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The Sheahan Diamond Literature Reference Compilation - Scientific and Media Articles based on Major Keyword - Carbon
The Sheahan Diamond Literature Reference Compilation is compiled by Patricia Sheahan who publishes on a monthly basis a list of new scientific articles related to diamonds as well as media coverage and corporate announcements called the Sheahan Diamond Literature Service that is distributed as a free pdf to a list of followers. Pat has kindly agreed to allow her work to be made available as an online digital resource at Kaiser Research Online so that a broader community interested in diamonds and related geology can benefit. The references are for personal use information purposes only; when available a link is provided to an online location where the full article can be accessed or purchased directly. Reproduction of this compilation in part or in whole without permission from the Sheahan Diamond Literature Service is strictly prohibited. Return to Diamond Keyword Index
Sheahan Diamond Literature Reference Compilation - Scientific Articles by Author for all years
Each article reference in the SDLRC is tagged with one or more key words assigned by Pat Sheahan to highlight the main topics of the article. In an effort to make it easier for users to track down articles related to a specific topic, KRO has extracted these key words and developed a list of major key words presented in this Key Word Index to which individual key words used in the article reference have been assigned. In most of the individual Key Word Reports the references are in crhonological order, though in some such as Deposits the order is first by key word and then chronological. Only articles classified as "technical" (mainly scientific journal articles) and "media" (independent media articles) are included in the Key Word Index. References that were added in the most recent monthly update are highlighted in yellow.
Carbon is relevant to diamonds because that is what a pure diamond is made of, namely an octahedral carbon crystal that forms in a combination of pressure and temperature that does not exist at the earth's surface. The diamond form of carbon is the hardest known naturally occurring material, while the hexagonal crystal form of carbon known as graphite is among the softest materials. Articles with the key word "carbon" encompass a wide range of carbon related topics, of which the most relevant to diamond deal with diamond formation, the carbon cycle within the earth as opposed to the gaseous form of carbon dioxide in so far as the inputs for diamond formation are concerned, and the transition between diamond and graphite as defined by the pressure-temperature regime at the center of which is the diamond stability field.
Mathematics and the Buckyball.The elaborate symmetries of this soccer ball shaped molecule allow many of its properties to be calculated from firstprinciples
American Scientist, Vol. 81, January-February pp. 56-70
Graphite melt equilibration temperatures during mantle melting: constraints on CO2 in Mid Ocean Ridge Basalt (Mid Ocean Ridge Basalt (MORB))magmas and carbon content..
Chemical Geology, Vol. 147, No. 1-2, May 15, pp. 89-98.
Maeda, F., Ohtani, E., Kamada, S., Sakamaki, T., Ohishi, Y., Hirao, N.
The reactions in the MgCO3-SiO2 system in the slabs subducted into the lower mantle and formation of deep diamond.
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 1p. Abstract
Abstract: Aqueous subduction-zone fluids contain CO2 and methane. New calculations indicate that these fluids also host a wide array of organic carbon species, in concentrations sufficient to influence the deep carbon cycle.
Fe Ni Cu C S phase relations at high pressures and temperatures - the role of sulfur in carbon storage and diamond stability at mid to deep upper mantle.
Earth and Planetary Science Letters, Vol. 412, pp. 132-142.
Doklady Earth Sciences, Vol. 464, 2, pp. 1062-1065.
Russia
Carbon
Abstract: Results of the study of carbon material (CM) discovered in kimberlite-like rocks of the Charteskii Complex (Subpolar Urals) are considered. It is shown that CM is represented by partially oxidized graphite and optically transparent amorphous CM (presumably diamond-like carbon). The data obtained are important for estimation of the diamond potential of this object, as well as for understanding of the new mechanism of the formation of diamond-like carbon and diamond.
Abstract: The relative abundance of light elements in the Earth’s core has long been controversial. Recently, the presence of carbon in the core has been emphasized, because the density and sound velocities of the inner core may be consistent with solid Fe7C3. Here we report the longitudinal wave velocity of liquid Fe84C16 up to 70?GPa based on inelastic X-ray scattering measurements. We find the velocity to be substantially slower than that of solid iron and Fe3C and to be faster than that of liquid iron. The thermodynamic equation of state for liquid Fe84C16 is also obtained from the velocity data combined with previous density measurements at 1 bar. The longitudinal velocity of the outer core, about 4% faster than that of liquid iron, is consistent with the presence of 4-5 at.% carbon. However, that amount of carbon is too small to account for the outer core density deficit, suggesting that carbon cannot be a predominant light element in the core.
Abstract: Carbon from Earth’s interior is thought to be released to the atmosphere mostly via degassing of CO2 from active volcanoes1, 2, 3, 4. CO2 can also escape along faults away from active volcanic centres, but such tectonic degassing is poorly constrained1. Here we use measurements of diffuse soil CO2, combined with carbon isotopic analyses to quantify the flux of CO2 through fault systems away from active volcanoes in the East African Rift system. We find that about 4?Mt?yr?1 of mantle-derived CO2 is released in the Magadi-Natron Basin, at the border between Kenya and Tanzania. Seismicity at depths of 15-30?km implies that extensional faults in this region may penetrate the lower crust. We therefore suggest that CO2 is transferred from upper-mantle or lower-crustal magma bodies along these deep faults. Extrapolation of our measurements to the entire Eastern rift of the rift system implies a CO2 flux on the order of tens of megatonnes per year, comparable to emissions from the entire mid-ocean ridge system2, 3 of 53-97?Mt?yr?1. We conclude that widespread continental rifting and super-continent breakup could produce massive, long-term CO2 emissions and contribute to prolonged greenhouse conditions like those of the Cretaceous.
Diamond and Related Materials, Vol. 62, pp. 42-48.
Technology
Carbon
Abstract: It is known that carbon melts at temperatures around 4000 K or higher, and, therefore, this will be for the first time, when liquid carbon state formation preserved within diamond is documented in a carbon-carbonate system at the PT-conditions around 8.0 GPa and 2000 K, that is essentially far from the carbon diagram liquid field, so the newly reported liquid carbon was formed by neither fusion nor condensation. Based on a preponderance of such a strong circumstantial evidence, as morphological features of globular glass-like carbon inclusions within the globular-textured host diamond crystals resulting from liquid segregation process under synthesis conditions, it is suggested, that the produced carbon state has general properties of liquid and is formed through agglomeration alongside with diffusion process of carbon within carbonate melt solvent, and, thus, can potentially open a novel route for liquid carbon production and manufacturing of advanced high-refractory alloys and high-temperature compounds at lower than commonly accepted standard temperatures. A new model of diamond formation via metastable liquid carbon is presented.
Annual Review of Earth and Planetary Sciences, Vol. 43, pp. 273-298.
Mantle
Carbon
Abstract: Pyrogenic carbon (PyC; includes soot, char, black carbon, and biochar) is produced by the incomplete combustion of organic matter accompanying biomass burning and fossil fuel consumption. PyC is pervasive in the environment, distributed throughout the atmosphere as well as soils, sediments, and water in both the marine and terrestrial environment. The physicochemical characteristics of PyC are complex and highly variable, dependent on the organic precursor and the conditions of formation. A component of PyC is highly recalcitrant and persists in the environment for millennia. However, it is now clear that a significant proportion of PyC undergoes transformation, translocation, and remineralization by a range of biotic and abiotic processes on comparatively short timescales. Here we synthesize current knowledge of the production, stocks, and fluxes of PyC as well as the physical and chemical processes through which it interacts as a dynamic component of the global carbon cycle.
Abstract: Earth owes its oxygenated atmosphere to its unique claim on life, but how the atmosphere evolved from an initially oxygen-free state remains unresolved. The rise of atmospheric oxygen occurred in two stages: approximately 2.5 to 2.0 billion years ago during the Great Oxidation Event and roughly 2 billion years later during the Neoproterozoic Oxygenation Event. We propose that the formation of continents about 2.7 to 2.5 billion years ago, perhaps due to the initiation of plate tectonics, may have led to oxygenation by the following mechanisms. In the first stage, the change in composition of Earth's crust from iron- and magnesium-rich mafic rocks to feldspar- and quartz-rich felsic rocks could have caused a decrease in the oxidative efficiency of the Earth's surface, allowing atmospheric O2 to rise. Over the next billion years, as carbon steadily accumulated on the continents, metamorphic and magmatic reactions within this growing continental carbon reservoir facilitated a gradual increase in the total long-term input of CO2 to the ocean -atmosphere system. Given that O2 is produced during organic carbon burial, the increased CO2 input may have triggered a second rise in O2. A two-step rise in atmospheric O2 may therefore be a natural consequence of plate tectonics, continent formation and the growth of a crustal carbon reservoir.
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 10193 Abstract
Technology
Book - carbon
Abstract: As an academic historian of science, I am writing a history of the discovery of the interior workings of our dynamic planet. I am preparing a book, titled Carbon from Crust to Core: A Chronicle of Deep Carbon Science, in which I will present the first history of deep carbon science. I will identify and document key discoveries, the impact of new knowledge, and the roles of deep carbon scientists and their institutions from the 1400s to the present. This innovative book will set down the engaging human story of many remarkable scientists from whom we have learned about Earth's interior, and particularly the fascinating story of carbon in Earth. I will describe a great journey of discovery that has led to a better understanding of the physical, chemical, and biological behaviour of carbon in the vast majority of Earth's interior. My poster has a list of remarkable Deep Carbon Explorers, from Georgius Agricola (1494-1555) to Claude ZoBell (1904-1989). Come along to my poster and add to my compilation: choose pioneers from history, or nominate your colleagues, or even add a selfie! As a biographer, I am keen to add researchers who may have been overlooked in the standard histories of geology and geophysics. And I am always on the lookout for standout stories and personal recollections. I am equipped to do oral history interviews. What's your story? Cambridge University Press will publish the book in 2019.
Abstract: Diamonds are renowned as the record of Earth’s evolution history. Natural diamonds on the Earth can be distinguished in light of genetic types as kimberlitic diamonds (including peridotitic diamonds and eclogitic diamonds), ultrahigh-pressure metamorphic diamonds and ophiolitic diamonds. According to the inclusion mineralogy, most diamonds originated from continental lithospheric mantle at depths of 140-250 km. Several localities, however, yield ultradeep diamonds with inclusion compositions that require a sublithospheric origin (>~250 km). Ultradeep diamonds exhibit distinctions in terms of carbon isotope composition, N-concentration, mineral inclusions and so on. The present study provides a systematic compilation concerning the features of ultradeep diamonds, based on which to expound their genesis affinity with mantle-carbonate melts. The diamond-parental carbonate melts are proposed to be stemmed from the Earth’s crust through subduction of oceanic lithosphere. Ultradeep diamonds are classified into a subgroup attaching to kimberlitic diamonds grounded by formation mechanism, and present connections in respect of carbon origin to eclogitic diamonds, ultrahigh-pressure metamorphic diamonds and ophiolitic diamonds.
Abstract: Trapping inclusions in diamonds during growth experiments is used as a diagnostic to constrain natural diamond formation conditions in the Earth’s lithosphere. Isotopic signature of the new diamond grown areas close to those inclusions is also useful to identify the carbon source for the diamonds. In this study experiments were carried at conditions compatible with the Earth’s geotherm between 6-7 GPa (1300-1675°C) in multi-anvil presses from a few hours to a few days. Carbon-bearing starting materials are powders of carbonates and graphite. Results show that within the timescale of the experiments diamond growth occurs on preexisting seeds if water and alkali-bearing carbonates are present. The ?13C isotopic composition of the new diamond grown areas measured close to the inclusions show a different isotopic signature than that of the starting seeds (-29.6 to - 30.4±1.4‰). The new diamond carbon signatures are falling into the range of signatures of the starting carbonates used for the experiments (-4.8±0.1 to -16.2±0.1‰) but far away from the composition of the starting graphite (-26.4±0.1‰). This suggests that the carbon source for diamond growth at the conditions of the lithosphere must be the carbonates present either as CO3 2- ions dissolved in the melt or as carbon dioxide in the aqueous fluid. It is concluded that diamond growth occurred from carbonate reduction rather that from graphite dissolution in the melt.
Abstract: Knowledge about the viscosity and other transport properties of CaCO3 melts at high pressures and temperatures relevant to the Earth’s mantle is critically important for understanding the deep carbon cycle [1,2]. We have conducted First-Principles Molecular Dynamics Calculations of CaCO3 melts up to 52.5 GPa and 3000 K to provide atomistic insights into the mechanisms of diffusion and viscosity. Our calculated viscosities of CaCO3 melts at low pressures are in good agreement with those from experiments. In particular, viscosity is almost constant at low pressures but increases linearly with pressure above 10 GPa. The ultralow viscosity of CaCO3 melts at low pressures [1] is readily attributed to the uncorrelated diffusion of Ca2+ and CO3 2- ions (Fig. 1). In contrast, the motions of the Ca2+ cations and CO3 2- anions at pressures >10 GPa become increasingly correlated (Fig. 1), leading to higher viscosities. Compared to water, the viscosity of CaCO3 melts is not anomalously low. Rather, the viscosity of water is anomalously high, because water molecules are strongly H-bonded and behave like polymers.
Abstract: Estimates of carbon concentrations in Earth’s mantle vary over more than an order of magnitude, hindering our ability to understand mantle structure and mineralogy, partial melting, and the carbon cycle. CO2 concentrations in mantle-derived magmas supplying hotspot ocean island volcanoes yield our most direct constraints on mantle carbon, but are extensively modified by degassing during ascent. Here we show that undegassed magmatic and mantle carbon concentrations may be estimated in a Bayesian framework using diverse geologic information at an ocean island volcano. Our CO2 concentration estimates do not rely upon complex degassing models, geochemical tracer elements, assumed magma supply rates, or rare undegassed rock samples. Rather, we couple volcanic CO2 emission rates with probabilistic magma supply rates, which are obtained indirectly from magma storage and eruption rates. We estimate that the CO2 content of mantle-derived magma supplying Hawai‘i’s active volcanoes is 0.97?0.19+0.25 wt% -roughly 40% higher than previously believed-and is supplied from a mantle source region with a carbon concentration of 263?62+81?ppm. Our results suggest that mantle plumes and ocean island basalts are carbon-rich. Our data also shed light on helium isotope abundances, CO2/Nb ratios, and may imply higher CO2 emission rates from ocean island volcanoes.
Abstract: Estimates of carbon in the deep mantle vary by more than an order of magnitude. Coupled volcanic CO2 emission data and magma supply rates reveal a carbon-rich mantle plume source region beneath Hawai'i with 40% more carbon than previous estimates.
Geochemical Perspectives Letters, Vol. 3, pp. 230-237.
Canada, Quebec, Anticosti Island
carbon cycle
Abstract: Chemical weathering of silicate rocks is a primary drawdown mechanism of atmospheric carbon dioxide. The processes that affect weathering are therefore central in controlling global climate. A temperature-controlled “weathering thermostat” has long been proposed in stabilising long-term climate, but without definitive evidence from the geologic record. Here we use lithium isotopes (?7Li) to assess the impact of silicate weathering across a significant climate-cooling period, the end-Ordovician Hirnantian glaciation (~445 Ma). We find a positive ?7Li excursion, suggestive of a silicate weathering decline. Using a coupled lithium-carbon model, we show that initiation of the glaciation was likely caused by declining CO2 degassing, which triggered abrupt global cooling, and much lower weathering rates. This lower CO2 drawdown during the glaciation allowed climatic recovery and deglaciation. Combined, the data and model provide support from the geological record for the operation of the weathering thermostat.
Earth and Planetary Science Letters, Vol. 458, 1, pp. 141-151.
Mantle
carbon
Abstract: One of the most remarkable observations regarding volatile elements in the solar system is the depletion of N in the bulk silicate Earth (BSE) relative to chondrites, leading to a particularly high and non-chondritic C:N ratio. The N depletion may reflect large-scale differentiation events such as sequestration in Earth's core or massive blow off of Earth's early atmosphere, or alternatively the characteristics of a late-added volatile-rich veneer. As the behavior of N during early planetary differentiation processes is poorly constrained, we determined together the partitioning of N and C between Fe–N–C metal alloy and two different silicate melts (a terrestrial and a martian basalt). Conditions spanned a range of fO2 from ?IW?0.4 to ?IW?3.5 at 1.2 to 3 GPa, and 1400?°C or 1600?°C, where ?IW is the logarithmic difference between experimental fO2 and that imposed by the coexistence of crystalline Fe and wüstite. N partitioning ( ) depends chiefly on fO2, decreasing from to with decreasing fO2. also decreases with increasing temperature and pressure at similar fO2, though the effect is subordinate. In contrast, C partition coefficients () show no evidence of a pressure dependence but diminish with temperature. At 1400?°C, partition coefficients increase linearly with decreasing fO2 from to At 1600?°C, however, they increase from ?IW?0.7 to ?IW?2 ( to ) and decrease from ?IW?2 to ?IW?3.3 . Enhanced C in melts at high temperatures under reduced conditions may reflect stabilization of C–H species (most likely CH4). No significant compositional dependence for either N or C partitioning is evident, perhaps owing to the comparatively similar basalts investigated. At modestly reduced conditions (?IW?0.4 to ?2.2), N is more compatible in core-forming metal than in molten silicate ( ), while at more reduced conditions (?IW?2.2 to ?IW?3.5), N becomes more compatible in the magma ocean than in the metal phase. In contrast, C is highly siderophile at all conditions investigated (). Therefore, sequestration of volatiles in the core affects C more than N, and lowers the C:N ratio of the BSE. Consequently, the N depletion and the high C:N ratio of the BSE cannot be explained by core formation. Mass balance modeling suggests that core formation combined with atmosphere blow-off also cannot produce a non-metallic Earth with a C:N ratio similar to the BSE, but that the accretion of a C-rich late veneer can account for the observed high BSE C:N ratio.
Abstract: The continental lithosphere is a vast store for carbon. The carbon has been added and reactivated by episodic freezing and re-melting throughout geological history. Carbon remobilization can lead to significant variations in CO2 outgassing and release in the form of magmas from the continental lithosphere over geological timescales. Here we use calculations of continental lithospheric carbon storage, enrichment and remobilization to demonstrate that the role for continental lithosphere and rifts in Earth’s deep carbon budget has been severely underestimated. We estimate that cratonic lithosphere, which formed 2 to 3 billion years ago, originally contained about 0.25 Mt C km -3. A further 14 to 28 Mt C km -3 is added over time from the convecting mantle and about 43 Mt C km -3 is added by plume activity. Re-melting focuses carbon beneath rifts, creating zones with about 150 to 240 Mt C km -3, explaining the well-known association of carbonate-rich magmatic rocks with rifts. Reactivation of these zones can release 28 to 34 Mt of carbon per year for the 40 million year lifetime of a continental rift. During past episodes of supercontinent breakup, the greater abundance of continental rifts could have led to short-term carbon release of at least 142 to 170 Mt of carbon per year, and may have contributed to the high atmospheric CO2 at several times in Earth's history.
Earth and Planteray Science Letters, Vol. 489, pp. 84-91.
Mantle
carbonate
Abstract: Carbonate minerals are important hosts of carbon in the crust and mantle with a key role in the transport and storage of carbon in Earth's deep interior over the history of the planet. Whether subducted carbonates efficiently melt and break down due to interactions with reduced phases or are preserved to great depths and ultimately reach the core-mantle boundary remains controversial. In this study, experiments in the laser-heated diamond anvil cell (LHDAC) on layered samples of dolomite (Mg,?Ca)CO3 and iron at pressure and temperature conditions reaching those of the deep lower mantle show that carbon-iron redox interactions destabilize the MgCO3 component, producing a mixture of diamond, Fe7C3, and (Mg,?Fe)O. However, CaCO3 is preserved, supporting its relative stability in carbonate-rich lithologies under reducing lower mantle conditions. These results constrain the thermodynamic stability of redox-driven breakdown of carbonates and demonstrate progress towards multiphase mantle petrology in the LHDAC at conditions of the lowermost mantle.
Abstract: All living things contain carbon in some form, as it is the primary component of macromolecules including proteins, lipids, nucleic acids (RNA and DNA), and carbohydrates. As a matter of fact, it is the backbone of all organic (chemistry) compounds forming different kinds of bonds. Carbon: The Black, the Gray and the Transparent is not a complete scientific history of the material, but a book that describes key discoveries about this old faithful element while encouraging broader perspectives and approaches to its research due to its vast applications. All allotropes of carbon are described in this book, along with their properties, uses, and methods of procurement or manufacturing. Black carbon is represented by coal, gray carbon is represented by graphite, and transparent carbon is represented by diamond.
Abstract: The Earth’s surface inventory of carbon is critical for maintaining the planet’s habitability, yet the majority of Earth’s carbon is likely sequestered in the solid Earth. Understanding how Earth’s shallow carbon cycle evolved requires an assesment of the total carbon accreted, how it was distributed between Earth’s reservoirs, and how these reservoirs continue to exchange carbon. The low carbon content of Earth’s depleted upper mantle has been well constrained by primitive olivine hosted melt inclusions and the CO2/3He ratios of magmatic fluids. Using mass balance constraints we show that either the lower mantle is considerably more carbon rich, or the Earth has lost much of its initial carbon inventory. Distinguising between these scenarios is crucial for understanding the development and maintenance of Earth’s shallow carbon cycle. We assess the carbon content of the lower mantle using new melt inclusion datasets from Iceland, sampling both primordial and recycled mantle material. By comparing carbon concentrations with lithophile element concentrations we find evidence that carbon rich material is transported in the Iceland plume. Furthermore, we demonstrate that such datasets provide only a low bound on the true carbon content of the lower mantle, due to fundamental limits imposed by magma mixing, degassing and inclusion decrepitation. Using a global compilation of melt inclusion analyses we argue these processes occur ubiquitously and are likely to limit our ability to robustly resolve high mantle carbon using melt inclusion datasets. By combining these observations with global mass balance constraints we derive new estimates of the carbon content of primordial and recycled mantle material.
Abstract: Deep carbon is terrestrial carbon that is not in the atmosphere or oceans or on the surface. We have a great deal of knowledge about the properties of nearsurface carbon, but relatively little is known about the deep carbon cycle. The Deep Carbon Observatory, was founded in 2009, to address major questions about deep carbon. Where are the reservoirs of carbon? Is there significant carbon flux between the deep interior and the surface? What is deep microbial life? Did deep organic chemistry have a role in the origin of life? This project is directed toward documenting and describing of the history of deep carbon science. The narrative begins in 1601, when William Gilbert suggested that Earth’s interior behaves like a giant bar magnet. We trace across three centuries the slow evolution of thought that led to the establishment of the interdisciplinary field of Earth System Science. The concept and then development of the deep carbon cycle of burial and exhumation dates back at least two hundred years. We identify and document the key discoveries of deep carbon science, and assess the impact of this new knowledge on geochemistry, geodynamics, and geobiology. A History of Deep Carbon Science is in preparation for publication by Cambridge University Press in 2019. Its illuminating narrative highlights the engaging human stories of many remarkable researchers who have discovered the complexity and dynamics of Earth’s interior.
Abstract: Diamonds and the fluids that form them are important players in the deep carbon cycle that transforms carbon between mantle and surface reservoirs. However, the role of the high-density fluids (HDFs) that are found in microinclusions in diamonds is not limited to diamond formation. Examination of literature data on metasomatized rocks suggests that some may have formed by interaction of peridotites and eclogites with HDF-like melts. For example, silicic HDFs can explain the evoltion of an orthopyroxenerich vein in a garnet hartzburgite from Bulfontein,SA [1]. The composition that was added to the harzburgite and turned it into an orthopyroxene+olivine+phlogopite+garnet+carbonate +sulfide vein (green ellipse in the figure) lies at the extention of the array of silicic to low-Mg carbonatitic HDFs found in fibrous diamonds (pink diamonds). A silicic HDF (blue diamond) that contributed the added component would evolve into more carbonatitic compositions (arrow). Saline melts found in diamonds carry chloride, carbonate and silicate components, similar to saline hydrous fluids found in harzburgites xenoliths from Pinatubo, Phillipeens [2]. The higher water content in Pinatubo is, most probably, the result of lower temperatures and shallower level, but it attests for the role of saline fluids in metasomatism at the arc environment. In a companion abstract (Elazar et al., this volume) we report the finding of potassium-rich microinclusions in garnets in an eclogite xenolith from Robert Victor, SA. Their composition falls close to that of silicic to low-Mg carbonatitic HDFs in diamonds. Their lower potassium and higher aluminum content suggests derivation by higher degree of partial melting compared with the diamond forming fluids. All of the above observations support the important role of HDF-like melts and fluids in mantle processes.
Geochimica et Cosmochimica Acta, Vol. 238, pp. 477-495.
Mantle
carbon
Abstract: Constraining carbon (C) fractionation between silicate magma ocean (MO) and core-forming alloy liquid during early differentiation is essential to understand the origin and early distribution of C between reservoirs such as the crust-atmosphere, mantle, and core of Earth and other terrestrial planets. Yet experimental data at high pressure (P)-temperature (T) on the effect of other light elements such as sulfur (S) in alloy liquid on alloy-silicate partitioning of C and C solubility in Fe-alloy compositions relevant for core formation is lacking. Here we have performed multi-anvil experiments at 6-13?GPa and 1800-2000?°C to examine the effects of S and Ni on the solubility limit of C in Fe-rich alloy liquid as well as partitioning behavior of C between alloy liquid and silicate melt (). The results show that C solubility in the alloy liquid as well as decreases with increasing in S content in the alloy liquid. Empirical regression on C solubility in alloy liquid using our new experimental data and previous experiments demonstrates that C solubility significantly increases with increasing temperature, whereas unlike in S-poor or S-free alloy compositions, there is no discernible effect of Ni on C solubility in S-rich alloy liquid. Our modelling results confirm previous findings that in order to satisfy the C budget of BSE, the bulk Earth C undergoing alloy-silicate fractionation needs to be as high as those of CI-type carbonaceous chondrite, i.e., not leaving any room for volatility-induced loss of carbon during accretion. For Mars, on the other hand, an average single-stage core formation at relatively oxidized conditions (1.0 log unit below IW buffer) with 10-16?wt% S in the core could yield a Martian mantle with a C budget similar to that of Earth’s BSE for a bulk C content of ?0.25-0.9?wt%. For the scenario where C was delivered to the proto-Earth by a S-rich differentiated impactor at a later stage, our model calculations predict that bulk C content in the impactor can be as low as ?0.5?wt% for an impactor mass that lies between 9 and 20% of present day Earth’s mass. This value is much higher than 0.05-0.1?wt% bulk C in the impactor predicted by Li et al. (Li Y., Dasgupta R., Tsuno K., Monteleone B., and Shimizu N. (2016) Carbon and sulfur budget of the silicate Earth explained by accretion of differentiated planetary embryos. Nat. Geosci.9, 781-785) because C-solubility limit of 0.3?wt% in a S-rich alloy predicted by their models is significantly lower than the experimentally derived C-solubility of ?1.6?wt% for the relevant S-content in the core of the impactor.
Optimizing carbon capture and storage in kimberlite tailings for environmental benefit and operational efficiency.
Vancouver Kimberlite Cluster, Nov. 6, 1p. Abstract
Global
carbon
Abstract: Ultramafic mine tailings, including those from kimberlite-hosted diamond mines, offer potential operational and environmental benefit through reaction with carbon dioxide from air and power plant flue gas. The carbon dioxide is sequestered from the environment through the precipitation of carbonate minerals, thus reducing or offsetting the greenhouse gas emissions associated with mining. Additional benefits can include tailings stabilization, dust reduction, acid mine drainage prevention, and toxic metal encapsulation. In this talk I will present an overview of the processes and controls on carbonation reactions within tailings at active mines with a focus on acceleration of carbon sequestration within kimberlite tailings. Carbonation reactions can be limited by transport (rate of CO2 supply) and by reaction kinetics (mineral dissolution or mineral precipitation). Field studies of accidental passive carbonation within tailings at operating mines supplemented with laboratory experiment and reactive transport modelling has been key to identifying the rate limits to carbon sequestration at each mine site. With these limits identified, acceleration approaches can be tailored to the local climate, gangue mineralogy, and mine design, all of which can exert a primary control on carbon sequestration rates. The result is a methodology for evaluating the carbon sequestration potential of a mine site and a toolbox of acceleration strategies which together allow for site selection and project design. In the coming years, these systems will be deployed on site at active mines to further test and advance the technology. I will end with a perspective on the role that mining of ultramafic-hosted deposits can play in achieving net negative CO2 emissions as is projected to be required by the end of this century if we are to avoid net global warming in excess of two degrees centigrade.
Abstract: Carbon has been suggested as one of the light elements existing in the Earth's core. Under core conditions, iron carbide Fe7C3 is likely the first phase to solidify from a Fe-C melt and has thus been considered a potential component of the inner core. The crystal structure of Fe7C3, however, is still under debate, and its thermoelastic properties are not well constrained at high pressures. In this study, we performed synchrotron-based single-crystal X-ray diffraction experiment using an externally heated diamond-anvil cell to determine the crystal structure and thermoelastic properties of Fe7C3 up to 80 GPa and 800 K. Our diffraction data indicate that Fe7C3 adopts an orthorhombic structure under experimentally investigated conditions. The pressure-volume-temperature data for Fe7C3 were fitted by the high-temperature Birch-Murnaghan equation of state, yielding ambient-pressure unit-cell volume V0 = 745.2(2) Å3, bulk modulus K0 = 167(4) GPa, its first pressure derivative K0? = 5.0(2), dK/dT = -0.02(1) GPa/K, and thermal expansion relation ?T = 4.7(9) × 10-5 + 3(5) × 10-8 × (T - 300) K-1. We also observed anisotropic elastic responses to changes in pressure and temperature along the different crystallographic directions. Fe7C3 has strong anisotropic compressibilities with the linear moduli Ma > Mc > Mb from zero pressure to core pressures at 300 K, rendering the b axis the most compressible upon compression. The thermal expansion of c3 is approximately four times larger than that of a3 and b3 at 600 and 700 K, implying that the high temperature may significantly influence the elastic anisotropy of Fe7C3. Therefore, the effect of high temperature needs to be considered when using Fe7C3 to explain the anisotropy of the Earth's inner core.
Geochemical Perspectives, Vol. 7, no. 2, pp. 117-196. doi: 10.7185/geochempersp.7.2
Mantle
carbon
Abstract: As we struggle to cope with the ongoing buildup of CO2 produced by burning fossil fuels, can we acquire guidance from the geologic record? Although our ability to reconstruct past atmospheric CO2 content reliably is currently confined to the last 800 thousand years, we do have compelling evidence that this greenhouse gas played a key role throughout the Earth’s history. It certainly compensated for the young Sun’s lower luminosity. There is no question that it bailed us out of two snowball episodes or that it led to a brief 5 °C warming at the onset of the Eocene. Less certain is that diminishing atmospheric CO2 content was responsible for the global cooling that began 50 million years ago when the Indian subcontinent collided with Asia. Finally, it colluded with changing seasonality, ocean circulation re-organisation and iron fertilisation to generate the 100 thousand year glacial cycles that dominated the last half-million years.
Abstract: The Arctic is warming 2-3 times faster than the global average. The rapid increase of near-surface air temperatures at high latitudes is driving a loss of ice in oceans, rivers, mountain glaciers, and soil. Permafrost, the perennially frozen ground found in frigid climates, is estimated to store approximately 1,500 gigatons of carbon, or about half of the world’s underground stores. This carbon is slowly escaping from the soil as permafrost thaws; this thawing could release as much carbon into the atmosphere as current emissions from global land use change over the next 80 years. Like many other models of future conditions, uncertainty plagues the estimates of permafrost carbon release. Salmon et al. explored how nitrogen, an important contributor to this uncertainty, interacts with carbon in thawing soils. Nitrogen is an essential nutrient for plants and soil microbes but occurs in limited supply in tundra soils. This limitation restricts plant growth and microbial decomposition, which are critical pieces of the carbon cycle. The researchers drilled soil cores at the Eight Mile Lake site in interior Alaska to depths of 85 centimeters to evaluate the annually thawed active layer (0-55 centimeters) as well as the upper permafrost (below 55 centimeters). They then incubated the soil cores at 15°C for about 8 months and measured the subsequent nitrogen levels and microbial biomass. The data collected in the incubation informed statistical models that were used to analyze the effects of depth, time, and growing season conditions on nitrogen and carbon dynamics. The findings revealed that both carbon loss and microbial biomass decreased significantly with soil depth. Models predicted that soil decomposition would release the largest amount of mineral nitrogen from soils located in the middle of the active layer. Permafrost soils at the bottom of the soil profile, however, released a large flush of mineral nitrogen during the initial thaw but a small flux of mineral nitrogen during subsequent decomposition. These patterns indicate that microbes near the soil surface are nitrogen limited, whereas deep microbial communities are more limited by carbon. The team’s calculations estimate that mineral nitrogen released from the soil profile would increase tenfold during the first 5 years of permafrost thaw. Should permafrost continue to thaw in the Arctic, these results suggest that tundra ecosystems may experience an increase in nitrogen availability that exceeds plant and microbial demands. Excess nitrogen, in turn, could precipitate increased decomposition of soil carbon and increased levels of nitrogen in streams draining from thawing permafrost landscapes. The study offers critical insights into how warming temperatures in the Arctic could dramatically increase permafrost thaw and initiate profound changes in carbon and nitrogen cycling in tundra ecosystems.
Abstract: In this study, we present a number of experiments on the transformation of graphite, diamond, and multiwalled carbon nanotubes under high pressure conditions. The analysis of our results testifies to the instability of diamond in the 55-115 GPa pressure range, at which onion-like structures are formed. The formation of interlayer sp3-bonds in carbon nanostructures with a decrease in their volume has been studied theoretically. It has been found that depending on the structure, the bonds between the layers can be preserved or broken during unloading.
Abstract: Over geological timescales, CO2 levels are determined by the operation of the long term carbon cycle, and it is generally thought that changes in atmospheric CO2 concentration have controlled variations in Earth's surface temperature over the Phanerozoic Eon. Here we compile independent estimates for global average surface temperature and atmospheric CO2 concentration, and compare these to the predictions of box models of the long term carbon cycle COPSE and GEOCARBSULF. We find a strong relationship between CO2 forcing and temperature from the proxy data, for times where data is available, and we find that current published models reproduce many aspects of CO2 change, but compare poorly to temperature estimates. Models are then modified in line with recent advances in understanding the tectonic controls on carbon cycle source and sink processes, with these changes constrained by modelling 87Sr/86Sr ratios. We estimate CO2 degassing rates from the lengths of subduction zones and rifts, add differential effects of erosion rates on the weathering of silicates and carbonates, and revise the relationship between global average temperature changes and the temperature change in key weathering zones. Under these modifications, models produce combined records of CO2 and temperature change that are reasonably in line with geological and geochemical proxies (e.g. central model predictions are within the proxy windows for >~75% of the time covered by data). However, whilst broad long-term changes are reconstructed, the models still do not adequately predict the timing of glacial periods. We show that the 87Sr/86Sr record is largely influenced by the weathering contributions of different lithologies, and is strongly controlled by erosion rates, rather than being a good indicator of overall silicate chemical weathering rates. We also confirm that a combination of increasing erosion rates and decreasing degassing rates over the Neogene can cause the observed cooling and Sr isotope changes without requiring an overall increase in silicate weathering rates. On the question of a source or sink dominated carbon cycle, we find that neither alone can adequately reconstruct the combination of CO2, temperature and strontium isotope dynamics over Phanerozoic time, necessitating a combination of changes to sources and sinks. Further progress in this field relies on >108?year dynamic spatial reconstructions of ancient tectonics, paleogeography and hydrology. Whilst this is a significant challenge, the latest reconstruction techniques, proxy records and modelling advances make this an achievable target.
Journal of the Geological Society of London, Vol. 176, pp. 337-347.
Global
carbon
Abstract: Carbon is found in nature in a huge variety of allotropic forms and recent research in materials science has encouraged the development of technological materials based on nanocarbon. Carbon atoms with sp2 or sp3 hybridization can be thought of as building blocks. Following a bottom-up approach, we show how graphene and diamond molecules are built up and how their properties vary with size, reaching an upper limit with bulk graphite and diamond. Carbon atoms with sp2 hybridization give rise to an impressive number of different materials, such as carbon nanotubes, graphene nanoribbons, porous carbon and fullerene. As in any crystalline phase, the crystal structures of natural carbon allotropes (i.e. graphite and diamond) contain various types of imperfections. These so-called lattice defects are classified by their dimensions into 0D (point), 1D (line), 2D (planar) and 3D (volume) defects. Lattice defects control the physical properties of crystals and are often a fingerprint of the geological environment in which they formed and were modified. Direct observations of lattice defects are commonly accomplished by transmission electron microscopy. We present and discuss the ideal and real structures of carbon allotropes, the energetics of lattice defects and their significance in understanding geological processes and conditions.
Abstract: Hydrogen (H) and carbon (C) have probably been delivered to the Earth mainly during accretion processes at High Temperature (HT) and High Pressure (HP) and at variable redox conditions. We performed HP (1-15?GPa) and HT (1600-2300°C) experiments, combined with state-of-the-art analytical techniques to better understand the behavior of H and C during planetary differentiation processes. We show that increasing pressure makes H slightly siderophile and slightly decreases the highly siderophile nature of C. This implies that the capacity of a growing core to retain significant amounts of H or C is mainly controlled by the size of the planet: small planetary bodies may retain C in their cores while H may have rather been lost in space; larger bodies may store both H and C in their cores. During the Earth's differentiation, both C and H might be sequestrated in the core. However, the H content of the core would remain one or two orders of magnitude lower than that of C since the (H/C)core ratio might range between 0.04 and 0.27.
Journal of the Geological Society of London, Vol. 176, pp. 398-407.
Mantle
carbon
Abstract: On a planetary scale, the carbon cycle describes the movement of carbon between the atmosphere and the deep earth, which affects petrologic processes in a range of geologic settings and the long-term viability of life at the surface. In this context, volcanoes and their associated magmatic systems represent the interface through which carbon is transferred from the deep earth to the atmosphere. Thus, describing the CO2 budget of volcanic systems is necessary for understanding the deep carbon cycle. In this review, Kilauea volcano (Hawaii) is used as a case study, and we present several simple calculations that can be used to account for processes that affect the amount and distribution of CO2 in this relatively well-studied volcanic system. These processes include estimating the concentration of CO2 in a melt derived by partial melting of a source material, enrichment of CO2 in the melt during fractional crystallization, exsolution of CO2 from a fluid-saturated melt, trapping and post-entrapment modification of melt inclusions, and degassing from the volcanic edifice. Our goal in this review is to provide straightforward example calculations that can be used to derive first-order estimates regarding processes that control the CO2 budgets of magmas.
Journal of the Geological Society of London, Vol. 176, pp. 348-374.
Mantle
carbon
Abstract: Carbon is subducted to depths where metamorphism liberates water-bearing fluids. The C-bearing fluids facilitate partial melting of the upper mantle, generating magmas that may erupt as arc volcanics. Degassing of the magmas releases CO2 and other volatile species to the atmosphere. Over geological time, this process contributes to the composition of the atmosphere and planetary habitability. Here I summarize the background needed to carry out theoretical geochemical modelling of fluids and fluid-rock interactions from surficial conditions into the upper mantle. A description of the general criteria for predicting equilibrium and non-equilibrium chemical reactions is followed by a summary of how the thermodynamic activities of species are related to measurable concentrations through standard states and activity coefficients. Specific examples at ambient conditions involving dilute water are detailed. The concept of aqueous speciation and how it can be calculated arises from this discussion. Next, I discuss how to calculate standard Gibbs free energies and aqueous activity coefficients at elevated temperatures and pressures. The revised Helgeson-Kirkham-Flowers equations of state are summarized and the revised predictive correlations for the estimation of equation of state coefficients in the Deep Earth Water (DEW) model are presented. Finally, the DEW model is applied to the solubility and speciation of aqueous aluminium.
Journal of the Geological Society of London, Vol. 176, pp. 388-397.
Mantle
carbon
Abstract: The valence of carbon is governed by the oxidation state of the host system. The subducted oceanic lithosphere contains considerable amounts of iron so that Fe3+/Fe2+ equilibria in mineral assemblages are able to buffer the fO2 and the valence of carbon. Alternatively, carbon itself can be a carrier of redox budget when transferred from the slab to the mantle, prompting the oxidation of the sub-arc mantle. Also, the oxidation of sedimentary carbonaceous matter to CO2 in the slab could consume the available redox budget. Therefore, the correct use of intensive and extensive variables to define the slab-to-mantle redox budget by C-bearing fluids is of primary importance when considering different fluid/rock ratios. Fluid-mediated processes at the slab-mantle interface can be investigated also experimentally. The presence of CO2 (or CH4 at highly reduced conditions) in aqueous COH fluids in peridotitic systems affects the positions of carbonation/decarbonation reactions and of the solidus. Some methods to produce and analyse COH fluid-saturated experiments in model systems are introduced, together with the measurement of experimental COH fluids composition in terms of volatiles and dissolved solutes. The role of COH fluids in the stability of hydrous and carbonate minerals is discussed comparing experimental results with thermodynamic models.
Journal of the Geological Society, Vol. 176, pp. 348-374.
Mantle
carbon, subduction
Abstract: Carbon is subducted to depths where metamorphism liberates water-bearing fluids. The C-bearing fluids facilitate partial melting of the upper mantle, generating magmas that may erupt as arc volcanics. Degassing of the magmas releases CO2 and other volatile species to the atmosphere. Over geological time, this process contributes to the composition of the atmosphere and planetary habitability. Here I summarize the background needed to carry out theoretical geochemical modelling of fluids and fluid-rock interactions from surficial conditions into the upper mantle. A description of the general criteria for predicting equilibrium and non-equilibrium chemical reactions is followed by a summary of how the thermodynamic activities of species are related to measurable concentrations through standard states and activity coefficients. Specific examples at ambient conditions involving dilute water are detailed. The concept of aqueous speciation and how it can be calculated arises from this discussion. Next, I discuss how to calculate standard Gibbs free energies and aqueous activity coefficients at elevated temperatures and pressures. The revised Helgeson-Kirkham-Flowers equations of state are summarized and the revised predictive correlations for the estimation of equation of state coefficients in the Deep Earth Water (DEW) model are presented. Finally, the DEW model is applied to the solubility and speciation of aqueous aluminium.
Barry, P.H., de Moor, J.M., Giovannelli, D., Schrenk, M., Hummer, D.R., Lopez, T., Pratt, C.A., Alpizar Segua, Y., Battaglia, A., Beaudry, A., Bini, G., Cascante, M., d'Errico, G., di Carlo, M., Fattorini, D., Fullerton, K., H+Gazel, E., Gonzalez, G., Hal
Abstract: Carbon and other volatiles in the form of gases, fluids or mineral phases are transported from Earth’s surface into the mantle at convergent margins, where the oceanic crust subducts beneath the continental crust. The efficiency of this transfer has profound implications for the nature and scale of geochemical heterogeneities in Earth’s deep mantle and shallow crustal reservoirs, as well as Earth’s oxidation state. However, the proportions of volatiles released from the forearc and backarc are not well constrained compared to fluxes from the volcanic arc front. Here we use helium and carbon isotope data from deeply sourced springs along two cross-arc transects to show that about 91 per cent of carbon released from the slab and mantle beneath the Costa Rican forearc is sequestered within the crust by calcite deposition. Around an additional three per cent is incorporated into the biomass through microbial chemolithoautotrophy, whereby microbes assimilate inorganic carbon into biomass. We estimate that between 1.2 × 108 and 1.3 × 1010 moles of carbon dioxide per year are released from the slab beneath the forearc, and thus up to about 19 per cent less carbon is being transferred into Earth’s deep mantle than previously estimated.
Earth and Planetary Science Letters, Vol. 516, pp. 190-201.
Mantle
carbon
Abstract: A long-standing unresolved problem in understanding Earth's deep carbon cycle is whether crustal carbon is recycled beyond arc depths. While isotopic signatures of eclogitic diamonds and their inclusions suggest deep recycling of crustal material, the crustal carbon source remains controversial; seafloor sediment - the widely favored crustal carbon source - cannot explain the combined carbon and nitrogen isotopic characteristics of eclogitic diamonds. Here we examined the carbon and oxygen isotopic signatures of bulk-rock carbonate for 80 geographically diverse samples from altered mafic-ultramafic oceanic crust (AOC), which comprises 95 vol% of the crustal material in subducting slabs. The results show: (i) AOC contains carbonate with C values as low as ?24‰, indicating the presence of biogenic carbonate; (ii) carbonate in AOC was mainly formed during low-temperature (<100 °C) alteration processes. Modeling accounting for this newly recognized carbon source in the oceanic crust with formation temperatures <100 °C yields a global carbon influx of 1.5±0.3 × 1012 mol C/yr carried by subducting AOC into the trench, which is 50-90% of previous estimates, but still of the same order of the carbon influx carried by subducting sediments into the trench. The AOC can retain carbon better than sediment during subduction into the asthenosphere, transition zone and lower mantle. Mixing of asthenospheric and AOC fluids provides the first consistent explanation of the diverse record of carbon and nitrogen isotopes in diamonds, suggesting that AOC, instead of sediment, is the key carrier of crustal carbon into the deep mantle.
Earth and Planetary Science Letters, Vol. 519, pp. 213-222.
Mantle
carbon cycle
Abstract: The breakdown of carbonate minerals at high pressure is frequently cited as an important mechanism that leads to carbon release from subducted rocks. However, carbonate minerals in the subducting slab are predicted to be stable to depths that are greater than arc-generating magma depths of approximately 150 km, implying that breakdown of carbonate phases in dehydrated MORB may not be a major contributor to arc volcano carbon budgets. To account for this discrepancy, previous studies have suggested that addition of H2O-rich fluids promotes the breakdown of carbonate-rich lithologies, thus generating volatile C species that could be incorporated into arc magmas. Here, we explore the feasibility of H2O-mediated decarbonation with a simple thermodynamic model. We calculate equilibrium mineral assemblages and accompanying fluid H2O/CO2 ratios for typical subducted lithologies, assuming a range of subduction zone geotherms, and explore the implications of addition of external fluids that are generated from deserpentinization of ultramafic lithologies at various stages. Results suggest that the liberation of C along volcanic arcs is facilitated by either the breakdown of carbonate minerals due to thermodynamically favorable conditions in hotter subduction systems, or by the breakdown of carbonate minerals during periods of higher fluid productivity associated with deserpentinization at appropriate depths along colder subduction geotherms. A comparison of C fluxes measured at volcanic arcs shows that colder subduction zones generate higher C fluxes, implying that the depth at which deserpentinization reactions occur strongly controls the availability of aqueous fluids for slab decarbonation, and that fluid availability represents the dominant control on carbon volatility during subduction.
Abstract: The fate of subducted carbonates in the lower mantle and at the core-mantle boundary was modelled via experiments in the MgCO3-Fe0 system at 70-150 GPa and 800-2600 K in a laser-heated diamond anvil cell. Using in situ synchrotron X-ray diffraction and ex situ transmission electron microscopy we show that the reduction of Mg-carbonate can be exemplified by: 6MgCO3 + 19Fe = 8FeO +10(Mg0.6Fe0.4)O + Fe7C3 + 3C. The presented results suggest that the interaction of carbonates with Fe0 or Fe0-bearing rocks can produce Fe-carbide and diamond, which can accumulate in the D’’ region, depending on its carbon to Fe ratio. Due to the sluggish kinetics of the transformation, diamond can remain metastable at the core-mantle boundary (CMB) unless it is in a direct contact with Fe-metal. In addition, it can be remobilized by redox melting accompanying the generation of mantle plumes.
Abstract: Carbon is central to the formation and environmental impact of large igneous provinces (LIPs). These vast magmatic events occur over geologically short timescales and include voluminous flood basalts, along with silicic and low-volume alkaline magmas. Surface outgassing of CO2 from flood basalts may average up to 3,000 Mt per year during LIP emplacement and is subsidized by fractionating magmas deep in the crust. The large quantities of carbon mobilized in LIPs may be sourced from the convecting mantle, lithospheric mantle and crust. The relative significance of each potential carbon source is poorly known and probably varies between LIPs. Because LIPs draw on mantle reservoirs typically untapped during plate boundary magmatism, they are integral to Earth’s long-term carbon cycle.
Abstract: Human society's rapid release of vast quantities of CO2 into the atmosphere is a significant planetary experiment. An obvious natural process capable of similar emissions over geologically short time spans are very large bolide impacts. When striking a carbon-rich target, bolides significantly, and potentially catastrophically, disrupt the global biogeochemical carbon cycle. Independent factors, such as sulfur-rich targets, redox state of the oceans or encountering ecosystems already close to a tipping point, dictated the magnitude of further consequences and determined which large bolide strikes shaped Earth's evolution. On the early Earth, where carbon-rich sedimentary targets were rare, impacts may not have been purely destructive. Instead, enclosed subaqueous impact structures may have contributed to initiating Earth's unique carbon cycle.
Abstract: here 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.
Abstract: The isotopic "flavor" of Earth’s major volatiles, including carbon, can be compared to the known reservoirs of volatiles in the solar system and so determine the source of Earth’s carbon. This requires knowing Earth’s bulk carbon isotope value, which is not straightforward to determine. During Earth’s differentiation, carbon was partitioned into the core, mantle, crust, and atmosphere. Therefore, although carbon is omnipresent within the Earth system, scientists have yet to determine its distribution and relative abundances. This article addresses what we know of the processes involved in the formation of Earth’s carbon reservoirs, and, by deduction, what we know about the possible origins of Earth’s carbon.
Abstract: Carbon is one of the most important elements on Earth. It is the basis of life, it is stored and mobilized throughout the Earth from core to crust and it is the basis of the energy sources that are vital to human civilization. This issue will focus on the origins of carbon on Earth, the roles played by large-scale catastrophic carbon perturbations in mass extinctions, the movement and distribution of carbon in large igneous provinces, and the role carbon plays in icehouse-greenhouse climate transitions in deep time. Present-day carbon fluxes on Earth are changing rapidly, and it is of utmost importance that scientists understand Earth's carbon cycle to secure a sustainable future.
Abstract: A "happy accident" has yielded a new, stable form of pure carbon made from cheap feedstocks, researchers say. Like diamond and graphene, two other guises of carbon, the material seems to have extraordinary physical properties. It is harder than stainless steel, about as conductive, and as reflective as a polished aluminum mirror. Perhaps most surprising, the substance appears to be ferromagnetic, behaving like a permanent magnet at temperatures up to 125°C. The discovery, announced in a talk here at the International Symposium on Clusters and Nanomaterials, could lead to lightweight coatings, medical products, and novel electronic devices. The news has elicited both excitement and caution among the dozens of researchers attending the meeting. Experts note that carbon is much lighter than other ferromagnetic elements such as manganese, nickel, and iron. Moreover, carbon is nontoxic in the body—which could mean the substance could be used for making biosensors or drug-delivery carriers.
IN: Deep carbon: past to present, Orcutt, Daniel, Dasgupta eds., pp. 237-275.
Mantle
carbon
Abstract: This chapter provides a summary of the flux of carbon through various oceanic volcanic centers such as mid-ocean ridges and intraplate settings, as well as what these fluxes indicate about the carbon content of the mantle. By reviewing methods used to measure the carbon geochemistry of basalts and then to estimate fluxes, the chapter provides insight into how mantle melting and melt extraction processes are estimated. The chapter discusses how the flux of carbon compares with other incompatible trace elements and gases. From there, the chapter discusses whether the budget of carbon in the ocean mantle can be explained by primordial carbon or whether carbon recycling is required to balance the budget.
Abstract: A hidden carbon cycle exists inside Earth. Every year, megatons of carbon disappear into subduction zones, affecting atmospheric carbon dioxide and oxygen over Earth’s history. Here we discuss the processes that move carbon towards subduction zones and transform it into fluids, magmas, volcanic gases and diamonds. The carbon dioxide emitted from arc volcanoes is largely recycled from subducted microfossils, organic remains and carbonate precipitates. The type of carbon input and the efficiency with which carbon is remobilized in the subduction zone vary greatly around the globe, with every convergent margin providing a natural laboratory for tracing subducting carbon.
Abstract: For approximately the first 2?billion years of the Earth’s history, atmospheric oxygen levels were extremely low. It was not until at least half a billion years after the evolution of oxygenic photosynthesis, perhaps as early as 3?billion years ago, that oxygen rose to appreciable levels during the Great Oxidation Event. Shortly after, marine carbonates underwent a large positive spike in carbon isotope ratios known as the Lomagundi event. The mechanisms responsible for the Great Oxidation and Lomagundi events remain debated. Using a carbon-oxygen box model that tracks the Earth’s surface and interior carbon fluxes and reservoirs, while also tracking carbon isotopes and atmospheric oxygen levels, we demonstrate that about 2.5?billion years ago a tectonic transition that resulted in increased volcanic CO2 emissions could have led to increased deposition of both carbonates and organic carbon (organic?C)?via enhanced weathering and nutrient delivery to oceans. Increased burial of carbonates and organic?C would have allowed the accumulation of atmospheric oxygen while also increasing the delivery of carbon to subduction zones. Coupled with preferential release of carbonates at arc volcanoes and deep recycling of organic?C to ocean island volcanoes, we find that such a tectonic transition can simultaneously explain the Great Oxidation and Lomagundi events without any change in the fraction of carbon buried as organic?C relative to carbonate, which is often invoked to explain carbon isotope excursions.
Abstract: The carbon allotropes of diamond and graphene have different types of bonding that lead to their exceptional properties. Bakharev et al. pull off the impressive trick of making a monolayer carbon film that is diamond-like in its bonding. The authors accomplish this by attaching fluorine atoms to the carbon film, creating “F-diamane.” Diamane is a long-sought-after, but challenging to make, material that should have useful properties. F-diamane may find use in a variety of applications, from microelectronics as a semiconductor to a seed material for growing single-crystal diamond films.
Earth and Planetary Science Letters, Vol. 533, 11p. Pdf
Mantle
carbon
Abstract: Knowledge of the effect of water on the density of carbonate melts is fundamental to constrain their mobility in the Earth's interior and the exchanges of carbon between deep and surficial reservoirs. Here we determine the density of hydrous MgCO3 and CaMg(CO3)2 melts (10 wt% H2O) from 1.09 to 2.98 GPa and 1111 to 1763 K by the X-ray absorption method in a Paris-Edinburgh press and report the first equations of state for hydrous carbonate melts at high pressure. Densities range from 2.26(3) to 2.50(3) g/cm3 and from 2.34(3) to 2.48(3) g/cm3 for hydrous MgCO3 and CaMg(CO3)2 melts, respectively. Combining the results with density data for the dry counterparts from classical Molecular Dynamic (MD) simulations, we derive the partial molar volume (, ) and compressibility of H2O and CO2 components at crustal and upper mantle conditions. Our results show that in alkaline carbonate melts is larger and less compressible than at the investigated conditions. Neither the compressibility nor depend on carbonate melt composition within uncertainties, but they are larger than those in silicate melts at crustal conditions. in alkaline earth carbonate melts decreases from 25(1) to 16.5(5) cm3/mol between 0.5 and 4 GPa at 1500 K. Contrastingly, comparison of our results with literature data suggests strong compositional effects on , that is also less compressible than in transitional melts (e.g., kimberlites) and carbonated basalts. We further quantify the effect of hydration on the mobility of carbonate melts in the upper mantle and demonstrate that 10 wt% H2O increases the mobility of MgCO3 melts from 37 to 67 g.cm?3.Pa?1s?1 at 120 km depth. These results suggest efficient carbonate melt extraction during partial melting and fast migration of incipient melts in the shallow upper mantle.
Abstract: Differential cycling of carbonate and organic carbon in the mantle may link the Great Oxidation Event and the subsequent increase in carbon isotope values, according to a model that links the Earth’s surface and interior.
Earth Science Reviews, in press available, 38p. Doi.org/1010.1016 /jearsciev.2019.103073
Global
carbon
Abstract: Although the deep recycling of carbon has been proposed to play a key role in producing intraplate magmatism, the question of how it controls or triggers mantle melting remains poorly understood. In addition, generation of incipient carbonated melts in the mantle and their subsequent reaction with the mantle are critical processes that can influence the geochemistry of intraplate basalts, but the details of such processes are also unclear. Here we present geochemical evidence for the existence of pervasive carbonate melt in the mantle source of Cenozoic continental intraplate highly alkali basalts (SiO2 < 45 wt%), which are volumetrically minor but widespread in eastern China. The primary magma compositions of these basalts cannot be explained by either partial melting of a single mantle source lithology or mixing of magmas derived from distinct mantle sources, but can be adequately explained by carbonate-fluxed melting of eclogite and subsequent reaction between silica-rich melts and peridotite that ultimately transformed the initial carbonated silica-rich melts into silica-undersaturated alkalic magmas. The source of the carbonate is in subducted eclogites associated with the Pacific plate, which stagnated in the mantle transition zone (MTZ). The spatial distribution of the alkali basalts is in accord with large-scale seismic low-velocity anomalies in the upper mantle above the MTZ. Similar scenarios in central-western Europe and eastern Australia lead us to propose that reaction between carbonated silica-rich melt and peridotite may be a pivotal mechanism for the generation of continental intraplate alkali basalts elsewhere in the world.
Abstract: The permafrost zone is expected to be a substantial carbon source to the atmosphere, yet large-scale models currently only simulate gradual changes in seasonally thawed soil. Abrupt thaw will probably occur in <20% of the permafrost zone but could affect half of permafrost carbon through collapsing ground, rapid erosion and landslides. Here, we synthesize the best available information and develop inventory models to simulate abrupt thaw impacts on permafrost carbon balance. Emissions across 2.5?million?km2 of abrupt thaw could provide a similar climate feedback as gradual thaw emissions from the entire 18?million?km2 permafrost region under the warming projection of Representative Concentration Pathway 8.5. While models forecast that gradual thaw may lead to net ecosystem carbon uptake under projections of Representative Concentration Pathway 4.5, abrupt thaw emissions are likely to offset this potential carbon sink. Active hillslope erosional features will occupy 3% of abrupt thaw terrain by 2300 but emit one-third of abrupt thaw carbon losses. Thaw lakes and wetlands are methane hot spots but their carbon release is partially offset by slowly regrowing vegetation. After considering abrupt thaw stabilization, lake drainage and soil carbon uptake by vegetation regrowth, we conclude that models considering only gradual permafrost thaw are substantially underestimating carbon emissions from thawing permafrost.
Geochimica et Cosmochimica Acta, Vol. 275, pp. 99-122.
Mantle
carbon
Abstract: Diamonds are unrivalled in their ability to record the mantle carbon cycle and mantle fO2 over a vast portion of Earth’s history. Diamonds’ inertness and antiquity means their carbon isotopic characteristics directly reflect their growth environment within the mantle as far back as ?3.5 Ga. This paper reports the results of a thorough secondary ion mass spectrometry (SIMS) carbon isotope and nitrogen concentration study, carried out on fragments of 144 diamond samples from various locations, from ?3.5 to 1.4 Ga for P [peridotitic]-type diamonds and 3.0 to 1.0 Ga for E [eclogitic]-type diamonds. The majority of the studied samples were from diamonds used to establish formation ages and thus provide a direct connection between the carbon isotope values, nitrogen contents and the formation ages. In total, 908 carbon isotope and nitrogen concentration measurements were obtained. The total ?¹³C data range from ?17.1 to ?1.9 ‰ (P = ?8.4 to ?1.9 ‰; E = ?17.1 to ?2.1‰) and N contents range from 0 to 3073 at. ppm (P = 0 to 3073 at. ppm; E = 1 to 2661 at. ppm). In general, there is no systematic variation with time in the mantle carbon isotope record since > 3 Ga. The mode in ?¹³C of peridotitic diamonds has been at ?5 (±2) ‰ since the earliest diamond growth ?3.5 Ga, and this mode is also observed in the eclogitic diamond record since ?3 Ga. The skewness of eclogitic diamonds’ ?¹³C distributions to more negative values, which the data establishes began around 3 Ga, is also consistent through time, with no global trends apparent. No isotopic and concentration trends were recorded within individual samples, indicating that, firstly, closed system fractionation trends are rare. This implies that diamonds typically grow in systems with high excess of carbon in the fluid (i.e. relative to the mass of the growing diamond). Any minerals included into diamond during the growth process are more likely to be isotopically reset at the time of diamond formation, meaning inclusion ages would be representative of the diamond growth event irrespective of whether they are syngenetic or protogenetic. Secondly, the lack of significant variation seen in the peridotitic diamonds studied is in keeping with modeling of Rayleigh isotopic fractionation in multicomponent systems (RIFMS) during isochemical diamond precipitation in harzburgitic mantle. The RIFMS model not only showed that in water-maximum fluids at constant depths along a geotherm, fractionation can only account for variations of <1‰, but also that the principal ?¹³C mode of ?5 ± 1‰ in the global harzburgitic diamond record occurs if the variation in fO2 is only 0.4 log units. Due to the wide age distribution of P-type diamonds, this leads to the conclusion that the speciation and oxygen fugacity of diamond forming fluids has been relatively consistent. The deep mantle has therefore generated fluids with near constant carbon speciation for 3.5 Ga.
Earth and Planetary Science Letters, Vol. 529, 115848 12p. Pdf
Global
carbon
Abstract: At high temperatures, isotope partitioning is often assumed to proceed under equilibrium and trends in the carbon isotope composition within graphite and diamond are used to deduce the redox state of their fluid source. However, kinetic isotope fractionation modifies fluid- or melt-precipitated mineral compositions when growth rates exceed rates of diffusive mixing. As carbon self-diffusion in graphite and diamond is exceptionally slow, this fractionation should be preserved. We have hence performed time series experiments that precipitate graphitic carbon through progressive oxidization of an initially CH4-dominated fluid. Stearic acid was thermally decomposed at 800 °C and 2 kbar, yielding a reduced COH-fluid together with elemental carbon. Progressive hydrogen loss from the capsule caused CH4 to dissociate with time and elemental carbon to continuously precipitate. The newly formed C0, aggregating in globules, is constantly depleted by ‰ in 13C relative to the methane, which defines a temperature dependent kinetic graphite-methane 13C/12C fractionation factor. Equilibrium fractionation would instead yield graphite heavier than the methane. In dynamic environments, kinetic isotope fractionation may control the carbon isotope composition of graphite or diamond, and, extended to nitrogen, could explain the positive correlation of and sometimes observed in coherent diamond growth zones. 13C enrichment trends in diamonds are then consistent with reduced deep fluids oxidizing upon their rise into the subcontinental lithosphere, methane constituting the main source of carbon.
Minerals MDPI, Vol. 10, 267 doi: 10.23390/min10030267 14p. Pdf
Mantle
Melililite, carbon
Abstract: Understanding the viscosity of mantle-derived magmas is needed to model their migration mechanisms and ascent rate from the source rock to the surface. High pressure-temperature experimental data are now available on the viscosity of synthetic melts, pure carbonatitic to carbonate-silicate compositions, anhydrous basalts, dacites and rhyolites. However, the viscosity of volatile-bearing melilititic melts, among the most plausible carriers of deep carbon, has not been investigated. In this study, we experimentally determined the viscosity of synthetic liquids with ~31 and ~39 wt% SiO2, 1.60 and 1.42 wt% CO2 and 5.7 and 1 wt% H2O, respectively, at pressures from 1 to 4.7 GPa and temperatures between 1265 and 1755 °C, using the falling-sphere technique combined with in situ X-ray radiography. Our results show viscosities between 0.1044 and 2.1221 Pa•s, with a clear dependence on temperature and SiO2 content. The atomic structure of both melt compositions was also determined at high pressure and temperature, using in situ multi-angle energy-dispersive X-ray diffraction supported by ex situ microFTIR and microRaman spectroscopic measurements. Our results yield evidence that the T-T and T-O (T = Si,Al) interatomic distances of ultrabasic melts are higher than those for basaltic melts known from similar recent studies. Based on our experimental data, melilititic melts are expected to migrate at a rate ~from 2 to 57 km•yr?1 in the present-day or the Archaean mantle, respectively.
Diamonds & Related Materials, in press available, 31p. Pdf
Global
carbon
Abstract: Natural diamonds that have been partially replaced by graphite have been observed to occur in natural rocks. While the graphite-to-diamond phase transition has been extensively studied the opposite of this (diamond to graphite) remains poorly understood. We performed high-pressure and temperature hydrous and anhydrous experiments up to 1.0?GPa and 1300?°C using Amplex premium virgin synthetic diamonds (20-40??m size) as the starting material mixed with Mg (OH)2 as a source of H2O for the hydrous experiments. The experiments revealed that the diamond-to-graphite transformation at P?=?1GPa and T?=?1300?°C was triggered by the presence of H2O and was accomplished through a three-stage process. Stage 1: diamond reacts with a supercritical H2O producing an intermediate 200-500?nm size “globular carbon” phase. This phase is a linear carbon chain; i.e. a polyyne or carbyne. Stage 2: the linear carbon chains are unstable and highly reactive, and they decompose by zigzagging and cross-linking to form sp2-bonded structures. Stage 3: normal, disordered, and onion-like graphite is produced by the decomposition of the sp-hybridized carbon chains which are re-organized into sp2 bonds. Our experiments show that there is no direct transformation from sp3 C-bonds into sp2 C-bonds. Our hydrous high-pressure and high-temperature experiments show that the diamond-to-graphite transformation requires an intermediate metastable phase of a linear hydrocarbon. This process provides a simple mechanism for the substitution of other elements into the graphite structure (e.g. H, S, O).
Brovarone, A.V., Butch, C.J., Ciappa, A., Cleaves, H.J., Elmaleh, A., Faccenda, M., Feineman, M., Hermann, J., Nestola, F., Cordone, A., Giovannelli., D.
American Mineralogist, Vol. 105, pp. 1152-1160. pdf
Mantle
carbon
Abstract: Water plays a key role in shaping our planet and making life possible. Given the abundance of water on Earth's surface and in its interior, chemical reactions involving water, namely hydration and dehydration reactions, feature prominently in nature and are critical to the complex set of geochemical and biochemical reactions that make our planet unique. This paper highlights some fundamental aspects of hydration and dehydration reactions in the solid Earth, biology, and man-made materials, as well as their connections to carbon cycling on our planet.
Abstract: The chemical and physical processes operating during subduction-zone metamorphism can profoundly influence the cycling of elements on Earth. Deep-Earth carbon (C) cycling and mobility in subduction zones has been of particular recent interest to the scientific community. Here, we present textural and geochemical data (CO, Sr isotopes and bulk and in-situ trace element concentrations) for a suite of ophicarbonate rocks (carbonate-bearing serpentinites) metamorphosed over a range of peak pressure-temperature (P-T) conditions together representing a prograde subduction zone P-T path. These rocks, in order of increasing peak P-T conditions, are the Internal Liguride ophicarbonates (from the Bracco unit, N. Apennines), pumpellyite- and blueschist-facies ophicarbonates from the Sestri-Voltaggio zone (W. Ligurian Alps) and the Queyras (W. Alps), respectively, and eclogite-facies ophicarbonates from the Voltri Massif. The Bracco oceanic ophicarbonates retain breccia-like textures associated with their seafloor hydrothermal and sedimentary origins. Their trace element concentrations and ?18OVSMOW (+15.6 to +18.2‰), ?13CVPDB (+1.1 to +2.5‰) and their 87Sr/86Sr (0.7058 to 0.7068), appear to reflect equilibration during Jurassic seawater-rock interactions. Intense shear deformation characterizes the more deeply subducted ophicarbonates, in which prominent calcite recrystallization and carbonation of serpentinite clasts occurred. The isotopic compositions of the pumpellyite-facies ophicarbonates overlap those of their oceanic equivalents whereas the most deformed blueschist-facies sample shows enrichments in radiogenic Sr (87Sr/86Sr?=?0.7075) and depletion in 13C (with ?13C as low as ?2.0‰). These differing textural and geochemical features for the two suites reflect interaction with fluids in closed and open systems, respectively. The higher-P-metamorphosed ophicarbonates show strong shear textures, with coexisting antigorite and dolomite, carbonate veins crosscutting prograde antigorite foliation and, in some cases, relics of magnesite-nodules enclosed in the foliation. These rocks are characterized by lower ?18O (+10.3 to 13.0‰), enrichment in radiogenic Sr (87Sr/86Sr up to 0.7096) and enrichment in incompatible and fluid-mobile element (FME; e.g., As, Sb, Pb). These data seemingly reflect interaction with externally-derived metamorphic fluids and the infiltrating fluids likely were derived from dehydrating serpentinites with hybrid serpentinite-sediment compositions. The interaction between these two lithologies could have occurred prior to or after dehydration of the serpentinites elsewhere. We suggest that decarbonation and dissolution/precipitation processes operating in ancient subduction zones, and resulting in the mobilization of C, are best traced by a combination of detailed field and petrographic observations, C, O and Sr isotope systematics (i.e., 3D isotopes), and FME inventories. Demonstration of such processes is key to advancing our understanding of the influence of subduction zone metamorphism on the mobilization of C in subducting reservoirs and the efficiency of delivery of this C to depths beneath volcanic arcs and into the deeper mantle.
Nature Communications, doi.org/10.1038/ s41467-020-18157-6 11p. Pdf
Mantle
carbon
Abstract: Magmatic systems play a crucial role in enriching the crust with volatiles and elements that reside primarily within the Earth’s mantle, including economically important metals like nickel, copper and platinum-group elements. However, transport of these metals within silicate magmas primarily occurs within dense sulfide liquids, which tend to coalesce, settle and not be efficiently transported in ascending magmas. Here we show textural observations, backed up with carbon and oxygen isotope data, which indicate an intimate association between mantle-derived carbonates and sulfides in some mafic-ultramafic magmatic systems emplaced at the base of the continental crust. We propose that carbon, as a buoyant supercritical CO2 fluid, might be a covert agent aiding and promoting the physical transport of sulfides across the mantle-crust transition. This may be a common but cryptic mechanism that facilitates cycling of volatiles and metals from the mantle to the lower-to-mid continental crust, which leaves little footprint behind by the time magmas reach the Earth’s surface.
Earth and Planetary Science Letters, Vol. 554, doi.org/10.1016/ j.epsl.2020. 116659 12p . Pdf
Mantle
carbon
Abstract: In the accreting Earth and planetesimals, carbon was distributed between a core forming metallic melt, a silicate melt, and a hot, potentially dense atmosphere. Metal melt droplets segregating gravitationally from the magma ocean equilibrated near its base. To understand the distribution of carbon, its partitioning between the two melts is experimentally investigated at 1.5-6.0 GPa, 1300-2000 °C at oxygen fugacities of ?0.9 to ?1.9 log units below the iron-wuestite reference buffer (IW). One set of experiments was performed in San Carlos olivine capsules to investigate the effect of melt depolymerization (NBO/T), a second set in graphite capsules to expand the data set to higher pressures and temperatures. Carbon concentrations were analyzed by secondary ionization mass spectrometry (SIMS) and Raman spectra were collected to identify C-species in the silicate melt. Partition coefficients are governed by the solubility of C in the silicate melt, which varies from 0.01 to 0.6 wt%, while metal melts contain ?7 wt% C in most samples. C solubility in the silicate melt correlates strongly with NBO/T, which, in olivine capsules, is mostly a function of temperature. Carbon partition coefficients DCmetal/silicate at 1.5 GPa, 1300-1750 °C decrease from 640(49) to 14(3) with NBO/T increasing from 1.04 to 3.11. For the NBO/T of the silicate Earth of 2.6, DCmetal/silicate is 34(9). Pressure and oxygen fugacity show no clear effect on carbon partitioning. The present results differ from those of most previous studies in that carbon concentrations in the silicate melt are comparatively higher, rendering C to be about an order of magnitude less siderophile, and the discrepancies may be attributed to differences in the experimental protocols. Applying the new data to a magma ocean scenario, and assuming present day mantle carbon mantle concentrations from 120 to 795 ppm, implies that the core may contain 0.4-2.6 wt% carbon, resulting in 0.14-0.9 wt% of this element for the bulk Earth. These values are upper limits, considering that some of the carbon in the modern silicate Earth has very likely been delivered by the late veneer.
Researchgate , DOI: 10.1017/ 9781108677950.007 26p. Pdf
mantle
carbon
Abstract: Significant investment in new capacities for experimental research at high temperatures and pressures have provided new levels of understanding about the physical properties of carbon in fluids and melts, including its viscosity, electrical conductivity, and density. This chapter reviews the physical properties of carbon-bearing melts and fluids at high temperatures and pressures and highlights remaining unknowns left to be explored. The chapter also reviews how the remote sensing of the inaccessible parts of the Earth via various geophysical techniques - seismic shear wave velocity, attenuation, and electromagnetic signals of mantle depths - can be reconciled with the potential presence of carbon-bearing melts or fluids.
Diamond & Related Materials, Vol. 113, 108284, 8p. Pdf
Global
carbon
Abstract: A new orthorhombic carbon crystal denoted oI20?carbon possessing the Immm space group was designed. Its structure is formed by stacking of a cage structure, which consists of 32 carbon atoms. Its stability and structural, mechanical and electronic properties were investigated by first-principles simulations. Density functional theory calculations show that this new carbon allotrope is thermodynamically stable (even more stable than synthesized T?carbon and supercubane). Ab initio molecular dynamics (AIMD) simulations show that it can maintain the structure above a temperature of 1000 K, indicating its excellent thermal stability. oI20?carbon can also maintain dynamic stability under a high pressure of 100 GPa. It is an anisotropic superhard material with a Vickers hardness of 46.62 GPa. Notably, the cage structure gives it a low density, which has a really small value among superhard carbon allotropes. In addition, it is worth noting that oI20?carbon has an indirect ultrawide band structure with a bandgap of 4.55 eV (HSE06), which is higher than that of most previously reported superhard carbon allotropes. All these outstanding properties show that it is a potential material for high-temperature, high-frequency electronic devices and the aerospace industry.
Geochimica et Cosmochimica Acta, Vol. 294. pp. 295-314. pdf
Canada
carbon
Abstract: The recent expansion of studies at hydrothermal submarine vents from investigation of abiotic methane formation to include abiotic production of organics such acetate and formate, and rising interest in processes of abiotic organic synthesis on the ocean-world moons of Saturn and Jupiter, have raised interest in potential Earth analogs for investigation of prebiotic/abiotic processes to an unprecedented level. The deep continental subsurface provides an attractive target to identify analog environments where the influence of abiotic carbon cycling may be investigated, particularly in hydrogeological isolated fracture fluids where the products of chemical water-rock reactions have been less overprinted by the biogeochemical signatures of the planet’s surficial water and carbon cycles. Here we report, for the first time, a comprehensive set of concentration measurements and isotopic signatures for acetate and formate, as well as the dissolved inorganic and organic carbon pools, for saline fracture waters naturally flowing 2.4?km below surface in 2.7 billion year-old rocks on the Canadian Shield. These geologically ancient fluids at the Kidd Creek Observatory were the focus of previous investigations of fracture fluid geochemistry, microbiology and noble gas-derived residence times. Here we show the fracture waters of Kidd Creek contain high concentrations of both acetate and formate with concentrations from 1200 to 1900?µmol/L, and 480 to 1000?µmol/L, respectively. Acetate and formate alone account for more than 50-90% of the total DOC - providing a very simple "organic soup". The unusually elevated concentrations and profoundly 13C-enriched nature of the acetate and formate suggest an important role for abiotic organic synthesis in the deep carbon cycle at this hydrogeologically isolated site. A variety of potential abiotic production reactions are discussed, including a radiolytically driven H, S and C deep cycle that could provide a mechanism for sustaining deep subsurface habitability. Scientific discoveries are beginning to reveal that organic-producing reactions that would have prevailed on Earth before the rise of life, and that may persist today on planets and moons such as Enceladus, Europa and Titan, can be accessed in some specialized geologic settings on Earth that provide valuable natural analog environments for the investigation of abiotic organic chemistry outside the laboratory.
Journal of Asian Earth Sciences, Vol. 214, 104772, 8p. Pdf
Mantle
carbon
Abstract: The temperature of the upper mantle was a principal factor controlling the style of plate tectonics and influencing magmatism and metamorphism on Earth over geological history. Recent studies emphasized that Earth’s tectonic style has transited into the modern plate tectonics since the late Neoproterozoic, which is characterized by a global network of plate boundaries with deep and cold oceanic plate subduction. However, the consequence of the establishment of modern plate tectonics to Earth’s mantle temperature and deep carbon cycle has not been fully understood. Here we apply statistical analysis on the geochemical data of continental igneous rocks and identify an increased magnitude of nephelinitic volcanism at the end of the Ediacaran. Nephelinitic rocks, a silica-undersaturated high-alkaline rock group, are mostly formed by low-degree melting of carbonated mantle sources. We link their widespread emergence with an enhanced mantle cooling event and a dramatically increased flux of crustal carbonates transporting to the mantle. The rapid cooling of the mantle was ascribed to the onset of modern-style plate tectonics with global-scale cold oceanic and continental subduction since the late Neoproterozoic. The declined upper-mantle temperature could not only favor the low-degree melting but also allow the subduction of carbonates into the deep mantle without decarbonation at shallow depth. Considering the high oxygen fugacity feature of the nephelinitic rocks and some other high-alkaline volcanism, the establishment of modern plate tectonics and thereafter enhanced mantle cooling and deep carbon cycle might contribute to the high-level atmospheric oxygen content during the Phanerozoic.
Abstract: The episode of widespread organic carbon deposition marked by peak black shale sedimentation during the Palaeoproterozoic is also reflected in exceptionally abundant graphite deposits of this age. Worldwide anoxic/euxinic sediments were preserved as a deep crustal reservoir of both organic carbon, and sulphur in accompanying pyrite, both commonly >1 wt %. The carbon- and sulphur-rich Palaeoproterozoic crust interacted with mafic magma to cause Ni-Co-Cu-PGE mineralization over the next billion years, and much uranium currently produced is from Mesoproterozoic deposits nucleated upon older Palaeoproterozoic graphite. Palaeoproterozoic carbon deposition has thus left a unique legacy of both graphite deposits and long-term ore deposition.
Abstract: Evaluating carbon’s candidacy as a light element in the Earth’s core is critical to constrain the budget and planet-scale distribution of this life-essential element. Here we use first principles molecular dynamics simulations to estimate the density and compressional wave velocity of liquid iron-carbon alloys with ~4-9 wt.% carbon at 0-360 gigapascals and 4000-7000 kelvin. We find that for an iron-carbon binary system, ~1-4 wt.% carbon can explain seismological compressional wave velocities. However, this is incompatible with the ~5-7 wt.% carbon that we find is required to explain the core’s density deficit. When we consider a ternary system including iron, carbon and another light element combined with additional constraints from iron meteorites and the density discontinuity at the inner-core boundary, we find that a carbon content of the outer core of 0.3-2.0 wt.%, is able to satisfy both properties. This could make the outer core the largest reservoir of terrestrial carbon.
Carbon in Earth's Interior, Geophysical Monograph , Vol. 249, Chapter 26, pp. 329- 12p. Pdf
Mantle
carbon
Abstract: The concept of a deep hydrocarbon cycle is proposed based on results of experimental modeling of the transformation of hydrocarbons under extreme thermobaric conditions. Hydrocarbons immersed in the subducting slab generally maintain stability to a depth of 50 km. With deeper immersion, the integrity of the traps is disrupted and the hydrocarbon fluid contacts the surrounding ferrous minerals, forming a mixture of iron hydride and iron carbide. This iron carbide transported into the asthenosphere by convective flows can react with hydrogen or water and form an aqueous hydrocarbon fluid that can migrate through deep faults into the Earth's crust and form multilayer oil and gas deposits. Other carbon donors in addition to iron carbide from the subducting slab exist in the asthenosphere. These donors can serve as a source of deep hydrocarbons that participate in the deep hydrocarbon cycle, as well as an additional feed for the general upward flow of the water-hydrocarbon fluid. Geological data on the presence of hydrocarbons in ultrabasites squeezed from a slab indicate that complex hydrocarbon systems may exist in a slab at considerable depths. This confirms our experimental results, indicating the stability of hydrocarbons to a depth of 50 km.
