to incipient melting of carbonated rocks in Earth’s mantle.
Keywords: Barium carbonate, melting point, density crossover, phase transition, negative melting
slope, post-aragonite structure; Earth in Five Reactions: A Deep Carbon Perspective

Introduction studies suggest that BaCO3 decomposes in the solid state to BaO
and CO2 (Arvanitidis et al. 1996). Data on the melting behavior
Alkaline earth carbonates, primarily CaCO3 and MgCO3, play of BaCO3 at higher pressures are not available. In this study, the
important roles in transporting carbon into the deep mantle through melting curve of BaCO3, as well as the phase boundary between
subducting slabs (e.g., Dasgupta 2013). A recent study suggests a the aragonite and post-aragonite phases of BaCO3, were investi-
local minimum in the melting curve of CaCO3 near 13 GPa, likely gated experimentally at upper mantle conditions. The results were
resulting from a phase transition that intersects the melting curve, applied to examine the influence of solid-solid transitions on the
but the inferred negative melting slope is not clearly resolved (Li shape of the melting curve and explore the implications for the
et al. 2017). Another alkaline earth carbonate, BaCO3, is shown to melting behavior of carbonated rocks in Earth’s mantle.
undergo similar pressure-induced aragonite to post-aragonite phase
transition as CaCO3 but at lower pressures (Shatskiy et al. 2015). Methods
Investigating the melting behavior of BaCO3 will allow for testing
the occurrence of solid-liquid density crossover in compressed Fine powder of high-purity BaCO3 (Alfa Aesar 10645, 99.997%) was used as
alkaline earth carbonates. Furthermore, systematic comparison the starting material. Prior to experiments, the sample was kept in a vacuum oven
of the structure and stability of alkaline earth carbonates is useful at 400 K to remove moisture.
for constructing thermodynamic models to predict the melting
behavior of complex mantle rocks in a petrologically relevant Multi-anvil experiments
pressure-temperature-composition space (Hurt and Wolf 2018).
Ionic conductivity experiments were performed at pressures between
The melting behavior of BaCO3 at 1 bar is currently unre- 3 and 11 GPa using a 1000-ton Walker-type multi-anvil press at the University of
solved. The reported melting point ranges from 1084 K in the Michigan. Toshiba-Tungaloy F-grade tungsten carbide cubes with 5 mm truncation
Material Safety Data Sheets (MSDS) provided by Alfa Aesar edge length (TEL) and the COMPRESS 10/5 assembly (Leinenweber et al. 2012)
(ThermoFisher Scientific 2010), 1653 K in the National Standard were used to generate high pressures and high temperatures. Closed high-pressure
Reference Data System (Stern and Weise 1969) to 1828 K in the cell assemblies were dried in a vacuum oven at 400 K for 8–24 h before loading
CRC Handbook of Chemistry and Physics (Rumble 2018). Some into the multi-anvil press. The uncertainty in pressure measurement is estimated to
be ±7%. This includes the precision of pressure calibration of ±5% estimated on
* E-mail: [email protected]. Present address: Department of Earth and the basis of duplicate experiments, and systematic errors of ±5% arising from the
Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, U.S.A. effect of temperature on pressure calibration and pressure drift during heating and
Orcid 0000-0003-1114-9348. cooling (Li and Li 2015). A standard type-C thermocouple (TC) was used to monitor
† Orcid 0000-0003-4761-722X. temperature. The uncertainty in the measured temperature is estimated to be ±50 K.
‡ Present address: Hawaii Institute of Geophysics and Planetology, University This includes the precision in the thermocouple calibration and the position of the
of Hawai‘i at Mānoa, Honolulu, Hawaii, U.S.A. Orcid 0000-0003-2409-151X. TC junction relative to the sample (Li and Li 2015) but ignores the effect of pressure
§ Special collection papers can be found online at http://www.minsocam.org/MSA/ on the electromotive force (emf) of the TC. Limited data suggest that the type-C
AmMin/special-collections.html. TC underestimates temperature and that systematic error generally increases with
pressure and temperature, rising to tens of degrees at 10 GPa and above 2000 K (Li
et al. 2003). As a result, the measured melting points of BaCO3 at >10 GPa may be

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on 10 October 2019

672 DONG ET AL.: MELTING CURVE MINIMUM OF BARIUM CARBONATE

a MgO temperature of each experiment, the sample was quenched by turning off the power.
Al2O3 To cross-validate the ionic conductivity measurements, Pt sphere experiments
W5%Re ZrO2
W26%Re BaCO3 without electrodes were conducted independently using the same multi-anvil press.
MgO filler During sample loading, a Pt sphere of 100 to 200 μm in diameter was placed near
Pt the top of the sample. After heating and recovering the sample, the location of the
Pt sphere was used to determine whether the sample was molten or not at the target
b Fluke Mastech temperature. Pt sphere experiments at 5.5 and 8 GPa used the standard COMPRESS
ammeter 10/5 assembly with Pt capsule, and the experiment at 1.4 GPa used cast octahedra and
Fansteel tungsten carbide cubes with 8 mm TEL. At the target pressure, the sample
power supply was heated to the target temperature and held for 5 min before quenched by turning
off the power. The multi-anvil press is calibrated for pressures above 2 GPa, and
A therefore a larger pressure error may be present in the experiment at 1.4 GPa where
some ceramic parts may not be fully equilibrated under compression.
V
The experimental products were recovered and examined for texture, composi-
+ - AC output tion, and structure. An optical microscope was used to check the position of the
+- electrode tips and thermocouple junctions and locate the Pt sphere in the sample.
Type-C Raman spectra were collected on a Renishaw Raman microscope for phase identifica-
thermocouple tion. Backscattered-electron (BSE) images and energy-dispersive spectra (EDS) were
obtained on a JOEL 7800 FLV field emission SEM in the Central Campus Electron
V Micro-beam Analysis Laboratory (EMAL) at the University of Michigan. The BSE
and EDS results were inspected to make sure that products were free of contamination
... Pt from the four-bore Al2O3 tubing, Pt parts or other components in the assembly, which
were in direct contact with the sample or might diffuse into the sample.
WC cubes electodes + -
Diamond-anvil cell experiments
with cell Keithley
Synchrotron XRD measurements were conducted to investigate the phase
voltameter stability of BaCO3 at high pressures and high temperatures using a laser-heated
diamond-anvil cell (DAC). A symmetric cell with 400 μm culet diamond anvils was
Figure 1. Experimental configuration of ionic conductivity used to generate high pressures. A Re gasket was pre-indented to ~35 μm thickness
measurements in a multi-anvil press. (a) Configuration of the modified and drilled to form a sample chamber with 200 μm diameter. Fine powders of BaCO3
COMPRES 5 mm cell assembly. The electrodes and the thermocouple were mixed with about 5 wt% Pt powder and dried in vacuum oven at 400 K over-
were placed symmetrically along the rotational axis of the cell assembly night before loading into the DAC. The Pt powder was used as a laser absorber and
for reliable measurements of melting temperature. (b) Pictorial diagram secondary pressure standard. Two ~10 μm ruby spheres were loaded as the primary
of the circuit with a type-C thermocouple and a pair of Pt electrodes. pressure standard. The sample was immersed in neon as the pressure transmitting
medium and thermal insulator.
lower than the real values by a few tens of degrees.
The cell assembly for the ionic conductivity measurements (Fig. 1a) was modi- Laser heating and angular-dispersive XRD measurements were conducted at
the Advanced Photon Source (APS), Beamline 16-ID-B of HPCAT. The sample was
fied from the COMPRESS 10/5 assembly, similar to that of Li et al. (2017). Two heated from both sides by two identical Nd:YLF lasers (λ = 1053 nm) with 30 μm
pairs of slots were cut at both ends of the Re furnace and the LaCrO3 sleeve to fit laser spots. Temperatures were determined by fitting the thermal radiation spectrum
the TC and Pt electrode wires. A pair of Pt wires were inserted into one of the four- to the gray body radiation function (Meng et al. 2015). For a 30 s acquisition time,
bore Al2O3 tubing and served as the electrodes. Each 4-bore alumina (Al2O3) tubing the temperature measurements yielded readings of 1000–1200 K. The acquisition
was enclosed in a Pt tubing, which was further surrounded by a magnesia (MgO) time was reduced to 15 s at temperatures above 1200 K. The temperature of the
sleeve. The electrode tips and TC junction were placed along the rotational axis of heated samples was measured with an accuracy of ±100 K (Errandonea et al. 2003).
the cylindrical-shaped heater and at the same distance to the equator of the heater so The X‑ray beam was monochromatized to a wavelength of 0.4066 Å and focused
that the TC measured the temperature at the electrode tips. Both the TC junction and to an area of 5.3 × 4.4 μm. Diffraction images were recorded for 15 s with a MAR
electrode tips were positioned within the middle third of the heater length, where the CCD detector.
temperature gradient is estimated to be less than 100 K (Leinenweber et al. 2012).
Synchrotron XRD patterns of BaCO3 were recorded at temperatures up to 1500 K
The circuit for ionic conductivity measurements (Fig. 1b) includes a Mastech and at pressures up to 30 GPa. At several pressure points near the phase boundary,
variable transformer and a Fluke 289 multi-meter. External electromagnetic interfer- the sample was laser-heated at a small power step until a temperature reading could
ence, including heating current and the pressure control motor, is less than a few be obtained. A series of XRD patterns of the heated spot were recorded at different
microamps and negligible compared with the ionic current through molten BaCO3. temperatures and after quenching to 300 K. The 2D images were integrated into
1D patterns using Dioptas (Prescher and Prakapenka 2015) and refined using the
In a typical experiment, the ionic current through the compressed sample was PDIndexer software (Seto et al. 2010).
monitored during multiple heating and cooling cycles at a given pressure, and the
same recording procedure was repeated at several pressures along its compressional Results and discussion
path. The sample was pressurized at a rate of 1 to 3 GPa per hour to the target pres-
sure and then heated at a rate of 60 K per minute. In some experiments, the assembly Melting points of BaCO3 from ionic conductivity and Pt
was sintered at 1273 K for 1–2 h, and then further heated until a current jump was sphere experiments
detected. After heating, the sample was cooled at the rate of 180 K per minute to
1473 K and then heated up again for more heating cycles to repeat the melting detec- The melting points of BaCO3 between 3 and 11 GPa were
tion. At each pressure, at least two heating cycles were completed before the sample determined in five ionic conductivity experiments (Table 1). At a
was compressed to the next target pressure. Melting measurements were repeated given pressure, melting was detected on the basis of a steep rise in
multiple times at different pressures in each experiment. At the highest pressure and the ionic current through the sample (Fig. 2 and Supplementary1
Data). Upon heating, the current through the samples remained
at a fairly low value of a few to a few tens of microamps before
soaring to several hundred microamps near the melting point.
Further heating led to a plateau or smaller rise in current. The
current-temperature relation reversed upon cooling, with a steep
decrease usually 50 to 100 K lower than the melting point detected

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DONG ET AL.: MELTING CURVE MINIMUM OF BARIUM CARBONATE 673

Table 1. Experimental conditions and results of ionic conductivity or end of the steep rise. For consistency, the middle point of the
experiments steepest segment of the current-temperature curve, where dI/dT
reached the maximum value, was taken as the melting point. The
Exp. ID Pa (GPa) T b (K) melting points measured in multiple heating cycles at a given
pressure typically differ by less than 20 K and the average values
M072815 3.3 2063 are reported (Table 1).Among different experiments, the measured
4.1 2028 melting temperature at a given pressure agree to within 100 K.
5.0 1948
7.0 1961 In an attempt to measure the melting point of BaCO3 at 1 bar,
9.0 2240 we heated BaCO3 in a Pt crucible to 1173 K using an electric
10.0c 2343 furnace at the rate of 60 K per minute and then cooled it in the
11.0c 2453 air to room temperature. The sample was then weighed using a
M040716 3.3 2087 Mettler-Toledo balance and examined under a Leica microscope
4.0 1986 for mass and textural change. Its weight loss clearly indicated
5.0 1863 decomposition, possibly in combination with melting. The Alfa
6.0 1933 Aesar value of 1084 K is similar to the witherite-trigonal phase
7.0 2083 transition at ~1093 K and 1 bar (Antao and Hassan 2007), or it
M072717 3.0 2149 may correspond to the eutectic melting between BaCO3 and BaO
6.0 1843 at a specific partial pressure of CO2. A furnace with controlled
M080317 5.5 1965
6.5 2020
8.0 2197
M080617 7.0 1961
8.0 2148
9.0 2253
10.0c 2368

a Pressure uncertainty is ±7%. CO2 partial pressure is required to determine the melting and
b Temperatures are averages of melting points measurements from at least two decomposition behavior of BaCO3. Here we take the CRC value
heating cycles and uncertainty is ±50 K. at 1828 K as the nominal melting point of BaCO3 at 1 bar.
c The melting points measured above 10 GPa may have large errors likely due
to the melting of Pt. Melting point data from 1 bar to 5 GPa were used to fit a Kechin

during heating. The rise and fall of sample current were repeatedly melting equation (Kechin 2001) (Eq. 1):
measured over multiple heating and cooling cycles at a given pres-
sure. The rapid rise in ionic current is attributed to the dissociation Tm = T0 ⋅⎝⎜⎜⎜⎛1+ P− P0 ⎟⎟⎠⎟⎟⎞1b ⋅ e−c⋅(P−P0) (1)
a

of crystalline ionic bonding, which is endothermic and reversible. and melting data above 5.5 GPa were fitted to a second-order
The hysteresis can be explained by supercooling due to the kinetic polynomial (Eq. 2) from 5.5 to 11 GPa:
barrier in nucleation (Galiński et al. 2006), which exists for freez-

ing but not melting, hence we located the melting points based on Tm = p·P2 + q·P + w (2)
the steep current rise in the heating cycles.

As reported previously (Li et al. 2017), the pre-melting rise in where Tm and T0 are the melting point and the reference temperature

ionic conductivity measurements introduces uncertainties in locat- in K, and P and P0 are the pressure and the reference pressure in

ing the melting point (Fig. 2). The acnu#dr6rre8an9mt1spRtsaerutvspistaoitoinannc1rientoacsrAeemaastein5rig0can BGMaPiCna.eOHr3aealroteg1,i tbshtaer,rTef0e=re1n8c2e8c KonadnitdioPn0 is the nominal melting point of
to 100 K below the melting point = 1 bar ~0 GPa. Fitted melting

rate toward melting, likely due to crystal defects created at high curve parameters are a = 0.0382017, b = 11.6106, c = 0.0760715
te1m48peratures (HaSyyenscahnrdotHrountcXhiRngDs p1a9t8te9r)n. Ds oifffeBreanCtOcr3iwteeriraemreacyordeadndatpte=m–p2e.r6a8t4u,reqs=up15to3.61,5w00=K10an8d8.at

b1e4a9doptepdretosspulraecseutphetoon3s0etGoPfam. eAlttinsegvaetrtahlepbreegssinunreinpgo,imntisddnleea,r the phBaosuenbdosuonndtahreym, tehletinsagmppoilnetwwaesre obtained from four Pt sphere
experiments at 1.4, 5.5, and 8 GPa (Table 2). In M120117 and

Figure 2M. R12e3pr1e1s7e,ntPatisvpehceurrersensta-ntekmapnedraitunrdeicated melting below 2023 K

measureamt 5e.n5tsGdPuari,nagnhdebaetilnogwcy2c2l2e3s  aKt haitg8h GPa. Experiment M112117 at
pressure5cso.5.nMtGro0P7lal2ew8r.1aT5sh(qeruePdetnsscqphuheaedrereas)taa2lns0od11sa Knkdiunetthoistheexpfaeirliumreenotf, the heating
suggesting
M08061th7a(tbtlhuee cmirecllteisn)g. Tphoeinmtealttin5g.5poGinPtaisis likely lower than 2011 K.

locatedIbnyMth1e1m0a9x1i7m,utmheofPthsepfhiresrte remained at the top of the sample,

derivatiivnedoicfatthiengcutrhreant tth(deI/mdTe)l.tiTnhge pdoasinhet dat 1.4 GPa is above 2073 K.
Although more Pt sphere experiments are required to bracket

rectangtlhee(bmlueel)tisnhgowcusrfvleucitnudaetipoennidnetnhtely, the bounds from the existing Pt

currentsapftheerrme exltpinegriamt e1n0tsGaPrae,binrodaicdaltyincgonsistent with the results of ionic

meltingcoofntdhuecPtitvcitaypsmueleasourreelmecetrnotdsews.ithin experimental uncertainties and
support the adopted criterion for locating the melting point from

the current jumps (Fig. 3).

All the recovered samples were confirmed to be BaCO3 and

h(dbee11lar55uitFve01inaicggtiiruvccerylXecleoaslRe2f)s.s.eDtTRhra-hteephehpacerimtugaetrshetereerlepntdnnirtnsaetagtsto(isvdpfaueIor/tseicdhnmsuTet.r)ariMh.selelTn0laotph7-tcteo2eea8wdmtde1aedps5srpehb(rosreyaettdtedtuwhprrseeeequcmmrutneaaeatnrairxelgesismulca)erouaet(renmmbdmdleeuonMpdefte)st0arhds8taehu0tdoruf6iiiwrfn1rsefg7sterreeandMTEMtxia11tpneb11.gml02IDe91cp 112o77e.u rRaledtusbureeltssooabnftaPditnasefPptd(he51G.r..e54PAr qae)u seeexnrpiceehrsiimnogfentots T (K) Result

2073 not sink
2011 sink
fthl1uec5Pt2utactiaopn3su0inl0etohKre.ecTluehrcreterno2td-aeDfst.eimr maegletisngwaetre10inGtePga,rainteddicaintitnog 1m-eDltipnagttoefrns MMus11i22n30g1111D77 ioptas (Presche58..r05 and 2023 sink
2223 sink

153 Prakapenka, 2015) and refined using the PDIndexer software (Seto et al. 2010).

American Mineralogist, vol. 104, 2019

154

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#6891 Revision 1 to American Mineralogist

168 for freezing but not melting, hence we located the melting points based on the steep current rise

169 in the heatin6g74cycles. DONG ET AL.: MELTING CURVE MINIMUM OF BARIUM CARBONATE

170 As reported previously (Li et al. 2017), pre-melting rise in ionic conductivities introduces

Table 3. Lattice parameters of BaCO3 at high pressures and tem-
peratures

P T Space a b c V
(GPa) (K) group (Å) (Å) (Å) (Å3)

6.3 300 Pmcn 4.983(10) 5.425(10) 4.509(8) 121.9(8)
6.3 1026 Pmmn 5.080(5) 5.444(5) 4.532(4) 125.3(4)
8.0 300 Pmcn 4.931(2) 5.399(2) 4.520(3) 120.3(2)
8.0 1100 Pmmn 4.996(1) 5.415(1) 4.529(1) 122.5(1)
9.3 300 Pmmn 4.914(8) 5.381(10) 4.523(18) 119.6(11)

Note: Values in parentheses are uncertainties on the last digits.

mFiegausruer3em. MenetlsctiuinnrgvTFceauibriglvseuee1rosf(tefaBibl3laleC.idsOMhc3iereadcltltefhirniwoggihmthcpurteehrrsrevoseuriorbeonasfr.i)cTB, hcbaeyoCnmfiOdtetuli3tncigantgtitvhhcieutiygmrvhmeeltipeisnraeegsssutpsaroubeilrnmiestshse.efnrdTotsfmhroien1mmbTtahaereblttloiieon5ng1iGc Pcoanto7dou–uac1itia0vniGtdy PSahaanhdroruoro2m01te0m; Tpoerwantusreen(dOentoal2.020071;3O; Wnoaentgael.t 2008; Za-
al. 2015).
Kechin meltin(gfilelqeudatcioinrc(lKeewchiitnh, 2e0r0r1o)rabnadra),sebcyonfdit-toirndgertphoelymnoemltiianlgfropmoi5n.t5stofr1o1mGP1a .baTrhetomeltinAglctuhroveugh we could not measure temperatures below 1000 K to
between 1 bar5anGdP3aGtPoaa(dKotetecdhliinnem) ieslbtianckg-eexqtruaaptoiloanted(Kfreomchtihne 2K0e0ch1i)n amnedltiangsceucrovned(E-oqr.1d)ebrut hads entoet rmine the transition temperature below 9 GPa, our data suggest

been confirmepdoelyxpneormimieanltaflrloym. A5m.5elttiong1m1iGnimPau.mTihs elomcateeldtinnegarc5u.r5vGePbaeatnwde1e8n491 Kb,awrhaenredt3he twothat the boundary between the aragonite and post-aragonite phases

segments of thGePfiatte(ddomteteltdinglicnuer)veiscrobsasc. kR-eesxulttrsaopfoPlat tsepdherferoemxpetrhimeeKntes c(thriianngmleedltoiwnng: ncoutrvsienk; trioanfgBleauCp:O3 likely has a negative slope (Fig. 5). The possibility of a
sink) are plott(eEdqfo. r1c)robsust-vhaalisdantiootn.bTeheenmceolntinfigrmcuerdveesxopf ePrti(mdaesnhteadlliyn.eAs, Emrrealntdinognema 2in01im3 aunmd Kavnpeorsaintdive boundary (Shatskiy et al. 2015), however, cannot be ruled
Jeanloz 1998)iscrlooscsattheedfintteinagr m5.e5ltiGngPcauarvnedo1f 8B4aC9 OK3,mwulhtieprlee ttihmeetsw, noeasre1gbmaer,n3tsGoPfa,thanedfi1t0te-1d2 GPao.ut because the transition may have been kinetically hindered at
dbuaeendflcooeepcwrtttseatdihcnertteoimeac31mnpstue9oelGeilr9tdantlvcPit8saneieil)anntosg,cgtkchhoarpa;iocenfgottusidrhiPonsrinanvt1gtttnehse0(atemged–hnctla1efepdroiso2tehmfurtsriGaepsmanemd.tgPl=uetRpalilmrinst.seeniigsesnneuulgsk(pptlHi)ton=asaiaatgtnoryEtatechfernu(erspPFrailvabtinnogneesdtc.tgdprooe2eihndfHna)eeBn.sfuraoiiTeatnnrcC2hgeghc0exO,irr1npmoca33egutsmierssrdai-rumtdn1veoldl9nateweil8tnp,iKa9dstlorset)aadra.tv(trsieDitntomrnsmeinidafret.efnosToelag,trinfhilnnenedetenchgardtmJe,ereoclaes1aiwrstklin etebtneileenaolarpy=rgzit,ard5imu0seeatbrlp.oyotoFoeowo1btoswc0meTtrr-0tyeahesreKtmeaetnagmpalroveopnreoeairmtrtaeaugtrsteueterrsmtue.hcpeaterunmrrdaeat1iuls0reee2xs6atpin maKdnas1atie1otd0n60t.oc3 KobGeeafPt3fi8a.c9iG(ae8nnP)dta×o21(fT.03a–B(5b2aKl)Ce–×1O3b1)3.e0Tti–wn5heKethes–nee1
171
172
173
174

175 dicnoI/nmdsTuislrtteeipanlccehytaceh,hhsdeteheawtetmahietnemoilgcmlmiadcaaldyisxcclcietolmhrenpaesutotiamatioenmt xtaovitoafnguflaBiurvtteehaie,oeo:nwCnfspta:atreOsheneestdpaisesrukBsereatacenpsCotpeayvOgrspeomti3rxhceedaeindimldtmlysoanaeftdolmetittiflhnpyfdeegle1reccp:,buo1oysrim:rhn3eloetnp..swtEoTs-tseDhteehdemSaomnnpraoeeu2nrl0nsaatiitldnKguyegrnseraepgoncsoodfu,inrtvhtsee,sp1matw9rrveea9haelie5srinamur)eegriedeandnabdryyMavdgadCliutOieos3na(aLrleistycanosocmhvproeattrraoablnl.eX2t0Ro0D8th)eabxt upotefrcCiamaneCnbOtes3.b(eWtteur et al.
176 con-
177

178 values are rienpcoorntegdru(Teanbtlme 1e)l.tiAngm.oInngsdeivfeferraelnctoenxdpuercitmiveintytsetxhepemriemaseunretsd, mtheeltcinugr-tempeMratuerletaitng curve minimum and solid-solid phase
179 a given presrseunrteraegardeientgo bweitchaimn 1e0u0nKst.able when the temperature approached
transition to post-aragonite structure
ceainixrrduaictmcoibairntloeeeIodndtmodaueJtp23Mt1nnthecee1o0dGmaoeam70i1etmnn3trP83ppmeltpa0ea)emKoooerr6zLofapalsfua1ttriet1tiustBlui7nith9lircoonsra,ega9aeng.Cmbv8t,mTpheaweO)peohni.teaocwea3eiLssrenxsucomsesiltceikrusaerbeecacmoeenrltoeyarwtyfrphdpetichcbliteienehenhseefdmteefewucotd,sortedwraPtthnlmihrs(mtaoletoiFelbctnaehpPreii1gmsgenlept2seanpap.eectm0atooa2widttn oirrhsKe)ndeotnu.elentitdtrgAwdbioroeeenahfextitdP.tegtilteBhcdouAttmhtaoerwmmeutaCifaemsslee1l6bOitctlnhl0pp0lith3tginyairenaKaneGgtrangtgsaaog.ppP1MstffeTpeuoutab.reeoPhriraInteerimeittrnntl,s,(e(AittonKwEtwhreshl-fuxefreeTaaetaBrpievtohagmA,eman2lenhearCaee1dtedientsrm8Olloeldaottd0anriies3btnnn ehsvBnKaagdgaealcttaanllCuneceacOoremctao3loeyaotflimeennn11dldB0tpta8ii8inee4ntP4rtu9gitwanoK tcKeuuuersrenlavyste3fu5rpma.o5nmtidonGi221mP141au45Gm93(F  PK,Kiagt,h,a.wetB33ia)mt.ChGeOOaPltnai3fnimtttgohteeep1dlo8tlmeoi4dnw9etl os-KtpvionreafegrtsBca5suua.l5CrraevrGOegsPe3imdadre.iaenOncoimgrnfeeuatthhsomeeef
180
181
182
183
184

(Errandonea 2013) and 2137 K9(oKf 2a5vner and Jeanloz 1998). No
sample was recovered from these experiments because the Pt cap-

sules melted. The crossing between the melting curves of BaCO3
and Pt limits the application of this experimental configuration at

pressures below 3 GPa and above 10–12 GPa (Fig. 3).

Phase boundary between aragonite and post-aragonite Figure 4. Synchrotron XRD experiments show that the aragonite
phase BaCO3 transforms into the post-aragonite phase at 6.3 GPa, 1026 K,
Synchrotron XRD measurements (Table 3) revealed the phase and the post-aragonite phase was preserved when quenching to ambient
transformation of BaCO3 from the aragonite structure (Pmcn) to temperature. The stars represent the Pt mixed with the sample as a heat
the post-aragonite structure (Pmmn) at high pressures and/or high absorber. The tick marks show the calculated peak position for aragonite
temperatures. The post-aragonite phase can be readily recognized [a = 5.227(4) Å, b = 8.889(12) Å, and c = 5.839(4) Å] and post-aragonite
by two distinct peaks at 6.36° and 6.70° in the XRD pattern (Fig. 4). phases [a = 5.800(5) Å, b = 5.444(5) Å, and c = 4.532(4) Å at 1026 K; a =
Upon compression at 300 K, the phase transformation took place 4.983(10) Å, b = 5.425(10) Å, and c = 4.509(8) Å at 300 K], respectively.
at a pressure between 8 and 9.5 GPa and room temperature. Upon
heating to 1026 K at 6.3 GPa, the sample transformed fully to the
post-aragonite phase. At 8.1 GPa, the transformation was already
complete at 1000 K. The post-aragonite phase remained meta-
stable when the sample was cooled to 1000 K at 6.3 GPa. It was
also a metastable phase when the sample was quenched to room
temperature at both pressures. Upon decompression to ambient
pressure, the post-aragonite phase transformed back to witherite.
Our results are consistent with the results from the literature that
BaCO3 transforms from aragonite to post-aragonite structure at

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DONG ET AL.: MELTING CURVE MINIMUM OF BARIUM CARBONATE 675

Figure 5. Phase diagram of BaCO3. The boundary between the the aragonite and the trigonal phase (Rapoport and Pistorius
witherite and post-aragonite BaCO3 (dotted negative slope in black) is 1967). The cubic BaCO3 has not been studied at high pressures
drawn according to the synchrotron XRD measurements in Table 3 (open and its stability field remains unconstrained. The transition to the
square = post-aragonite BaCO3; filled square = aragonite BaCO3). The dotted cubic structure at 1249 K is thought to be driven by the increased
positive slope in gray is the same phase boundary proposed in Shatskiy et rotational activity of the CO23– groups (Lander 1949). Because the
al. (2015). The phase transition from witherite to trigonal BaCO3 (solid = rotation is more restricted under compression, the cubic structure
Rapoport and Pistorius1967) intersects with this proposed witherite-post- may become thermodynamically less favored at elevated pressures.
aragonite boundary near 5 GPa and 1200 K. Cubic BaCO3 forms at 1 bar For these reasons, we postulate that the trigonal phase is likely
and high temperatures, and its stability field at high pressures is not known. the high-temperature BaCO3 polymorph on the melting curve just
below 5.5 GPa. On the high-pressure side of the melting curve
high-pressure side, it increases from 1849 K at 5.5 GPa to 2453 K at minimum, the BaCO3 polymorph below the melting curve is not
11 GPa. The fitted melting curve changes at a rate of ~–125 K/GPa fully resolved but the post-aragonite structure has been shown to
from 3 GPa, and after reaching the local minimum near 5 GPa, be stable at high temperature over a broad range of pressures (e.g.,
the melting curve increases at a rate of ~110 K/GPa to 11 GPa. Townsend et al. 2013). Assuming the post-aragonite BaCO3 is the
The intersection of the fitted melting curves was determined as the only stable phase on the high-pressure side of the melting curve
local minimum at 1849 K, 5.5 GPa, where the sign of the melting minimum, the aragonite-trigonal boundary intersects with the
slope flips from negative to positive (Fig. 5). negative phase boundary between the aragonite and post-aragonite
at a triple point near 5 GPa and 1200 K, suggesting the presence of
The local minimum is a prominent feature in the melting curve a boundary between trigonal and post-aragonite, which is defined
of BaCO3. According to the Claudius-Clapeyron equation, dT/dP by the triple point and the melting curve minimum. This bound-
= ΔV/ΔS = TΔV/ΔH, the slope of the melting curve is governed by ary implies that the melting curve minimum may correspond to
the volume of fusion (ΔV = Vliquid – Vsolid) and the entropy of fusion a triple point where liquid, trigonal, and post-aragonite phase of
(ΔS = Sliquid – Ssolid). Melting of a single component is usually an BaCO3 coexist (Fig. 5).
endothermic process with positive ΔH and ΔS, hence the sign of
the melting slope is determined by the ΔV term: A positive sign The coordination number of Ba is 6 in the trigonal structure and
implies that the solid is denser than the liquid, and vice versa. A 12 in the post-aragonite structure (Ono et al. 2008). The difference
sign change indicates a density crossover between the liquid and in the coordination of Ba would make the post-aragonite phase
solid. In BaCO3, the flip of the melting slope from negative to denser than the trigonal phase and could explain the density jump
positive near 5 GPa indicates a density jump (volume collapse) at the melting minimum near 5 GPa (Fig. 6). Current knowledge
in the solid phase. of solid-solid phase transformations of BaCO3 is insufficient
to map out the phase diagram, and therefore the melting curve
We postulate a trigonal to post-aragonite phase transition minimum may be associated with other solid-solid phase transi-
is responsible for the density crossover between the solid and tions involving different coordination number of Ba. In addition,
liquid near 5.4 GPa. The structures of solid phases along the several metastable phases of BaCO3 have been observed experi-
low-pressure segment of the melting curve are not known. At mentally. A P2122 rhombohedral phase was also recovered from
least three solid polymorphs of BaCO3 occur at pressures up to the experiment at 15 GPa and 1273 K (Lin and Liu 1997), while
6 GPa, including witherite in the aragonite structure, a trigonal a phase transition to the trigonal phase (P31c) was observed at 7.2
calcite structure and a cubic structure (Fig. 5). Upon heating at GPa and room temperature (Holl et al. 2000; Chaney et al. 2015).
the ambient pressure, aragonite BaCO3 transforms to the trigonal
structure at 1084 K (Antao and Hassan 2007), and then to a cubic Melting curve maximum and possible change in liquid
structure at 1249 K (Lander 1949; Antao and Hassan 2007; Nie structure
et al. 2017). No further phase change has been observed up to
1573 K at ambient pressure. Previous experiments at pressures The presence of a melting curve maximum in BaCO3 is sug-
up to 3.6 GPa found a slightly positive phase boundary between gested by our experimental data and existing constraints on the
melting point at 1 bar. According to the 98th edition of the CRC
Handbooks of Chemistry and Physics, the melting point at 1 bar is
1828 K, whereasAlfaAesar Materials Safety Data Sheet (Thermo-
Fisher Scientific 2010) listed 1084 K as the melting point, which
is most likely the boundary between the aragonite and trigonal
phase. Some studies found that BaCO3 started decomposing at
1200 K to produce BaO and CO2 vapors (L’vov and Novichikhin
1997) and decomposition proceeds in a melt after 1300 K (Gal-
wey and Brown 1999). The prevailing CO2 pressure is known to
influence its melting behavior (Judd and Pope 1972). Despite the
uncertainties, the melting point of BaCO3 at 1 bar is likely equal
to or less than 1828 K. Our Pt sphere experiment indicates that
the melting point at 1.4 GPa is above 2073 K, and therefore the
melting curve has a positive slope at elevated pressures near 1 bar.
Between 3 and 5.4 GPa, a negative melting slope determined by
our conductivity measurements implies that a local maximum of

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676 DONG ET AL.: MELTING CURVE MINIMUM OF BARIUM CARBONATE

resolved when data from different experiments are combined (Li
et al. 2017). The negative slope of BaCO3 from 3 to 5 GPa and the
positive slope at pressures above 6 GPa are both steep and have
been clearly observed experimentally. In contrast, limited data sug-
gest that the melting point of MgCO3 increases monotonically up
to 15 GPa (Irving and Wyllie 1973; Katsura and Ito 1990; Müller
et al. 2017) and tends to flatten at higher pressures up to 80 GPa
(Solopova et al. 2015).

The occurrence or absence of the solid-solid phase transitions
below the melting curve in MgCO3, CaCO3, and BaCO3 follow
the expected inverse relation between the pressure of structure
transformation (Prewitt and Downs 1998; Redfern 2000) and
ionic radius (Ba2+ > Ca2+ > Mg2+). At the ambient temperature,
BaCO3 (witherite) undergoes the aragonite to post-aragonite phase

a

Figure 6. Schematic density profiles of liquid BaCO3, post-aragonite b
BaCO3, and a less dense BaCO3 phase, likely the trigonal phase, at a
constant temperature. On the high-pressure side, the phase transformation Figure 7. Systematic comparison of melting behavior of alkaline
from aragonite BaCO3 (witherite) to post-aragonite BaCO3 causes an abrupt earth carbonates. (a) The melting curves of CaCO3 (Li et al. 2017) and
density change in the solid. The post-aragonite BaCO3 becomes denser BaCO3 (this study) do not increase monotonically but have local minima,
than liquid BaCO3 above 5.5 GPa, whereas the compressed liquid BaCO3 which significantly decrease melting point at high pressures and affect the
is denser than the solid from 3 to 5.5 GPa (solid line), which explains order of melting of alkaline earth carbonates. Phase relations of CaCO3
the melting curve minimum near 5 GPa. On the low-pressure side, the are based on Li et al. (2017) and Bayarjargal et al. (2018). (b) No local
compressed liquid is less dense than the solid (dashed line), which explains minimum has been observed in MgCO3, however, a similar melting
the inferred melting curve maximum. curve minimum may exist in MgCO3 resulting from the predicted phase
transformation to the magnesite II phase (black open square) at megabar
the melting curve occurs between 1 bar and 3 GPa. pressures (Isshiki et al. 2003) and possibly generate carbonate melt near
The local maximum implies another density crossover be- core-mantle boundary or even in the lower mantle. The melting curve of
MgCO3 (red) is a preliminary fit of the existing data to the Kechin melting
tween solid and liquid, which can be attributed to continuous equation (a = 0.0005086, b = 11.06, c = –0.0003971, T0 = 875 K, and P0
changes in the liquid structure. Without long-range orders, the = 1 bar). Experimental data on MgCO3 were compiled from the literature:
liquid structure is more flexible and may allow the coexistence blue for Irving and Wyllie (1973), magenta for Katsura and Ito (1990),
of multiple coordination numbers (Ghiorso 2004; Stixrude and cyan for Isshiki et al. (2003), yellow for Solopova et al. (2015), and green
Karki 2005) and polyhedra configurations (Liu et al. 2007). As for Müller et al. (2017) (open circle = liquid MgCO3; filled circle = solid
pressure increases, the average coordination number of BaCO3 in MgCO3). Mantle adiabats were estimated based on Herzberg et al. (2007)
the liquid may increase continuously whereas the solid phase has a and Putirka et al. (2007) with a constant slope of dT/dP = 8 K/GPa.
fixed coordination number until a pressure-induced, discontinuous
phase transition takes place. As a result, the liquid would be more
compressible than the solid and have a density equals to that of
the solid at the melting point maximum. If the density crossover
arises from the more compressible liquid, the volume difference
between the solid and liquid would increase continuously away
from the local maximum, and therefore the slopes of the melting
curves are expected to flatten gradually near the turning point.

However, the occurrence of liquid structure change in BaCO3
need to be further confirmed by future theoretical and/or experi-
mental investigation, and this particular hypothesis remains highly
speculative.

Systematic comparison of alkaline earth carbonates

The shape of the melting curve of BaCO3 resembles that of
CaCO3, but the melting curve minimum is more pronounced and
occurs at lower pressure (Fig. 7a). In CaCO3, a local maximum was
observed near 8 GPa, and a local minimum occurs near 13 GPa.
The negative melting slope in CaCO3 between 8 and 13 GPa is
indicated by melting points collected at multiple pressures in indi-
vidual conductivity experiment, but the sign of the slope cannot be

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DONG ET AL.: MELTING CURVE MINIMUM OF BARIUM CARBONATE 677

transition at 9 GPa and the same transition occurs at much higher rocks. It also dictates the composition of the incipient melt. There-
pressure of near 40 GPa for CaCO3, whereas the trigonal phase fore, the solidus of a carbonated rock may have variable slopes,
of MgCO3 (magnesite) remains stable up to 80 GPa (Fiquet et al. and its composition may change considerably with pressure.
2002; Isshiki et al. 2004). The melting minimum in CaCO3 at 13
GPa is attributed to a transition from sixfold-coordinated calcite V Discontinuous change in the slope of the melting curve is a gen-
to ninefold-coordinated aragonite phase (Fig. 7a), whereas that in eral feature of silicate and alkaline earth carbonates, and therefore
BaCO3 near 5 GPa likely results from a trigonal to post-aragonite melting curves cannot be extrapolated beyond the measurement
transition. The lack of a melting curve minimum in MgCO3 up to range without considering adjacent solid-solid transitions. The
80 GPa is consistent with the stability of the trigonal phase (Fiquet empirical Simon equation has been widely used to fit high-pressure
et al. 2002; Isshiki et al. 2004). melting curves (e.g., Li and Li 2015). This melting equation has the
advantage of not requiring any knowledge of the solid’s equation
Although the systematics of the solid structures of alkaline of state and works well for interpolation. For a negative segment of
earth carbonates is consistent at moderate pressures, the formation the melting curve, however, the empirical Simon equation must be
of tetrahedrally coordinated carbon at high pressures indicates modified to describe the negative pressure dependence of melting
a deviation from the systematic behavior at megabar pressures temperature (Kechin 2001). Lindemann’s law provides a semi-
(Boulard et al. 2015). Synchrotron XRD measurements (Townsend empirical scaling relation to fit discrete measurements of melting
et al. 2013) and ab initio calculations (Arapan et al. 2007) showed temperatures for interpolation, and in the absence of data it is often
that that the post-aragonite structure of BaCO3 remained the used with an equation of state to predict melting temperatures at
thermodynamically favored phase up to at least 300 GPa. The high pressures (e.g., Li and Li 2015). Because Lindemann’s law
phase transformation to the pyroxene-type (C2221) BaCO3 was does not consider the liquid behavior, it is inadequate to represent
predicted to occur at 76 GPa (Zaoui and Shahrour 2010), whereas flat or negative melting slope associated with structural changes
it was not observed experimentally at the pressure of at least 150 in the liquid.
GPa (Townsend et al. 2013). This observation seems to violate
the expected systematic trend that isostructural compounds Constraints on the melting curves shed light on the adjacent
exhibit the same type of pressure-induced phase transformation high-temperature solid polymorphs. For BaCO3 the melting curve
and that the transition pressure is lower for larger cation (Prewitt minimum is interpreted as a triple point among liquid, trigonal,
and Downs 1998). and post-aragonite phases. The inferred boundary between the
trigonal and post-aragonite phases need to be mapped out by
Implications measurements. Furthermore, the boundary between aragonite and
post-aragonite may be narrowed down through XRD or Raman
In this study, we found that the melting curve of BaCO3 measurements using externally heated diamond-anvil cells, where
involves a local minimum near 5 GPa and may contain a local temperatures between room temperature and 1500 K can be more
maximum between ambient pressure and 3 GPa. Density cross- precisely controlled and reliably measured to allow evaluation of
overs at the turning points are attributed to structural changes of kinetic effects and reversal of phase transitions.
the relevant phases along the melting curve, including solid-solid
transition with an abrupt change in the coordination number of Knowledge of the melting curves can also be used to estab-
divalent cation and/or gradual increase in the average coordina- lish the equation-of-state of carbonate melts and help construct
tion number in the liquid. The experimentally observed melting thermodynamic models to predict the behavior of carbonate-
curve minima in both CaCO3 and BaCO3 imply that the solid-state bearing rocks inside the Earth (e.g., Liu and Lange 2003). This ap-
transitions in other alkaline earth carbonates may be used to predict proach is less straightforward in the multi-component melt (Walker
the occurrence of turning points in their melting curves. Given et al. 1988) but has been shown to work for carbonate melt (e.g.,
the systematic similarity of phase transformation in compressed Liu et al. 2007). For BaCO3, additional experiments are required
carbonates, a minimum may occur in MgCO3 at the megabar pres- to determine the melting curve between 1 bar and 3 GPa in the
sure range where its melting curve intersects the phase transition piston-cylinder press, to test the presence of a local maximum and
from magnesite to magnesite II (Fig. 7b). resolve its exact location. In particular, the experimental configu-
ration of the ionic conductivity method needs to be modified for
Accordingly, we may expect a melting curve maximum in measurements at a few gigapascals using a piston-cylinder press.
MgCO3 resulting from the predicted transition from trigonal More refractory materials such as Ir are needed to replace the Pt
phase to a denser polymorph with higher coordination number capsule and electrodes for ionic conductivity measurements on
at megabar pressures (Shatskiy et al. 2015; Isshiki et al. 2004) BaCO3 at pressures below 3 GPa and beyond 11 GPa.
if the comparative crystal chemistry rules still hold (Hazen et al.
2000). Previous studies suggest that the decomposition bound- Funding and Acknowledgments
ary of magnesite contains a minimum near 115 GPa (Isshiki et
al. 2004). It is conceivable that a similar minimum exists in the This work was supported by Alfred P. Sloan Foundation grant G-2016-7157
melting curve of MgCO3. and grant G-2017-9954, and by National Science Foundation grant EAR 1763189,
and grant AST 1344133. In addition, we thank Cassandra Seltzer for her assistance
The occurrence of melting curve minima at different pressures with the platinum sphere experiments; David Walker and Matthew Brennan for
implies that the order of alkaline-earth carbonate melting points discussions and comments on the manuscript.
changes with pressure. BaCO3 is less refractory than MgCO3 and
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American Mineralogist, Volume 104, pages 1083–1091, 2019

Experimental investigation of FeCO3 (siderite) stability in Earth’s lower mantle using
XANES spectroscopy

Valerio Cerantola1,2,*,†, Max Wilke3, Innokenty Kantor4, Leyla Ismailova5,
Ilya Kupenko6, Catherine McCammon2, Sakura Pascarelli1, and Leonid S. Dubrovinsky2

1European Synchrotron Radiation Facility, 71 Avenue des Martyrs, 38000 Grenoble, France
2Bayerisches Geoinstitut, Universität Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany
3Institut für Erd- und Umweltwissenschaften, Universität Potsdam, Karl-Liebknecht-Straße 24, 14476 Potsdam, Germany
4Danmarks Tekniske Universitet, Anker Engelunds Vej 1 Bygning 101A, 2800 Kgs. Lyngby, Denmark
5Skolkovo Institute of Science and Technology, Skolkovo Innovation Center, Building 3, Moscow, 143026, Russia

6Institut f. Geowissenschaften, Universität Münster, Schlossplatz 2, 48149 Münster, Germany

Abstract

We studied FeCO3 using Fe K-edge X‑ray absorption near-edge structure (XANES) spectroscopy
at pressures up to 54 GPa and temperatures above 2000 K. First-principles calculations of Fe at the
K-edge in FeCO3 were performed to support the interpretation of the XANES spectra. The variation
of iron absorption edge features with pressure and temperature in FeCO3 matches well with recently
reported observations on FeCO3 at extreme conditions, and provides new insight into the stability of
Fe-carbonates in Earth’s mantle. Here we show that at conditions of the mid-lower mantle, ~50 GPa
and ~2200 K, FeCO3 melts and partially decomposes to high-pressure Fe3O4. Carbon (diamond) and
oxygen are also inferred products of the reaction. We constrained the thermodynamic phase boundary
between crystalline FeCO3 and melt to be at 51(1) GPa and ~1850 K. We observe that at 54(1) GPa,
temperature-induced spin crossover of Fe2+ takes place from low to high spin such that at 1735(100) K,
all iron in FeCO3 is in the high-spin state. A comparison between experiment and theory provides a
more detailed understanding of FeCO3 decomposition observed in X‑ray absorption spectra and helps
to explain spectral changes due to pressure-induced spin crossover in FeCO3 at ambient temperature.

Keywords: Deep carbon cycle, siderite, decomposition, melting, spin transition; Earth in Five
Reactions: A Deep Carbon Perspective

Introduction already understood in a general way. However, a more detailed
understanding requires investigation of the stability of each sub-
Subduction zones are descending limbs of Earth’s lithosphere ducted phase to constrain the physics and chemistry of subducting
that, together with ascending mantle plumes, are part of the active plates at different depths inside Earth.
geodynamics of Earth that influence its physical and chemical
evolution (e.g., Tackley et al. 1993; Zhao 2003; Walter et al. Carbonates are one of the major components of the sedimen-
2011; Chang et al. 2016). Subduction zones are Earth’s largest tary layers (Rea and Ruff 1996). Their presence inside Earth is
recycling system. They deliver crustal material to Earth’s inte- supported by laboratory experiments (Stagno et al. 2011; Liu
rior, where re-equilibration with the surrounding mantle takes et al. 2015; Cerantola et al. 2017) and through observations
place mainly via complex physical mechanisms and chemical of natural samples, for example inclusions in diamonds from
reactions (e.g., Saunders and Tarney 1984; Keppler 1996; Motti the upper and lower mantle (e.g Kvasnytsya and Wirth 2009;
et al. 2004; Bebout 2014). Material that is not recycled in the Kaminsky 2012; Kaminsky et al. 2016).
upper few hundred kilometers of subduction zones will subduct
deeper, ultimately toward the core-mantle boundary (CMB), The three major carbonate components in the crust and
where different temperatures, pressures, and oxygen fugacities upper mantle are CaCO3 (calcite), MgCO3 (magnesite), and
(fO2) govern the stability of subducted material (e.g., Bina and FeCO3 (siderite). Their presence in subducting plates (Rea and
Helffrich 1994; Dubrovinsky et al. 2003; McCammon 2005; Ruff 1996) and recycled banded iron formations (Klein 2005;
Rohrbach and Schmidt 2011; Stagno et al. 2011; Bykova et al. Konhauser et al. 2017), and their stability in experiments that
2016). The general structure of subducting slabs is well known simulate the paragenesis of carbonated eclogites (i.e., Kiseeva et
and can be summarized as a layered sequence of sedimentary al. 2012, 2013) suggest that they are the major source of carbon
rocks, oceanic crust, and altered peridotite. The fate of slabs influx into the deep Earth.
exposed to the extreme conditions present in Earth’s interior is
Previous high-pressure studies on the carbonate end-members
* E-mail: [email protected] CaCO3, MgCO3, and FeCO3 revealed high-pressure phase transi-
† Special collection papers can be found online at http://www.minsocam.org/MSA/ tions in all three phases (i.e., Isshiki et al. 2004; Ono et al. 2005;
AmMin/special-collections.html. Lavina et al. 2009; Boulard et al. 2012; Smith et al. 2018; Li et
al. 2018), as well as in solid solutions between these phases (i.e.,
Mao et al. 2011; Merlini et al. 2012; Liu et al. 2015; Soloma-

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1084 CERANTOLA ET AL.: INVESTIGATION OF FECO3 STABILITY IN EARTH’S LOWER MANTLE

tova and Asimow 2017). In particular, iron plays a fundamental be involved. In parallel, major advances in ab initio modeling
role in the behavior of carbonates at extreme conditions (e.g., techniques enable calculation of l-projected density of states,
Boulard et al. 2012; Liu et al. 2015). Iron can radically change even in complex systems such as transition-metal oxides, to
the thermodynamic stability of carbonates, preserving them obtain theoretical simulations of the absorption spectra beyond
from breaking down at pressures and temperatures of the lower the multiple scattering formalism (Joly 2001). The theoretical
mantle. This behavior may be a direct consequence of pressure- work complements our experimental findings in qualitatively
induced electronic spin crossover (Lavina et al. 2009; Lin et al. reproducing the observed spectral features. Discussion of results
2012; Lobanov et al. 2015; Cerantola et al. 2015), which was focuses on two issues: (1) the effect of extreme conditions on
reported at ~43 GPa and room temperature for the end-member FeCO3 absorption spectra and (2) the comparison between ob-
FeCO3, increasing to above 50 GPa at ~1200 K (Liu et al. 2014). served and calculated XANES spectra with focus on FeCO3 spin
crossover. Finally, we extend the discussion of our experimental
During subduction the majority of slab geotherms intersect a observations to Earth’s interior and relate their importance to
deep depression along the melting curve of carbonated oceanic the fate of carbonates within the dynamics of subduction zones.
crust at depths of approximately 300 to 700 km (Thomson et al.
2016). FeCO3 melting in the mantle has also been confirmed by Methodology
other experimental studies (Tao et al. 2013; Kang et al. 2015) that
investigated its stability from ambient conditions up to 20 GPa Sample synthesis
and ~2150 K. At pressures below ~6.8 GPa, FeCO3 does not melt
but decomposes through an auto redox dissociation reaction to Single crystals of 57FeCO3 were grown from 57FeCO3 powder at 18 GPa
Fe3O4, a carbon polymorph and CO2 (Tao et al. 2013; Kang et al. and 1600 °C in a 1200-t Sumitomo press at Bayerisches Geoinstitut (Bayreuth,
2015). At pressures above ~6.8 GPa FeCO3 melts, with a minor Germany). FeCO3 powder was synthesized using 57Fe-oxalate (57FeC2O4) as a pre-
quenched Fe3+-rich phase interpreted to be the result of redox cursor, which in turn was obtained via chemical reactions starting from 57Fe-metal
dissociation of FeCO3-liquid, leading to dissolved Fe3+ and CO2 (see Cerantola et al. 2015 for more details). Single crystals with an average size
in the carbonate melt (Kang et al. 2015). At P > 33 GPa, however, of 0.015 × 0.015 × 0.010 mm3 were loaded together with small ruby chips 5 to
non-molecular CO2 does not melt but dissociates to carbon and 10 mm in diameter (for pressure estimation) into BX90-type diamond-anvil cells
oxygen, which indicates that FeCO3 melting at lower mantle (DACs) (Kantor et al. 2012) and high-pressure membrane cells from the European
conditions produces Fe-oxides + C (diamond) + O2 in the melt Synchrotron Radiation Facility (ESRF). The Dewaele et al. (2008) pressure scale
rather than Fe-oxides + molecular CO2 (Litasov et al. 2011). It is was used to estimate pressure from the fluorescence line of the ruby spheres.
likely, however, that O2 in natural settings reacts further to form Diamonds with culet sizes of 250 mm and rhenium gaskets with 120 mm starting
other phases, i.e., bonding with available cations as a network diameter holes were employed in all experiments. Neon was used as a pressure-
former or modifier in the melt or forming stable crystalline phases transmitting medium and loaded at Bayerisches Geoinstitut (Kurnosov et al. 2008)
such as oxides. Cerantola et al. (2017) recently showed that and ESRF. High temperatures were achieved using the double-sided YAG laser
FeCO3 melting extends up to ~70 GPa, where the transformation heating system at the ID24 beamline at ESRF. Temperature uncertainties (error
of FeCO3 to high-pressure carbonate (HP-carbonate) structures bars) were estimated using the difference between the measured temperature from
(CO44– groups) begins (Oganov et al. 2008; Boulard et al. 2011, the upstream and downstream sides of the double-sided laser heating system. To
2012, 2015; Merlini et al. 2015; Cerantola et al. 2017; Merlini et test for pressure gradients, in some experiments we placed two to three ruby chips
al. 2017). In the same work, Cerantola et al. (2017) unambigu- in different positions inside the pressure chamber. In all cases, pressure differences
ously showed that, after melting, FeCO3 partially recrystallizes measured by different ruby spheres loaded in the same gasket chamber were not
as a-Fe2O3 (hematite) and HP-Fe3O4 (Bykova et al. 2016) at more than 1 GPa along the entire pressure range investigated (up to 54 GPa).
pressures below and above 25 GPa, respectively. As subproducts
of the reactions the presence of other Fe-oxides (minor phases) XANES spectroscopy
and diamond was observed (Cerantola et al. 2017). The change in
the redox state of the system is likely caused by the stabilization Fe K-edge XANES measurements were performed at ESRF at the energy
of HP-Fe3O4 above 25 GPa (Dubrovinsky et al. 2003; Bykova dispersive X‑ray absorption spectroscopy (XAS) beamline ID24 (e.g., Pascarelli
et al. 2016). The kinetics of decomposition upon melting is still et al. 2016). The beam was focused horizontally using a curved polychromator Si
under debate and will not be discussed here. 111 crystal in Bragg geometry and vertically with a bent Si mirror. The obtained
cross section at the full-width half-maximum (FWHM) is about 3 × 5 mm2 in
In this study, we performed an experimental and theoretical horizontal and vertical directions, respectively. The Bragg diffraction peaks aris-
investigation on the high-pressure high-temperature behavior ing from the diamond anvils were removed from the energy range of interest by
of synthetic FeCO3 using X‑ray absorption near-edge struc- changing the orientation of the DAC and following in real time the intensity of the
ture (XANES) spectroscopy at the Fe K-edge. Experimental transmitted beam on a two-dimensional detector. The measured XANES spectra
conditions covered pressures and temperatures down to the were normalized using the Athena software package (Ravel and Newville 2005),
shallow-mid lower mantle and XANES data complement X‑ray and the second-order polynomial for the pixel to energy conversion parameters
diffraction studies of FeCO3 stability in Earth’s mantle. In par- was calibrated using a reference a-Fe foil spectrum.
ticular, XANES spectroscopy in dispersive mode is an extremely
powerful experimental technique capable of detecting decompo- Ab initio calculations
sition and phase transformation reactions on a millisecond time
scale through the characteristic XANES features of the material. Theoretical XANES calculations were performed using the FDMNES code
We acknowledge, however, the limitation of XANES in phase (Joly 2001; Bunau and Joly 2009). In FDMNES, the Schrödinger equation is solved
identification for which other complementary techniques should by the finite difference method (FDM) within the local density approximation.
For calculation of the spectra, structural models of FeCO3 and HP-Fe3O4 were
used that were based on high-pressure single-crystal X‑ray diffraction (SXRD)
data measured on a single crystal of FeCO3 from the same synthesis run used to
produce the samples for XANES (Table 1). SXRD measurements were carried out
at the high-pressure X‑ray diffraction beamline ID09a (now ID15b) at the ESRF
(MAR555 detector, l = 0.4126 Å). A FeCO3 single crystal with an average size
of 0.015 × 0.015 × 0.010 mm3 was loaded and compressed to different pressures
in a standard ESRF membrane cell using neon as a quasi-hydrostatic pressure
medium. X‑ray diffraction images were collected during continuous rotation of
DACs typically from –38° to +38° on w, while data collection experiments were

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CERANTOLA ET AL.: INVESTIGATION OF FECO3 STABILITY IN EARTH’S LOWER MANTLE 1085

Table 1. Details of crystal structure of synthetic FeCO3 and HP-Fe3O4 single crystal at different pressures and ambient temperature and atomic
coordinates used for simulations

Sample P (GPa) a b c Volume a b g

FeCO3 (R3c, Z = 6) 4(1) 4.661(1) / 15.09(5) 283.8(4) 90 90 120
37(1) 4.530(4) / 13.46(19) 239.2(3) 90 90 120
55(1) 4.346(1) / 12.4(4) 202.8(2) 90 90 120
HP-Fe3O4 (Bbmm, Z = 4) 51(1) 9.230(13) 9.168(4) 2.6775(12) 226.6(3) 90 90 90

FeCO3-4(1) GPa FeCO3-37(1) GPa
Site x y z Site x y z
Fe 0.0000 0.0000 0.0000 Fe 0.0000 0.0000 0.0000
C 0.0000 0.0000 0.2500 C 0.0000 0.0000 0.2500
O 0.27525 0.0000 0.2500 O 0.27912 0.0000 0.2500

FeCO3-55(1) GPa HP-Fe3O4-51(1) GPa
Site x y z Site x y z
Fe 0.0000 0.0000 0.0000 Fe(1) 0.1328 0.0744 0.0000
C 0.0000 0.0000 0.2500 Fe(2) 0.3809 0.2500 0.0000
O 0.2895 0.0000 0.2500 O(1) 0.5000 0.0000 0.0000
O(2) 0.0300 0.2500 0.0000
O(3) 0.2210 –0.1176 0.0000

performed by narrow 0.5–1° scanning of the same w range. Typical images and position at 7112 eV is preserved. Heating to 1890(100) K and
integrated XRD patterns are illustrated in Cerantola et al. (2017) who collected higher completely changes the spectral shape. The maximum of
comparable data on similar samples. Indexing and refinement of the unit-cell the pre-edge shifts from 7112 to 7113.5 eV (Fig. 3), and peak 1
parameters were performed using CrysAlisPro software (CrysAlisPro 2014), and shifts to slightly higher energies and “merges” with peak 2, the
the complete procedure for data analysis is described in Cerantola et al. (2017). latter becoming a weak “hump” of the first peak. The ambient
temperature spectrum after quenching from 2025(100) K (Fig.
The cluster radius for the calculations was set to 6 Å and was checked for 2a, top) preserves the same features observed at high temperature,
convergence beforehand. The electron density of the potential was optimized in a but they appear sharper due to the absence of heating.
self-consistent manner using the same cluster size. Calculations included quadrupo-
lar transitions. The natural core-hole broadening of 1.4 eV was used and the value After quenching, the same sample was re-annealed up to
of the Fermi energy set to include excitations to d-states in the pre-edge region. 1830(100) K. Annealing at lower temperature caused partial
Finally, calculated spectra were shifted in energy to match the experimental ones. recrystallization of low-spin (LS) FeCO3 as seen by the increase
Vibrational disorder from finite temperature was not included. in intensity of peak 2 (Cerantola et al. 2015) up to 1675(100) K
and a change in spectral shape at 1830(100) K. The XANES
Results region of the room temperature spectrum after annealing (Fig.
2b, top) is similar to the one before heating (Fig. 2a, bottom)
FeCO3 stability as a function of pressure and temperature and is characterized by distinctive peaks marked “1”, “2”, and
“3” in the spectra. The EXAFS region, however, is clearly dif-
The temperature effect on FeCO3 stability at 36(1) GPa ob-
served by XANES spectroscopy is shown in Figure 1. At room Figure 1. Experimental normalized XANES spectra of the Fe
temperature before heating, a typical FeCO3 XANES spectrum K-edge of FeCO3 at 36(1) GPa and measured in situ at the indicated
is observed with iron in the high-spin state, showing a weak temperatures. Peaks marked “1” and “2” are characteristic for FeCO3
pre-edge peak at 7112 eV and two main peaks, identified in the and are used to monitor the state of the material. After heating at
figures as “1” and “2” (Wilke et al. 2001; Cerantola et al. 2015). 2270(200) K the spectra radically change, indicating chemical and/or
Heating the sample to 1890(100) K causes a decrease in the inten- physical changes in the system.
sity of peak 2 and a smoothing of the EXAFS oscillations above
7160 eV. Upon further heating at 2270(200) K the spectrum radi-
cally changes, manifested by the loss of the double-peak structure
at the main edge and the appearance of a new peak at 7113.5 eV.
New features at 7155, 7173, and 7210 eV appear, drastically
modifying the beginning of the EXAFS region. The flattening
of the spectrum is a clear consequence of high temperature, and
suggests complete or at least partial melting of the sample (e.g.,
Aquilani et al. 2015). The spectrum after quench shows identical
features to the high-temperature spectrum, but more pronounced
due to the absence of thermal damping.

Representative XANES spectra of FeCO3 at 51(1) GPa and
increasing temperature are plotted in Figure 2. In particular,
Figure 2a shows low-spin FeCO3 at room temperature with the
characteristic pre-edge at 7112 eV and an additional feature at
7116.5 eV. Peak 2 is more intense than peak 1 and a pronounced
hump is observed at 7155 eV (feature 3, Cerantola et al. 2015).
The temperature effect is evident at 1600(100) and 1775(100) K,
where an exchange of intensity between peaks 1 and 2 and a
smoothing of the first EXAFS oscillation are observed. However,
despite the high temperature, the characteristic pre-edge peak

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1086 CERANTOLA ET AL.: INVESTIGATION OF FECO3 STABILITY IN EARTH’S LOWER MANTLE

Figure 2. Experimental normalized FeCO3 XANES spectra at Figure 4. Experimental normalized FeCO3 XANES spectra at
51(1) GPa and measured in situ at the indicated temperatures. (a) At 54(1) GPa and measured in situ at the indicated temperatures. With
room temperature, features characteristic of low-spin FeCO3 (including increasing temperature at constant pressure, iron atoms undergo a low-
the peak marked “3”; Cerantola et al. 2015) are visible. Increasing spin to high-spin transition, marked by a change in relative intensity of
temperature stabilizes the high-spin state (marked by the intensity peaks 1 and 2 and disappearance of peak 3 (Cerantola et al. 2015). There
exchange of peaks 1 and 2 and the disappearance of peak 3), but above is no signature indicative of decomposition reactions, and the spectrum
1775(150) K the spectra change more drastically. (b) Annealing of the after quenching matches closely to the spectrum before heating.
run product(s) at moderate temperatures shows nearly full recovery of
the original spectrum after annealing but with two new humps at ~7185 Similar to Figures 1 and 2, the high temperature tends to flatten
and ~7210 eV. the spectra, especially in the EXAFS region, where the charac-
teristic features become smoother and less pronounced. Note
that the room temperature spectrum after quenching matches
the spectrum before heating extremely well.

Figure 3. Expanded view of the pre-edge region of ambient XANES spectra calculated below and above spin crossover
temperature spectra at 51(1) GPa (a) normalized and (b) normalized with
background subtracted. The black solid line shows the spectrum before FeCO3 exhibits space-group symmetry R3c (calcite-group
heating in a and the red solid line shows the spectrum after quenching rhombohedral carbonates), where in the hexagonal setting, iron
from 2025(100) K in a. The black curve shows a pre-edge at ~7112 eV is located at the cell origin (6b), oxygen is at x, 0, 1⁄4 (18e), and
and a sharp feature at ~7116.5 eV, which matches well with the pre-edge carbon is at 0, 0, 1⁄4 (6a) (Bragg 1913). The atomic arrangement
region in low-spin FeCO3 (see Cerantola et al. 2015). The red curve has can be envisioned as a distorted rocksalt structure with Fe2+ as
one broad hump at ~7113.5 eV that dominates the spectrum, indicating the cation and CO32– groups as the anions. The CO32– groups form
the presence of one or more Fe-containing phase(s). Note that both pre- planes perpendicular to the c axis with Fe occupying the intersti-
edges in Figure 3b were normalized following the same background tial octahedral voids between the planes. No bond or polyhedral
subtraction procedure. (Color online.) edge is parallel to the c axis.

ferent at about 7200 eV, displaying two new humps at ~7185 We calculated XANES spectra of FeCO3 by ab initio simula-
and ~7210 eV that are both absent in the FeCO3 spectrum before tion using the structural lattice parameters measured by SXRD
heating (Fig. 2a, bottom). at 4, 37, and 55 GPa at ambient temperature (Fig. 5, Table 1).
Qualitatively the features of the experimental spectra are re-
The evolution of the XANES spectrum of low-spin FeCO3 produced well by the calculations: (1) both experimental and
at 54(1) GPa and moderate temperature is shown in Figure 4. calculated spectra are characterized by two main peaks (identi-
Peak 2, which is the most intense feature in the spectrum at fied as “1” and “2”) that shift to higher energies with increasing
ambient temperature, becomes progressively less intense upon pressure, (2) their relative intensities also change similarly with
heating, while peak 1 becomes the dominant peak at 1735(100) increasing pressure, such that at 55 GPa, peak 2 is more intense
and 1840(100) K. Moreover, high temperature causes the loss of than peak 1 (Fig. 5), and (3) the pre-edge feature observed at
peak 3 and the disappearance of the pre-edge peak at 7116.5 eV. 7117 eV in the simulated spectra at 37 and 55 GPa coincides with
the pre-edge feature at 7116.5 eV of the experimental spectra at
similar pressures. There are some differences, however: (1) the
pre-edge peak at 7112 eV in the experimental spectra (black) is
not well represented in the calculated spectra (red), where instead
two strong pre-edges are observed at 7115 and 7117 eV (Figs.
5a, 5b, and 5c), and (2) spectra calculated at 37 and 55 GPa are
characterized by a dip at 7125 eV visible on the left side of peak 1

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CERANTOLA ET AL.: INVESTIGATION OF FECO3 STABILITY IN EARTH’S LOWER MANTLE 1087

that becomes sharper with pressure, but this feature is nonexistent
in the experimental spectra, which instead display a hump at
7155 eV (peak 3) that is completely absent in the simulations.

Figure 5. Experimental (black) and simulated (red) XANES spectra Discussion
of FeCO3 at different pressures. The insets magnify the pre-edge region.
(a) Spectra show features characteristic of FeCO3 in the high-spin state, FeCO3 melting
where peak 1 is more intense than peak 2 and peak 3 is absent. Note that
the pre-edge in the calculated spectrum is at 7117 eV, 5 eV higher than in In a recent study, Cerantola et al. (2017) investigated the
the experimental spectrum, with a lower intensity feature at 7113.5 eV. (b) stability of FeCO3 up to 110 GPa and T > 2500 K using SXRD
Spectra are characteristic for high-spin FeCO3 just before spin crossover, and energy-domain synchrotron Mössbauer source (SMS) spec-
where peak 2 is slightly more intense than peak 1. In the experimental troscopy. At the conditions relevant for this study, P > 36 GPa
spectrum a new feature appears at ~7116.5 eV, whereas the simulated and T > 1850 K, FeCO3 melts, partially dissociating to HP-Fe3O4,
spectrum has a pre-edge at 7117 eV and the same broad low-intensity diamond, and oxygen (i.e., Litasov et al. 2011). The presence of
feature at ~7113.5 eV. (c) After iron spin crossover, peak 2 is more intense other Fe-oxide phases cannot be excluded (Cerantola et al. 2017).
than peak 1 and peak 3 appears in the experimental spectrum at ~7155 eV. Electron microprobe analyses (EMPA) (Tao et al. 2013; Kang et
Analogous to spectra at 37 GPa, the pre-edge peak in the calculated al. 2015), SXRD, and Mössbauer spectroscopy (Cerantola et al.
spectrum is at higher energy than in the experimental spectrum. The broad 2017) confirmed that even after prolonged heating the decom-
feature at ~7113.5 eV becomes slightly more pronounced. Note the absence position is never complete, and after quenching the presence of
of peak 3 in the calculated spectrum. The onset energy of each calculated recrystallized FeCO3 and HP-Fe3O4 is always observed. Note
spectrum was adjusted according to the variation in the Fermi energy and that, incomplete decomposition could be caused by pressure
in the s core level energy shift. (Color online.) and temperature gradients in the DAC, however we believe this
is not the case since similar behavior was observed in several
studies from different authors and using various experimental
techniques, such as DACs and multi-anvil apparatus (Tao et al.
2013; Kang et al. 2015; Cerantola et al. 2017).

The experimental results of the present study are consistent
with Cerantola et al. (2017) (Fig. 6). At 36(1) GPa, temperatures
higher than 1890(100) K (the last temperature where FeCO3
XANES features were clearly observed, Fig. 1) cause a drastic
change to the spectrum, which is interpreted to indicate com-
plete or partial melting of FeCO3. Previous work has shown that
melting is not stoichiometric due to partial redox dissociation of
liquid FeCO3, leading to dissolved Fe3+ and CO2 in the carbonate
melt below ~33 GPa, whereas above ~33 GPa the carbonate melt
recrystallizes after quenching as FeCO3, HP-Fe3O4, C (diamond),
and O2 (Litasov et al. 2011; Cerantola et al. 2017). A similar case
is observed at 51(1) GPa and temperatures up to ~2000 K (Fig. 2).
The intensity exchange between peaks 1 and 2 at 1600(100) and
1775(150) K is evident (Fig. 2), which is caused by the low- to
the high-spin transition of Fe2+ (Liu et al. 2014, 2015; Cerantola
et al. 2015). The characteristic pre-edge peak position at 7112
eV and the position of peaks 1 and 2 clearly indicate that at these
conditions, FeCO3 is stable and no decomposition has taken
place. In contrast, the spectra at higher temperatures [collected
above 1890(100) K] show a collapse of the FeCO3 characteristic
features, the most evident change is in the pre-edge that shifts
from 7112 to 7113.5 eV (Fig. 3). Again, the observed changes are
inferred to be caused by non-stoichiometric recrystallization due
to a self-oxidation reaction during melting with the consequent
formation of HP-Fe3O4 (Kang et al. 2015; Cerantola et al. 2017).

Interestingly, annealing the sample at moderate temperatures
between 1610(100) and 1675(100) K caused the back reaction
to occur, where partial recrystallization of LS-FeCO3 from a
mixture of HP-Fe3O4, C (diamond) + O2 is observed (Fig. 2b).
We suspect the presence of HP-Fe3O4 and possibly other Fe-oxide
phases due to two humps at ~7185 and ~7210 eV in the spectra
of recrystallized samples that are absent in the initial LS-FeCO3
spectrum before heating (Fig. 2a). At 51(1) GPa, the changes
in the spectra caused by heating below 1830(100) K and above

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1088 CERANTOLA ET AL.: INVESTIGATION OF FECO3 STABILITY IN EARTH’S LOWER MANTLE

Figure 6. Updated stability diagram of FeCO3 at high pressure and high temperature (modified from Cerantola et al. 2017). Symbols from this
study are: dark green diamonds with yellow dot = FeCO3; light blue squares with red dot = oxide(s) and recrystallized FeCO3. Symbols from Cerantola
et al. (2017) are: light green diamonds = FeCO3; blue squares = oxide(s) and recrystallized FeCO3; red triangles = tetrairon (III) orthocarbonate
Fe4C3O12; orange inverse triangles = diiron (II) diiron (III) tetracarbonate Fe4C4O13 + Fe4C3O12 + oxide(s); yellow hexagons = Fe4C4O13 + oxides. The
shading is as follows: gray region = FeCO3 decomposition to Fe3O4 + C + CO2 (Tao et al. 2013; Kang et al. 2015); orange region = high-spin FeCO3,
red region = low-spin FeCO3 (Liu et al. 2015); blue region = melting of FeCO3; green region = formation of high-pressure carbonates Fe4C3O12
and Fe4C4O13 (Cerantola et al. 2017). Black dashed curve = expected mantle geotherm (Katsura et al. 2010); thick red dashed line = new boundary
proposed in this study between crystalline FeCO3 and incongruent melt; brown and violet dashed lines = spin transition region in magnesio-siderite
from Liu et al. (2015) and consistent with Müller et al. (2017). A vertical dotted blue line separates the regions in which the formation of a-Fe2O3
and HP-Fe3O4 was observed upon melting of FeCO3. (Color online.)

1890(100) K allow us to locate the thermodynamic phase bound- the spectral shape. We estimated a run product containing 50
ary between crystalline FeCO3 and melt at about 1850 K. Results wt% HP-Fe3O4 and 50 wt% LS-FeCO3, but the actual amount of
of the experiment performed at 54(1) GPa and high temperature each phase is not known. The difference in the pre-edge region
are consistent with this conclusion (Fig. 4). can be explained by the inability of the code to simulate this
feature because the pre-edge represents quadrupolar excitations
To verify our experimental observations, we simulated the to localized states.
XANES spectrum of HP-Fe3O4 at 51 GPa using structural data
from Table 1. Figure 7a shows the calculated spectra of HP- The updated FeCO3 stability diagram based on XANES
Fe3O4 at 51 GPa (blue) and LS-FeCO3 at 55 GPa (black). To data (Fig. 6) is largely consistent with the FeCO3 stability fields
our knowledge, there are no experimental XANES spectra of reported by Cerantola et al. (2017). One slight difference is the
pure HP-Fe3O4 in the literature. The calculated FeCO3 spectrum position of the melting curve, which is around 200 K lower at
matches well with the experimental one at energies above the its highest point in the updated version compared to the original
absorption edge (see also Fig. 5). Figure 7b shows the experi- diagram. While this difference is largely within the uncertainties
mental XANES spectrum of FeCO3 obtained after laser heating of the temperature measurements, we note that XANES spec-
at 51(1) GPa and 2025(100) K and the sum of the calculated troscopy in dispersive mode is capable of detecting reactions on
spectra for LS-FeCO3 at 55 GPa and HP-Fe3O4 at 51 GPa. The a millisecond timescale and hence the in situ measurements in
similarities are evident: (1) peak 1 is more intense than peak 2, the present study may provide a more accurate determination of
and (2) both peaks in calculated and experimental spectra are melting compared to studies on quenched samples.
located at the same energy position. The calculated spectrum
also shows a third peak (peak 3), which is clearly not present in Spin crossover features from simulated spectra
the experimental spectrum. However, it is difficult to estimate
the relative abundance for each component, which can affect Cerantola et al. (2015) first reported spin crossover in FeCO3
observed by XANES. Here, we compare experimental data with

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CERANTOLA ET AL.: INVESTIGATION OF FECO3 STABILITY IN EARTH’S LOWER MANTLE 1089

Figure 7. (a) Comparison between calculated XANES spectra of LS-FeCO3 (black) and HP-Fe3O4 (blue). (b) Comparison between the sum
of HP-Fe3O4 and LS-FeCO3 calculated spectra (see Fig. 7a) (black) and the experimental spectrum after heating FeCO3 at 2025(100) K (blue).
The presence of recrystallized HP-Fe3O4 after FeCO3 (partial) melting at 51(1) was already observed using SXRD by Cerantola et al. (2017). The
presence of other Fe-oxide phases cannot be excluded. (Color online.)

ab initio simulations of XANES spectra using the onset of the 37 GPa due to the shift of the main edge. At higher energy, the
edge and the first EXAFS maximum. The similarity between presence of the hump at ~7155 eV (feature 3) in the experimental
simulated and experimental spectra is quite remarkable (Fig. 5), spectra above 37 GPa is not observed in the simulations, which
which can be attributed to the fact that the FDMNES code over- suggests that this feature stems from multiple scattering and
comes limitations from the Muffin Tin Approximation (Rehr and cannot be adequately reproduced by the calculation performed
Albers 2000). In particular, the double peak feature at the main here (Fig. 5). Overall, the theoretical spectra and analysis of the
edge (peaks 1 and 2) has been reproduced, which is the one l-projected density of states show that the changes in the main-
most indicative for the change in Fe spin state. The changes in edge XANES region of the spectra are mainly related to the shift
the spectra, the broadening of the peaks, and their shift to higher of p-states to higher energies, which is induced by the reduction
energies with pressure are related to changes in the electronic of the Fe-O distance.
structure due to shortening of Fe-O distances. The shortening is
also directly seen from the shift of the first EXAFS maximum Methods for ab initio simulation of XANES spectra still
between 7170 and 7190 eV to higher energy (~20 eV), which have shortcomings when compared to experiments (Rehr and
may be described by the relation DE × R2 = const. (e.g., Bianconi Albers 2000; Joly 2001; Bunau and Joly 2009). Interactions of
et al. 1983; Wilke et al. 2007), where DE is the energy difference the photoelectron with electronic states of the host close to the
between the onset of the edge and the first EXAFS maximum edge are quite complex and thus the fine structure cannot always
and R is the Fe-O distance. be fully replicated. In particular, excitations to localized states
are known to be less well-described by real-space calculations,
The pre-edge region at 7112 eV in the experimental spectra particularly if they are quadrupolar in nature. Furthermore, the
and at 7115 eV in the simulated spectra is related to localized core-hole screening is only approximated. This may substantially
1s → 3d transitions, which are quadrupolar in nature and only shift or suppress features in the calculated spectrum. Finally,
become dipole-allowed through hybridization of p and d orbitals these calculations did not include effects caused by thermal
for non-centro-symmetric sites. In theory, the pre-edge represents disorder to avoid broadening of observed features that were the
a region of the spectrum that is sensitive to changes in Fe spin as main focus of our study.
shown by Westre et al. (1997). Simulated spectra do show slight
differences in this energy region between high-spin and low-spin Implications
state; however, a comparison with experimental spectra is dif-
ficult due to low resolution and low statistical quality of the data Carbonate-bearing subducting slabs have different thermal
in this region. The feature at 7116.5 eV, which emerges in the profiles, which mainly vary based on slab age and velocity of sub-
experimental spectra at and above spin crossover, is likely related duction. Colder slabs subduct faster mainly due to their enhanced
to excitations of 1s to 4p electron states of the valence band (e.g., density (e.g., Syracuse et al. 2010). Recently, it has been proposed
Caliebe et al. 1998). This feature becomes better resolved at that the majority of slab geotherms intersect a deep depression
higher pressures because the main edge shifts to higher energy along the melting curve of carbonated oceanic crust at depths of
due to decreasing Fe-O distance. In the simulations, this feature approximately 300 to 700 km during subduction (Thomson et al.
is also at ~7117 eV and becomes better resolved due to the shift 2016). At those depths, FeCO3 melts, forming a-Fe2O3 below ~25
of the main edge to higher energy, similar to experiment. Like- GPa (Kang et al. 2015; Cerantola et al. 2017) and HP-Fe3O4 at
wise, the depression at around 7125 eV present in the simulated higher pressure (depths below 750 km) (Cerantola et al. 2017).
spectrum at 37 and 55 GPa is also related to the shortening of These redox dissociations are accompanied by the formation of
the Fe-O distance upon compression and spin crossover, so that other compounds such as diamond, oxygen, and possibly Fe-
the feature is more pronounced and sharper at 55 GPa than at oxides with different stoichiometry. FeCO3 incongruent melting
has important implications for the influx of carbon via Fe-car-

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1090 CERANTOLA ET AL.: INVESTIGATION OF FECO3 STABILITY IN EARTH’S LOWER MANTLE

bonates inside Earth, because it provides a mechanism for FeCO3 Cerantola, V., Bykova, E., Kupenko, I., Merlini, M., Ismailova, L., McCammon,
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Earth’s mantle. For instance, FeCO3-rich carbonated liquids that Chang, S-J., Ferreira, A.M.G., and Faccenda, M. (2016) Upper- and mid-mantle
escape from subducting plates can recrystallize to solid FeCO3 in interaction between the Samoan plume and the Tonga–Kermadec slabs. Nature
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that subducting slabs with average surface temperature (~1250 K
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at 15 GPa; Syracuse et al. 2010) will undergo carbonate-melting R., Osorio-Guillen, J.M., Dmitriev, V., Weber, H-P., Bihan, T.L., and Johansson,
B. (2003) The structure of the metallic high-pressure Fe3O4 polymorph: experi-
processes (Thomson et al. 2016) that result in partial FeCO3 mental and theoretical study. Journal of Physics: Condensed Matter, 15, 45.
decomposition to oxides and diamond. In contrast, subduct-
ing slabs with lower surface temperature (~1050 K at 15 GPa; Isshiki, M., Irifune, T., Hirose, K., Ono, S., Ohishi, Y., Watanuki, T., Nishibori, E.,
Takata, M., and Sakata, M. (2004) Stability of magnesite and its high-pressure
Syracuse et al. 2010 or 1500 K at 70 GPa; e.g., Kaneshima and form in the lowermost mantle. Nature, 427, 60–63.

Helffrich 2003; Komabayashi et al. 2009) could survive subduc- Joly, Y. (2001) X‑ray absorption near-edge structure calculations beyond the
muffin-tin approximation. Physical Review B, 63, 125120.
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Kaminsky, F. (2012) Mineralogy of the lower mantle: A review of ‘super-deep’min-
pressures and temperatures were high enough to cause trans- eral inclusions in diamond. Earth and Planetary Science Letters, 110, 127–147.
formation of CO32–-carbonates to their high-pressure structures
characterized by CO44– tetrahedra above 70 GPa (Merlini et al. Kaminsky, F.V., Ryabchikov, I.D., and Wirdth, R. (2016) A primary natrocarbon-
2015, 2017; Cerantola et al. 2017). atitic association in the Deep Earth. Mineralogy and Petrology, 110, 387–398.
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Acknowledgments and Funding
Kaneshima, S., and Helffrich,.G (2003) Subparallel dipping heterogeneities in
We acknowledge the ESRF for provision of beam time at ID24 and the the mid-lower mantle. Journal of Geophysical Research, 108, B5. DOI:
Sample Environment Service-HP lab for the technical support of the loan pool of 10.1029/2001JB001596
diamond-anvil cells. L.D., C.M., and M.W. thank the German Research Foundation
(Deutsche Forschungsgemeinschaft, DFG, Research UNIT FOR2125 CarboPat) Kang, N., Schmidt, M.W., Poli, S., Franzolin, E., and Connolly, J.A.D. (2015)
and the Federal Ministry of Education and research (BMBF, Germany) for fund- Melting of siderite to 20 GPa and thermodynamic properties of FeCO3-melt.
ing. The authors acknowledge partial support from the Sloan Foundation grant Chemical Geology, 400, 34–43.
G-2016-7157. We acknowledge J. Li for editorial handling as well as anonymous
reviewers for their constructive comments. Kantor, I.Yu., Prakapenka, V., Kantor, A., Dera, P., Kurnosov, A., Sinogeikin, S.,
Dubrovinskaia, N., and Dubrovinsky, L. (2012) BX90: A new diamond anvil
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Manuscript received December 9, 2017
Manuscript accepted April 11, 2019
Manuscript handled by Jie Li

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American Mineralogist, Volume 104, pages 1365–1368, 2019

Carbonation and the Urey reactionk 

Louise H. Kellogg1, Harsha Lokavarapu2, and Donald L. Turcotte2,*,†

1Department of Earth and Planetary Science, University of California, Davis, 1 Shields Avenue, Davis, California 95616, U.S.A.
Orcid 0000-0001-5874-0472

2Department of Earth and Planetary Science, University of California, Davis, 1 Shields Avenue, Davis, California 95616, U.S.A.

Abstract
There are three major reservoirs for carbon in the Earth at the present time, the core, the mantle,
and the continental crust. The carbon in the continental crust is mainly in carbonates (limestones,
marbles, etc.). In this paper we consider the origin of the carbonates. In 1952, Harold Urey proposed
that calcium silicates produced by erosion reacted with atmospheric CO2 to produce carbonates, this
is now known as the Urey reaction. In this paper we first address how the Urey reaction could have
scavenged a significant mass of crustal carbon from the early atmosphere. At the present time the Urey
reaction controls the CO2 concentration in the atmosphere. The CO2 enters the atmosphere by volcanism
and is lost to the continental crust through the Urey reaction. We address this process in some detail.
We then consider the decay of the Paleocene-Eocene thermal maximum (PETM). We quantify how
the Urey reaction removes an injection of CO2 into the atmosphere. A typical decay time is 100 000 yr
but depends on the variable rate of the Urey reaction.
Keywords: Urey reaction, deep carbon, chemical geodynamics, carbonation; Earth in Five Reac-
tions: A Deep Carbon Perspective

Introduction time are the core, mantle, and continental crust. We assume that

The continental crust is a major reservoir for carbon in the the core is an isolated reservoir and neglect its role. About 1%
Earth. A major question in geology is the origin of the carbon,
principally in calcium carbonates. The first successful attempt to of the carbon in the Earth is in the continental crust. Wedepohl
explain the origin of calcium carbonates (limestones, marbles)
in the continental crust was given by Urey (1952). The basic (1995) has given a comprehensive study of the composition of the
equation he gave was of the form
continental crust with an emphasis on carbon. He gives an estimate

for the total mass of carbon (c) in the continental crust (cc) at the

present time (p) of Mc = 4.2 × 107 Gt. Hayes and Waldbauer
ccp

(2006) have reviewed the literature on carbon in the continental

CaSiO3 + CO2 ↔ CaCO3 + SiO2. (1) crust and suggest that it may be as high as Mc = 108 Gt. DePaolo
ccp

He proposed that atmospheric CO2 combines with a calcium (2015) gives a range of 6 to 7 × 107 Gt. In this paper, we take a
silicate to generate a calcium carbonate plus silica. A direct
quote from his paper states: “As carbon dioxide was formed it representative value to be Mc = 5 × 107 Gt. The mass of carbon
reacted with silicates to form limestone. Of course, the silicates ccp
may have been a variety of minerals, but the presence of CO2
was always kept at a low level by this reaction or similar reac- in the ocean is about a factor of 103 less than the mass of carbon
tions just as it is now.”
in the continental crust (Houghton 2007).
In the current literature an expanded version of the Urey reac-
tion is given (Blättler and Higgins 2017). To include the role of Urey (1952, 1956) clearly recognized that the reaction he
acid rain the Urey reaction takes the form
proposed would efficiently remove CO2 from the Earth’s atmo-
sphere, but at that time little was known about the early atmo-

sphere. Although the mass of carbon in the atmosphere today

is small (850 Gt), the mass may have been much higher in the

past. One of the major differences between Venus and the Earth

is atmospheric composition. The atmospheric pressure on Venus

CaSiO3 + 2CO2 + H2O → Ca2+ + 2HCO3– + SiO2 is about a factor of 100 greater than the atmospheric pressure
→ CaCO3 + SiO2 + CO2 + H2O.
(2) on Earth and is 96% carbon dioxide. The mass of carbon in the

The carbonation takes place when carbon dioxide (carbonic acid) Venus atmosphere (a) at the present time (p) is cMap = 1.28 × 108
in acid rain dissolves calcium silicate (wollastonite) sediments to Gt. Scaling the atmospheric carbon masses to the overall masses
give calcium, bicarbonate, and silica. The resulting calcium and
bicarbonate ions flow in rivers to the oceans where either organic of Venus and the Earth gives an estimate of the mass of carbon
or inorganic precipitation produces the calcium carbonate.
(c) in the early atmosphere (t = 0) of the Earth. The estimated
The three large reservoirs for carbon in the Earth at the present
value is cMa0 = 1.57 × 108 Gt (Kasting and Ackerman 1986).

* E-mail: [email protected] Carbon from the atmosphere to the

† Special collection papers can be found online at http://www.minsocam.org/MSA/ continental crust

AmMin/special-collections.html. One hypothesis for the origin of the carbon in the continental
kOpen access: Article available to all readers online. crust is that it was extracted directly from the atmosphere rela-
tively early in Earth’s history. The estimated mass of carbon (c)
in the early atmosphere (a) given above, cMa0 = 1.57 × 108 Gt,

0003-004X/19/0010–1365$05.00/DOI: https://doi.org/10.2138/am-2019-6880 1365

1366 KELLOGG ET AL.: CARBONATION AND THE UREY REACTION

is substantially larger than the total estimated carbon in the

continental crust given above, Mc = 5 × 107 Gt. The hypothesis
ccp

of direct extraction from the atmosphere has been discussed in

some detail by Kramers (2002) and by Lowe and Tice (2004).

The basic hypothesis is that the mass flux of carbon from the

atmosphere to the continental crust, Jc is controlled by the
a-cc,

availability of calcium silicates. In order for the Urey reaction

to extract CO2 from the atmosphere the early Earth must have
had continental crust to generate surface deposits of calcium

silicate. In addition, the Earth must have had oceans in order for

the acid rain to catalyze the Urey reaction between atmospheric

CO2 and the service deposits of calcium silicates. Little data

are available for timing the initiation of the extraction of CO2

from the atmosphere. We will assume that the process begins at

a time t0 after the early bombardment and the solidification of Figure 1. Dependence of the mass of carbon in the continental crust
the magma ocean at about 4.4 Ga. We further assume that the
cMcc on time. Two limiting models are given for adding the present mass
Urey reaction extracted carbon from the atmosphere at a constant
Mc = 5 × 107 Gt. (1) Addition from the atmosphere beginning at t0 = 1
rate Jc until the concentration of CO2 in the atmosphere was ccp
a-cc
Gyr. All atmospheric carbon is transferred in ta-cc = 1 Gyr at a constant
reduced to a very low level. During the time, t0 < t < t0 + ta-cc,
flux Jc = 50 Mtyr-1. (2). Addition from the mantle beginning at t0 = 1
the Urey reaction extracts atmospheric carbon to the continental a-cc

Gyr. Carbon is added at a constant flux Jc = 14.7 Mtyr-1 to the present.
m-cc

crust. We will specify the mass of carbon extracted from the

atmosphere and obtain

Jc  c Ma0 . (3) is proportional to the mass of carbon in the atmosphere cMa. The
a  cc acc characteristic time tu takes account of the rate at which acid rain

can interact with calcium silicate sediments, and although we

We assume that the mass of carbon in the atmosphere cMa de- assume that tu is constant it clearly can be a function of time.
creases linearly in time from cMa0 to zero during the time period A comprehensive model for the variability of atmospheric
ta-cc and the mass of carbon in the continental crust increases
CO2 over Phanerozoic times has been given by Berner and
linearly in time. Kothavala (2001). This model, GEOCARB III, is complex

Assuming all the carbon in the continental crust Mc was and involves both organic and inorganic processes. Transport
ccp

extracted from the atmosphere the dependence on time is of carbon between the atmosphere, oceans, and continental

given by crust is quantified on the million year timescale. The balance

is dominated by the exchange of carbon between carbonates

cMcc = 0 0 ≤ t ≤ t0 in the continental crust and carbon in the surficial reservoirs

cMcc = Mc [(t – t0)/(ta-cc)] t0 ≤ t ≤ t0 + ta-cc (4) (oceans and atmosphere) and organic carbon (Berner and Cal-
ccp

cMcc = Mc t0 + ta-cc ≤ t ≤ tp deira 1997). When erosion is high, the Urey reaction extracts
ccp

Taking Mc = 5 × 107 Gt, t0 = 1 Gyr, and ta-cc = 1 Gyr, the de- CO2 from the atmosphere adding carbonates to the continental
ccp crust. High erosion rates are associated with low sea level and

pendence of cMcc on t is given in Figure 1. The required flux of large continental areas. When erosion is low, the Urey reaction

carbon from the atmosphere to the continental crust is Jc = 50 operates in the opposite direction (from right to left in Eq. 1)
a-cc

Mtyr-1. It must be emphasized that the value of ta-cc is uncertain with carbonates decomposing to give CO2. An example of this
metamorphic process is the subduction of carbonate sediments
and the flux Jc is expected to have considerable variability in
a-cc

time. However it is quite clear that the extraction of carbon from and the generation and return to the atmosphere of CO2 in

the atmosphere to the continental crust would have been carried subduction zone volcanics (Frezzotti et al. 2011).

out early in Earth’s history. The present mass of carbon in the atmosphere is 860 Gt

Removal of the volcanic addition of carbon (400 ppmv CO2), but this is not a quasi-equilibrium value

to the atmosphere because of the anthropogenic addition at high fluxes (3.5

When excess carbon in the atmosphere has been depleted by Gtyr-1). We will take the 1900 value of 650 Gt (300 ppmv
the Urey reaction an approximate steady-state balance is estab-
lished between the volcanic input of carbon into the atmosphere CO2) as the present equilibrium value. This is a typical value
and the extraction by the Urey reaction. We approximate this for the current glacial epoch (0 to 50 Ma). Values given by the
balance by the relationship
GEOCARB III Model are generally consistent with observa-

tions (Royer 2014). Between 50 and 250 Ma, the average values

were about 3000 Gt. During the major glacial epoch between

cMa 250 and 350 Ma low observed values near 650 Gt are found.
u
Jc  . (5) Between 350 and 550 Ma, values were considerably higher,
a  cc
typically near 10 000 Gt. This variability reflects variations in

where Jc is the rate of volcanic input of carbon into the both of the variables in Equation 3, the volcanic flux Jc into
a-cc a-cc

atmosphere. We assume that this extraction rate is constant and the atmosphere and the characteristic time tu.

American Mineralogist, vol. 104, 2019

KELLOGG ET AL.: CARBONATION AND THE UREY REACTION 1367

Carbon from the mantle to the continental crust extraction of a significant mass of carbon from the atmosphere
to the continental crust was completed by t = 2 Gyr. However,
The second hypothesis for the origin of the carbon in the how large this mass was is uncertain.

continental crust is that it comes from the mantle. If the volcanic

flux of carbon out of the mantle at ocean ridges and hot spots Paleocene-Eocene thermal maximum

exceeds the carbon lost to the mantle at subduction zones, the The decay of the Paleocene-Eocene thermal maximum
(PETM) can be used to quantitatively constrain the role of the
difference will be added to the continental crust. Some of the Urey reaction. The PETM was a period of elevated global tem-
peratures (4 to 5 °C) and high atmospheric CO2 beginning at 56.3
volcanic carbon input will enter the atmosphere and will be Ma, the onset lasted less than 10 Kyr and the subsequent decay
lasted about 100 Kyr (Mclnerney and Wing 2011). Storey et al.
transferred to the continental crust through the Urey reaction. (2007) have made a strong case for associating the PETM with
flood volcanism resulting from the opening of the north Atlantic.
However, some will enter the oceans and will be converted
Isotope studies have quantitatively documented the PETM.
directly to carbonates without entering the atmosphere. These studies have been reviewed by Gutjahr et al. (2017). These
authors also provided estimates for the carbon content of the
Rates of carbon loss from the mantle by volcanism and lost atmosphere during the PETM. They suggest that the background
carbon mass in the atmosphere before and after the PETM was
by subduction will certainly vary over geologic time, but the cMab = 1400 Gt and the peak mass of carbon was cMa0 = 3050 Gt.

variations are uncertain. Again, we assume that the plate tectonic We now carry out an analysis of the decay of the PETM due
to the loss of CO2 from the atmosphere by the Urey reaction. We
processes required for carbon transfer began at a time t0 after the extend the balance given in Equation 5 to include the transient
solidification of the magma ocean at about 4.5 Ga. We further removal of carbon from the atmosphere and write

assume that the transfer of carbon out of the mantle has been

at a constant rate Jc until the present time tp. Assuming all
m-cc

the carbon in the continental crust has been extracted from the

mantle, the dependence on time is given by

cMcc = 0 0 ≤ t ≤ t0

cMcc = Mc [(t – t0)/(tp – t0)] t0 ≤ t ≤ tp + ta-cc. (6)
ccp

The mass of carbon in the continental crust increases linearly in d cM a Jc accb  cMa . (8)
time over the period t0 to tp. The required flux of carbon from dt u
the mantle to the continental crust is given y

Mc From Equation 5 the background mass of carbon in the atmo-
ccp
Jc  . (7) sphere is given by
mcc tp  t0

Taking Mc = 5 × 107 Gt, t0 = 1 Gyr, and tp = 4.4 Gyr the depen- cM ab  u c Jaccb . (9)
ccp
We prescribe an initial mass of carbon in the atmosphere at t = 0,
dence of cMcc on t is given in Figure 1. The required flux of carbon cMa0 and solve Equation 7 taking tu to be constant with the result

from the mantle to the continental crust is Jc = 14.7 Mtyr-1.
m-cc

We next consider the estimate for the present loss of carbon

from the mantle to the atmosphere. Dasgupta and Hirschmann cM a ( cM a0  cM ab )et/u  cM ab . (10)

(2010) have summarized the available data on the loss of carbon

from the mantle to the surface reservoirs and give values in the The excess mass of carbon in the atmosphere cMa0 – cMab decays
exponentially as the Urey reaction extracts carbon from the
range Jc = 36 ± 24 Mtyr-1. Just as carbon is lost from the mantle
m-s

by volcanism, carbon is returned to the mantle by subduction. atmosphere.

A detailed study of carbon fluxes at subduction zones has been We next obtain the dependence of atmosphere carbon mass

given by Kelemen and Manning (2015). These authors suggest on time during PETM based on the model dependence given in

that the downward flux of carbon at global subduction zones is Equation 10. Taking the values cMab = 1400 Gt and cMa0 = 3050
Gt with tu = 100 kyr the model results are given in Figure 2.
53 ± 13 Mtyr-1. However a substantial fraction of this carbon
We now return to Equation 9. This result relates the background
never makes it to the mantle due to subduction zone volcanism.

They suggested that 24 ± 24 Mtyr-1 reach the mantle. Clearly it atmospheric carbon mass cMab to the background rate of volcanic

is quite possible that all the carbon in the continental crust could input of CO2 carbon into the atmosphere Jc and the Urey reac-
(a-cc)b

have come from the mantle. This conclusion was also given by tion rate tu. During the PETM we have taken the background carbon

Hayes and Waldbauer (2006). mass cMab = 1400 Gt. Taking tu = 100 kyr we find from Equation 9

In Figure 1 we give examples of the two limiting cases, the that Jc = 14 Mtyr-1. This is an independent determination of the
(a-cc)b

carbon in continental crust comes entirely from the atmosphere volcanic flux of carbon into the atmosphere at that time. As dis-

and the carbon comes entirely from the mantle. In the first case cussed above, we take the present equilibrium mass of carbon in the

the addition is early in time and in the second case it is more atmosphere to be cMab = 6500 Gt. Assuming that Jc = 14 Mtyr-1
(a-cc)b

uniform in time. Observations of the mass of carbonates in the we require from Equation 9 that tu = 50 kyr. This is our estimated

continental crust as a function of age could distinguish between relaxation time for a carbon excursion today.

the two cases, but the data are sparse. Observations of the mass Discussion

of carbon in the atmosphere as a function of time could also be a Urey (1952) proposed the Urey reaction, Equation 1, to
explain the origin of carbonates in the continental crust. He
constraint. An example given by Rye et al. (1995) utilizing studies argued that the reaction would essentially remove all CO2 from

of paleosols concluded that the mass of carbon in the atmosphere

at 2.2 to 2.75 Ga was less than 105 Gt. The conclusion is that the

American Mineralogist, vol. 104, 2019

1368 KELLOGG ET AL.: CARBONATION AND THE UREY REACTION

bon could have been extracted either from the early atmosphere
or from the mantle over a longer period of time. Studies of the
concentration of carbon in the atmosphere and continental crust
over geologic time are required and should receive a high priority.

The second question we have addressed is the relaxation of
injections of carbon into the atmosphere back to equilibrium
values. We quantify this by studying the Paleocene-Eocene
thermal maximum (PETM). This has obvious implications for
the recovery from the process of anthropogenic injection of
carbon into the atmosphere. We find the relaxation time to be
about 50 000 years.

Acknowledgment

It is with great sadness that we note the death of Louise Kellogg onApril 14, 2019.

Figure 2. Dependence of the atmosphere carbon mass values cMa References cited

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We have addressed two major questions concerning carbon in
the atmosphere in this paper. The first is the origin of the carbon Manuscript received November 6, 2018
in the continental crust. We conclude that it is possible the car- Manuscript accepted June 24, 2019
Manuscript handled by Jie Li

American Mineralogist, vol. 104, 2019


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