Article

Experimental determination of carbon solubility in Fe-Ni-S melts

March 2018
Geochimica et Cosmochimica Acta 225

March 2018
225

DOI:10.1016/j.gca.2018.01.009

Authors:

J. Zhangzhou

Zhejiang University

Anette von der Handt

University of Minnesota Twin Cities

Marc Hirschmann

University of Minnesota Twin Cities

To investigate the effect of metal/sulfide and Ni/Fe ratio on the C storage capacity of sulfide melts, we determine carbon solubility in Fe-Ni-S melts with various (Fe + Ni)/S and Ni/Fe via graphite-saturated high-pressure experiments from 2–7 GPa and 1200–1600 °C. Consistent with previous results, C solubility is high (4–6 wt.%) in metal-rich sulfide melts and diminishes with increasing S content. Melts with near M/S = 1 (XS > 0.4) have <0.5 wt.% C in equilibrium with graphite. C solubility is diminished modestly with increased Ni/Fe ratio, but the effect is most pronounced for S-poor melts, and becomes negligible in near-monosulfide compositions. Immiscibility between S-rich and C-rich melts is observed in Ni-poor compositions, but above ∼18 wt.% Ni there is complete miscibility. Because mantle sulfide compositions are expected to have high Ni concentrations, sulfide-carbide immiscibility is unlikely in natural mantle melts. An empirical parameterization of C solubility in Ni-Fe-S melts as a function of S and Ni contents allows estimation of the C storage capacity of sulfide in the mantle. Importantly, as the metal/sulfide (M/S) ratio of the melt increases, C storage increases both because C solubility increases and because the mass fraction of melt is enhanced by addition of metal from surrounding silicates. Under comparatively oxidized conditions where melts are near M/S = 1, as prevails at <250 km depth, bulk C storage is <3 ppm. In the deeper, more reduced mantle where M/S increases, up to 200 ppm C in typical mantle with 200 ± 100 ppm S can be stored in Fe-Ni-S melts. Thus, metal-rich sulfide melts are the principal host of carbon in the deep upper mantle and below. Residual carbon is present either as diamond or, if conditions are highly reduced and total C concentrations are low, solid alloy.

Carbon in the deep upper mantle and transition zone under reduced conditions: Insights from high-pressure experiments and machine learning models

Article

Sep 2022
GEOCHIM COSMOCHIM AC

The storage of carbon in Earth’s mantle is an important consideration within the framework of the deep carbon cycle. In the deep (>250 km depth) reduced mantle, carbon storage mechanisms differ greatly from those in the oxidized shallow mantle. To investigate the stability of carbon-bearing phases in Earth’s deep mantle, we experimentally constrained compositional effects on phase stability in the Fe-Ni-S-C system at conditions relevant to the deep upper mantle and mantle transition zone. Our experiments suggest that carbide is absent at 10 GPa and 1450 °C in the Ni-poor (molar Ni/(Ni+Fe) = 0.2) portion of the metal-sulfide-carbon ternary, with carbon occurring as diamond or dissolved in the Fe-Ni-S-C melt. At 19 GPa and 1450 and 1600 °C, (Fe,Ni)7C3 saturates in the melt with C-rich (4.78–9.47 wt.%), S-poor (2.29–6.98 wt.%) bulk compositions. In comparison, Fe-Ni alloy only saturates with the C- and S-poor bulk composition 77.88 wt.% Fe, 19.47 wt.% Ni, 1.93 wt.% S, and 0.72 wt.% C. Based on these results, we trained machine learning models to predict carbon solubility in Fe-Ni-S-C melts. Compared to classical regression models, machine learning models significantly improve the accuracy of carbon solubility predictions. Combined, our experimental and machine learning results suggest that diamond and Fe-Ni-S-C melt are the primary hosts of carbon in the convecting deep upper mantle and throughout most of the mantle transition zone. In the deepest parts of the transition zone, however, carbide is likely to precipitate at adiabatic temperatures in C-rich mantle sources.

Preservation conditions of CLIPPIR diamonds in the earth’s mantle in a heterogeneous metal–sulphide–silicate medium (experimental modeling)

Article

Mar 2020

The genesis of CLIPPIR diamonds (Cullinan–like, large, inclusion–poor, pure, irregular, and resorbed) have attracted much interest due to their possible crystallization from metal melt in deep horizons of the earth’s mantle. These diamonds usually show a pronounced resorption and irregular morphology. The present paper reports new experimental data on the dissolution of diamond crystals at high P–T parameters in Fe–S melt containing large amounts of silicate components (5–20 wt%). The experiments were performed using a split–sphere multi–anvil apparatus (BARS) at a pressure of 4 GPa and a temperature of 1450 °C. The samples consisted of natural diamond crystals placed in mixtures of Fe, S, and kimberlite. Wide variations in dissolution rates of diamond crystals were obtained. The absence of diamond dissolution in a heterogeneous medium indicates that the amount of solid silicate phases present in metal melt plays a role in the preservation of diamonds. This study demonstrated how diamonds can be stored in natural environments due to the heterogeneity of the medium composition which could insulate diamonds from the metal–sulphide melt. The obtained results improve our understanding of processes that lead to preservation of CLIPPIR diamonds in the deep mantle.

INTERACTION OF CARBON-BASED REFRACTORIES WITH LIQUID PGM-FURNACE MELT

Thesis

Full-text available

Jan 2019

B.M. Thethwayo

The upper-sidewall of a typical PGM smelter is lined with synthetic graphite at the hot-face. Copper waffle coolers located behind the graphite extract the heat away from the refractory. The sufficient cooling provided by the cooled copper waffle coolers causes the process material to freeze at the hot-face of the graphite, forming a protective skull (freeze-lining). The protective skull retards the penetration of the process melt and gases through the graphite block. It is desired to extend the graphite block to the hot-face of the lower side-wall but the interaction between graphite and matte is not well understood. The principal aim of this work was to determine the prominent wear mechanism of graphite when in contact with a PGM furnace melt (slag and matte). Wettability tests and crucible tests were done at temperatures from 800 °C to 1550 °C to determine the effect temperature has on the interaction between graphite and primary PGM matte. The contact time was varied from 1 hour to 5 days, to determine the effect of exposure time on the wear of graphite by the primary PGM matte. Penetration and graphite dissolution were used as a measure of the compatibility of graphite with the liquid PGM matte. The melting behavior of pure sulfides (Cu2S, FeS, and Ni3S2) was assessed; the observed melting temperatures were in agreement with the published figures. Wettability of graphite by pure sulfides (Cu2S, FeS, and Ni3S2), synthetic matte and industrial PGM-furnace matte was determined using a sessile drop method. The interfacial contact-angle between graphite and all samples was >90°; therefore graphite is poorly wetted by all the tested materials except FeS, reactive wetting was observed between FeS and graphite. Synthetic matte did not penetrate through the graphite under all the tested conditions. The formation of the Fe-Ni alloy through the metallization of (Fe, Ni)S increased with operating temperature and contact time. Fe-Ni alloy dissolved up to 0.3 mass percent carbon, whereas the sulfide phase dissolved up to 0.03 mass percent carbon. The industrial-matte samples had silicates and oxide impurities. During the exposure of industrial matte to graphite, the impurities interacted with the matte and graphite, this lead to high consumption of graphite. The silicates were extracted to a sulfide phase according to the following reaction: [M]SiO3 + (Fe, Ni)S + 3C→ (Fe, Ni)Si + [M]S + 3CO This reaction caused severe wear of graphite owing to the formation of the CO gas; the forward reaction was favored by high temperature. Sulfides in their pure state were not corrosive towards graphite. The erosion of graphite at the slag/gas interface increased with exposure time. Penetration of industrial matte was observed only during the initial stages of melting, the penetration ceased as the contact time increased. The penetration of industrial matte through graphite was driven by the foaming of matte and the excess gas pressure that forced the liquid matte through the pores of the graphite wall. The crucible was cooled from one end to determine if the frozen skull would form on the graphite-matte interface. The matte formed a frozen skull at temperatures below 900 °C. Penetration of matte was not observed where the frozen skull had formed but at temperatures above 900 °C matte penetration occurred and no skull was observed on the graphite surface. The industrial PGM-slag did not wet graphite, the contact angle between graphite and slag samples was greater than 90°. The industrial PGM-slag was exposed to graphite using the crucible test method. Physical penetration of slag through the graphite wall was observed. Slag penetration was attributed to the slag foaming and gas generated during melting, excess gas pressure forced the slag into the graphite pores. Matte and slag were exposed in one graphite crucible to simulate the layout of the material in the industrial setup. Erosion of graphite was observed at the matte-slag interface. Graphite thickness of up to 1.5 mm was consumed after 12 hours of exposure. The prominent wear mechanism of graphite was the sulfidation of silicates, followed by the dissolution of graphite in metal and finally the physical penetration of graphite by matte and slag. The silicates and oxides were more reactive and corrosive towards graphite than the matte (sulfides). Cooled-graphite is currently used against slag in the industrial application; cooled-graphite has performed well at the slag-zone. Since the slag is more corrosive towards graphite than matte, it is envisaged that cooled-graphite can be used against a matte-layer if sufficient cooling is applied. Micropore-carbon was tested against matte for comparative reasons, to determine if micropore-carbon can out-perform graphite. Micropore-carbon reacted with both synthetic matte and industrial matte. A sulfide phase formed in the refractory matrix and the alumina and silicon compounds in the refractory dissolved into the matte. Micropore carbon cannot be used in contact with matte since it has poor resistance towards chemical attack by the matte.

Carbon storage in Fe-Ni-S liquids in the deep upper mantle and its relation to diamond and Fe-Ni alloy precipitation

Article

Jun 2019
EARTH PLANET SC LETT

Core-mantle fractionation of carbon in Earth and Mars: The effects of sulfur

Article

Jul 2018
GEOCHIM COSMOCHIM AC

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 (DCalloy/silicate). The results show that C solubility in the alloy liquid as well as DCalloy/silicate 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.

Early volatile depletion on planetesimals inferred from C–S systematics of iron meteorite parent bodies

Article

Full-text available

Mar 2021

Significance Habitable rocky worlds require a supply of essential volatile elements (C, H, N, and S). These are plentiful in early solar systems but depleted during processes leading to planet formation. Here, evidence for loss during differentiation of small precursor bodies (planetesimals) is derived from iron meteorites, which are samples of planetesimal cores. Reconstruction of the C and S contents of planetesimal cores indicates severe C depletions compared to inferred original planetesimal compositions. Modeling of depletion processes shows that preferential loss of C compared to S is transferred to cores during differentiation. Iron meteorites preserve evidence of a key devolatilization stage in the formation of habitable planets and suggest pervasive carbon loss is likely associated with the birth of terrestrial worlds.

Constraints on the Abundances of Carbon and Silicon in Mercury's Core From Experiments in the Fe‐Si‐C System

Article

Full-text available

May 2020

The composition of a planet's core has important implications for the thermal and magmatic evolution of that planet. Here, we conducted carbon (C) solubility experiments on iron‐silicon (Fe‐Si) metal mixtures (up to 35 wt% [~52 atom%] Si) at 1 GPa and 800–1800°C to determine the carbon concentration at graphite saturation (CCGS) in metallic melt and crystalline metal with varying proportions of Fe and Si to constrain the C content of Mercury's core. Our results, combined with those in the literature, show that composition is the major controlling factor for carbon solubility in Fe‐rich metal with minimal effects from temperature and pressure. Moreover, there is a strong anticorrelation between the abundances of carbon and silicon in iron‐rich metallic systems. Based on the previous estimates of <1–25 wt% Si in Mercury's core, our results indicate that a carbon‐saturated Mercurian core has 0.5–6.4 wt% C, with 6.4 wt% C corresponding to an Si‐free, Fe core and 0.5 wt% C corresponding to an Fe‐rich core with 25 wt% Si. The upper end of estimated FeO abundances in the mantle (up to 2.2 wt%) are consistent with a core that has <1 wt% Si and up to 6.4 wt% C, which would imply that bulk Mercury has a superchondritic Fe/Si ratio. However, the lower end of estimated FeO (≤0.05 wt%) supports CB chondrite‐like bulk compositions of Mercury with core Si abundances in the range of 5–18.5 wt% and C abundances in the range of 0.8–4.0 wt%.

Origin and Early Differentiation of Carbon and Associated Life-Essential Volatile Elements on Earth

Chapter

Full-text available

Oct 2019

Dissolution of synthetic diamonds to produce morphologies similar to natural diamonds: an experimental study

Article

Dec 2023

The results on dissolution of flat-faced synthetic diamond crystals of octahedral habit in an Fe-Ni-S melt at 4.0 GPa and 1 400°C are presented. It has been established that the resulting diamond morphology is similar to some natural kimberlitic diamonds and follows the particular sequence: flat-faced octahedron – laminar octahedron – trisoctahedroid with parallel striations in the <110> direction (“O1-D1”). Comparing the obtained results with earlier experimental works it is concluded that oxidisation of octahedral diamonds by means of ditrigonal etching layers and formation of tetrahexahedroid form is a result of diamond interaction with the fluidised kimberlite magma. We suggest that formation of octahedral diamonds with trigonal etching layers does not occur in kimberlite magma, and that diamonds of the O1-D1 morphological series avoided natural oxidation in kimberlite magma, but, like flat-faced octahedrons, were enclosed within xenoliths. Most probably, this dissolution process took place in the mantle prior to their capture by kimberlite. The results support an idea that metal-sulphide melts could be considered not only as a growth place for the world’s largest Cullinan-like diamonds found in South Africa (CLIPPIR type), but also as a mantle agent producing crystal morphologies typical for common kimberlitic diamonds: it depends upon carbon content in metal-sulphide melt – with supersaturation of the melt, the growth of diamond occurs while at the undersaturation conditions the dissolution begins.

Evaporation of moderately volatile elements from metal and sulfide melts: Implications for volatile element abundances in magmatic iron meteorites

Article

Full-text available

Sep 2023
EARTH PLANET SC LETT

Volatile element abundances in magmatic iron meteorites provide fundamental insights into the processing of volatile elements in the early solar system. Although Cu, Ge and Ag concentrations of magmatic iron meteorites deviate up to 4 log units between different magmatic iron meteorite groups, the role of evaporation on these volatile abundances is poorly constrained. Here, we for the first time experimentally assess the volatility of Cu, Ge, Ag, S, Cr, Co, Ni, Mo, Ru, Pd, W, Re and Ir from metal and sulfide melts as a function of pressure (10−4 and 1 bar), temperature (1573−1823 K) and time (5−120 min) for two end-member compositions (Fe versus FeS). These novel experiments demonstrate that the presence of S is a major parameter in establishing the volatility of Ge, Mo, Ag, Ru, W, Re and Ir. At constant P-T and time, the volatility of Ge, Mo, Ru, W, Re and Ir are greatly increased in the presence of S, whereas Ag is less volatile in the presence of S. At 1773 K and ~0.001 bar, the volatility of S is sufficiently high that the degassed FeS liquid showed immiscibility of a S-rich sulfide and a S-poor Fe melt. Combining the novel results allowed for deriving empirical equations that predict the volatility of Cu, Ge, Mo, Ag from Fe and/or FeS liquid as a function of temperature and time. A comparison of our volatility sequences with commonly applied 50% condensation temperature models shows that the latter models cannot be applied to sulfur-bearing Fe liquids and therefore magmatic iron meteorites. Application of our new models on previously derived depletions of the elements of interest in the IVB parent body shows that evaporation, if it occurred, cannot have taken place under S-rich conditions, as it would result in a depletion of Mo, which is not observed for the IVB irons. However, evaporation of a S-poor/S-free Fe liquid reproduces the observed volatility depletion trend for IVB irons under a wider range of temperature and evaporation times, demonstrating the importance of evaporative loss on the IVB parent body.

Morphological characteristics of diamond crystals arising as the result of dissolution in Fe-Ni-S MELT at high pressure

Article

Dec 2019

The first results on the dissolution of flat-faced diamond crystals of octahedral habit in Fe-Ni-S melt at 3,5 GPa and 1400 С are presented. It has been established that as a result of dissolution, flat-faceted diamond crystals are transformed into curve-faced individuals with morphological features similar to kimberlite diamonds. It is concluded that similar forms of natural diamonds could have been formed in reducing domains of the Earths mantle before entering the kimberlite magma.

Where did the largest diamonds grow? The experiments on percolation of Fe-Ni melt through olivine matrix in the presence of hydrocarbons

Article

Sep 2021
LITHOS

Recently it was found that large natural diamonds can grow from a metal liquid. One of the principal issues of the proposed hypothesis is the formation of so-called “pockets” filled with Fe-Ni melt and hydrocarbons in the Earth's mantle. The existing models of Fe migration imply percolation of liquid melt through interconnected interstices between silicate minerals, although these models face several fundamental problems in explaining the process of penetration of Fe melt between solid crystalline phases like silicate and oxide minerals. The aim of the present study is to contribute to the mechanism of Fe-Ni melt migration, and to elucidate the evolution of the “pockets” in the presence of hydrocarbons. The experiments were performed using a high-pressure apparatus “BARS” at pressures 3 and 5 GPa, and temperature 1600 °C. A silicate matrix consisting of natural olivine grains was used. The interstices in olivine were filled with anthracene that decomposes under high P-T into a complex hydrocarbon fluid. Percolation of Fe-Ni (64/36 wt%) melt through the interstices was demonstrated which occurred at relatively high rates. The basis of the proposed mechanism is “solubility-enhanced infiltration”: Fe-Ni occupies the space filled with light elements or substances that are soluble in the melt. It is suggested that the following simple, but efficient mechanism supports the growth of large diamonds as well as their resorption and storage within silicate mantle of the Earth for a long time.

Electrical Resistivity of Iron Phosphides at High‐Pressure and High‐Temperature Conditions With Implications for Lunar Core's Thermal Conductivity

Article

Full-text available

Jun 2019

Based on cosmochemistry evidence and element partitioning experiments, phosphorus is thought to be present in the iron‐rich cores of Earth and Moon. Phosphorus has a similar effect as silicon and sulfur on the electrical and thermal transport properties of iron at core conditions. However, the magnitude of the impurity scattering caused by phosphorus, the temperature dependence of iron phosphorus compounds, and the change across melting all have not been intensively investigated. We measured the electrical resistivity of Fe3P, Fe2P, and FeP using a four‐wire method at 1.3 to 3.2 GPa and temperatures up to 1800 K. We also identify the melting temperatures of FeP, Fe2P, and Fe3P by sudden changes in resistivity upon heating. The present experimental results demonstrate that phosphorus can enhance the electrical resistivity of iron more effectively than silicon. The resistivity of iron phosphides decreases with increasing pressures and decreasing phosphorus content. The resistivity of Fe‐P alloys obeys the Matthiessen's rule, which describes the positive linear correlation between resistivity and phosphorus content. This finding is comparable to previously observed atomic order‐disorder in Fe‐Si and Fe‐C systems. Furthermore, the resistivity of liquid Fe2P and Fe3P shows a negative linear correlation with temperatures. Different from pure iron, the calculated thermal conductivity of Fe3P increases by 33% upon melting. It is speculated that the thermal conductivity of the lunar solid inner core may be much lower than that of the liquid outer core when ordered iron light element compounds (e.g., Fe3C and Fe3P) are present in the solid core.

The phase diagram of the Fe-P binary system at 3 GPa and implications for phosphorus in the lunar core

Article

Apr 2019
GEOCHIM COSMOCHIM AC

Phosphorus is a potential candidate in the metallic core of the Moon. The phase diagram of the Fe-P binary system was investigated at the pressure of 3 GPa and temperatures of up to 1600 °C. Up to 3.0 wt% and 10.4 wt% phosphorus can dissolve in the solid iron and liquid Fe-P phases at 1100 °C and 3 GPa, respectively. The eutectic temperature on the iron-rich side was determined as 1085 °C at 3 GPa. The solubility of phosphorus in the iron decreases from ∼1.4 wt% at 1100 °C to ∼0.7 wt% at 1500 °C and 3 GPa. Structure of the solid iron in the quenched sample is the body-center cubic, corresponding to α-Fe phase. Extending the phosphorus solubility in the solid iron to the present lunar core conditions yields a maximum phosphorus concentration in a fully crystallized iron core of 0.85 ± 0.15 wt%. If there are Ni and C in the core, the value would be depressed to 0.4 ± 0.1 wt%. In addition, based on a simple siderophile mass balance between the bulk Moon (BM) and bulk silicate Moon (BSM) and a modeled phosphorus partition coefficient, D P-Mooncore/mantle (40–200) for the lunar magma ocean, a bulk silicate Earth-like P content (82 ± 8 ppm) in the initial Moon yields a lunar core with <0.3 wt% P. Some other potential light elements such as S and C could reduce the P content in the lunar core. Furthermore, the partition coefficient of phosphorus in the iron and liquid melt (D PSM/LM ) was found to be 0.10 ± 0.03 at 3 GPa. Taking the sulfur into account, the D PSM/LM increase to 0.18 ± 0.02 at 5 GPa in the S-rich liquid metal (∼8 wt%). In the case of a solid lunar inner core and S-bearing liquid outer core, their P contents were assessed to be less than 0.09 ± 0.01 wt% and 0.51 ± 0.01 wt%, respectively, when the lunar core's storage of P is <0.3 wt%. The moderate phosphorus solubility in the solid iron, combined with the assumption of abundant phosphorus in the bulk Moon, indicates that the phosphorus concentration in the lunar core could higher than previously thought.

Mercury

Chapter

Jan 2024

Metal Droplet Migration and Gathering in Semi-molten Sludge at Low Temperature Reduction

Article

Apr 2024

The stainless-steel pickling sludge contained chromium, nickel, and other metal elements, which not only increased the disposal costs but also wasted useful metal resources. The aim of the project was to find the factors that affected the metal droplets gathering from sludge during the semi-molten low temperature reduction. The reduced metals formed low-melting-point metal droplets after carburization. The result showed that the content of Fe, Cr, Ni, and C in metal alloy was 80.89%, 10.02%, 2.68%, and 5.41, respectively, over 1250 °C. The average time for the metal liquid in each microchannel gathering to form metal particles was 1.3*10−5 s based on the model of metal droplets gathering and growth. The compaction density of graphite–sludge mixture clumps and temperature had significant effect on metal droplets gathering and aggregation. The average size of metal particles was about 73 μm and the recovery degree of Fe, Cr, and Ni elements was 93.6%, 84.2%, and 91.8%, respectively, at 1300 °C as packing density of the sample was 0.53, respectively. The content of Cr in non-magnetic tailings was only 0.04%.

Investigation of Chemical Interaction and Melting Using Laser-Heated Diamond Anvil Cell

Chapter

Nov 2022

High pressure mineral physics is a field that has shaped our understanding of deep planetary interiors and revealed new material phenomena occurring at extreme conditions. Comprised of sixteen chapters written by well-established experts, this book covers recent advances in static and dynamic compression techniques and enhanced diagnostic capabilities, including synchrotron X-ray and neutron diffraction, spectroscopic measurements, in situ X-ray diffraction under dynamic loading, and multigrain crystallography at megabar pressures. Applications range from measuring equations of state, elasticity, and deformation of materials at high pressure, to high pressure synthesis, thermochemistry of high pressure phases, and new molecular compounds and superconductivity under extreme conditions. This book also introduces experimental geochemistry in the laser-heated diamond-anvil cell enabled by the focused ion beam technique for sample recovery and quantitative chemical analysis at submicron scale. Each chapter ends with an insightful perspective of future directions, making it an invaluable source for graduate students and researchers.

The Geochemical Legacy of Low-Temperature, Percolation-Driven Core Formation in Planetesimals

Article

Full-text available

Jul 2023
EARTH MOON PLANETS

Geoffrey Bromiley

Mechanisms for core formation in differentiated bodies in the early solar system are poorly constrained. At temperatures below those required to extensively melt planetesimals, core formation could have proceeded via percolation of metallic liquids. Although there is some geochemical data to support such ‘low-temperature’ segregation, experimental studies and simulations suggest that percolation-driven segregation might have only contributed to core formation in a proportion of fully-differentiated bodies. Here, the effects low-temperature core-formation on elemental compositions of planetesimal cores and mantles are explored. Immiscibility of Fe-rich and FeS-rich liquids will occur in all core-formation models, including those involving large fraction silicate melting. Light element content of cores (Si, O, C, P, S) depends on conditions under which Fe-rich and FeS-rich liquids segregated, especially pressure and oxygen fugacity. The S contents of FeS-rich liquids significantly exceed eutectic compositions in Fe–Ni–S systems and cannot be reconciled with S-contents of parent bodies to magmatic iron meteorites. Furthermore, there is limited data on trace element partitioning between FeS-rich and Fe-rich phases, and solid/melt partitioning models cannot be readily applied to FeS-rich liquids. Interaction of metallic liquids with minor phases stable up to low fraction silicate melting could provide a means for determining the extent of silicate melting prior to initiation of core-formation. However, element partitioning in most core-formation models remains poorly constrained, and it is likely that conditions under which segregation of metallic liquid occurred, especially oxygen fugacity and pressure, had as significant a control on planetesimal composition as segregation mechanisms and extent of silicate melting.

Experimental constraints on metal-silicate partitioning of chlorine and implications for planetary core formation

Article

Jun 2023
GEOCHIM COSMOCHIM AC

Partitioning of Ru, Pd, Ag, Re, Pt, Ir and Au between sulfide-, metal- and silicate liquid at highly reduced conditions: Implications for terrestrial accretion and aubrite parent body evolution

Article

Aug 2022
GEOCHIM COSMOCHIM AC

The abundances of highly siderophile elements (HSE) in planetary mantles and achondrites potentially provide important constraints on several aspects of planet formation, including the nature and composition of late accreted materials. Here, we experimentally and systematically assess the distribution of the HSE between silicate melts, sulfide and/or metal liquids at the highly to moderately reduced conditions thought to have characterized Earth accretion. The results show that the chalcophile behavior of all elements, except for Re, is strongly decreased at low FeO and/or high S concentrations in the silicate melt. There are considerable differences between how FeO and/or S contents of the silicate melt affect the D values of the various HSE, with the largest effects observed for Pd, Pt, Ir and Au. If liquid metal is Si-rich and S-poor, the siderophile behavior of the HSE mimics that in the presence of sulfide liquids, but with an offset due to differences in HSE activities in metal and sulfide liquids. Using our new experimental data, we quantify the relative effects of O in sulfide and S in silicate melt on the sulfide liquid-silicate melt partitioning behavior of the HSE using a thermodynamic approach. The resulting expressions were used to model the distribution of the HSE in highly reduced and differentiated EH- and EL chondritic parent bodies and during differentiation of the aubrite parent body. Our results show that even with their strongly decreased chalcophile and siderophile behavior at highly reduced conditions, HSE abundances in the mantles of these parent bodies remain extremely low. However, if such bodies accreted to Earth, any residual metal present in the parent body mantle and subsequently retained in Earth’s mantle would dramatically affect HSE abundances and produce chondritic ratios, making it impossible to track the potential accretion of a large reduced impactor to the BSE using HSE abundance systematics. In terms of the aubrite parent body, our results confirm previous hypotheses related to the importance of (un)differentiated core forming metals in establishing the HSE contents of unbrecciated aubrites. Finally, our results confirm that sulfides are likely a minor source of HSE abundances in aubrites, particularly for Re, consistent with sample observations.

Fate of Carbonates in the Earth’s Mantle (10-136 GPa)

Article

Full-text available

Mar 2022

Earth carbon cycle shapes the evolution of our planet and our habitats. As a key region of carbon cycle, subduction zone acts as a sole channel transporting supracrustal carbonate rocks down to the mantle, balancing carbon budget between the Earth’s surface and the interior, and regulating CO2 concentration of the atmosphere. How carbonates evolve at depth is thus, a most fundamental issue in understanding carbon flux and carbon sequestration mechanism in the Earth. This study reviews prominent progresses made in the field of crystal chemistry of carbonates along subduction geotherms. It clearly finds that, in addition to common carbonates in the Earth’s crust, several new polymorphs of carbonates have been discovered to be stable under high-pressure and high-temperature conditions. This opens possibilities for oxidized carbon species in the deep Earth. However, metamorphic decarbonatation and reduction reactions restrict subducting carbonates to the top-mid region of the lower mantle. Specifically, subsolidus decarbonatation in the form of carbonates reacting with silicates has been proposed as an efficient process releasing CO2 from slabs to the mantle. Besides, carbonate reduction in the metal-saturated mantle likely results in generation of super-deep diamonds and a considerable degree of carbon isotope fractionation. Review of these novel findings leads us to consider three issues in the further studies, including 1) searching for new chemical forms of carbon in the mantle, 2) determining the reduction efficiency of carbonates to diamonds and the accompanying carbon isotope fractionation and 3) concerning carbon cycle in subduction of continental crust.

P-V-T measurements of Fe3C to 117 GPa and 2100 K: Implications for stability of Fe3C phase at core conditions

Article

Aug 2021

We report the thermal Equation of State (EoS) of the non-magnetic Fe3C phase based on in situ X-ray diffraction (XRD) experiments to 117 GPa and 2100 K. High-pressure and temperature unit-cell volume measurements of Fe3C were conducted in a laser-heated diamond-anvil cell. Our pressure-volume-temperature (P-V-T) data together with existing data were fit to the Vinet equation of state with the Mie-Grüneisen-Debye thermal pressure model, yielding V0 = 151.6(12) Å3, K0 = 232(24) GPa, K0′= 5.09(46), γ0 = 2.3(3), and q = 3.4 (9) with θ0 = 407 K (fixed). The high-T data were also fit to the thermal pressure model with a constant αKT term, PTh = αKT(ΔT), and there is no observable pressure or temperature dependence, which implies minor contributions from the anharmonic and electronic terms. Using the established EoS for Fe3C, we made thermodynamic calculations on the P-T locations of the breakdown reaction of Fe3C into Fe7C3 and Fe. The reaction is located at 87 GPa and 300 K and 251 GPa and 3000 K. An invariant point occurs where Fe, Fe3C, Fe7C3, and liquid are stable, which places constraints on the liquidus temperature of the outer core, namely inner core crystallization temperature, as the inner core would be comprised by the liquidus phase. Two possible P-T locations for the invariant point were predicted from existing experimental data and the reaction calculated in this study. The two models result in different liquidus “phases” at the outer core-inner core boundary pressure: Fe3C at 5300 K and Fe7C3 at 3700 K. The Fe7C3 inner core can account for the density, as observed by seismology, while the Fe3C inner core cannot. The relevance of the system Fe-C to Earth’s core can be resolved by constructing a thermodynamic model for melting relations under core conditions as the two models predict very different liquidus temperatures.

Dissolution of Natural Octahedral Diamonds in an Fe–S Melt at High Pressure

Article

Nov 2020

Morphological Features of Diamond Crystals Resulting from Dissolution in a Fe–Ni–S Melt under High Pressure

Article

Dec 2019

Carbon in the Convecting Mantle

Chapter

Oct 2019

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.

An effect of reduced S-rich fluids on diamond formation under mantle-slab interaction

Article

Mar 2019
LITHOS

Experimental study, dedicated to understanding the effect of S-rich reduced fluids on the diamond-forming processes under subduction settings, was performed using a multi-anvil high-pressure split-sphere apparatus in Fe 3 C-(Mg,Ca)CO 3 -S and Fe ⁰ -(Mg,Ca)CO 3 -S systems at the pressure of 6.3 GPa, temperatures in the range of 900–1600 °C and run time of 18–60 h. At the temperatures of 900 and 1000 °C in the carbide-carbonate-sulfur system, extraction of carbon from cohenite through the interaction with S-rich reduced fluid, as well as C ⁰ -producing redox reactions of carbonate with carbide were realized. As a result, graphite formation in assemblage with magnesiowüstite, cohenite and pyrrhotite (±aragonite) was established. At higher temperatures (≥1100 °C) formation of assemblage of Fe ³⁺ -magnesiowüstite and graphite was accompanied by generation of fO 2 -contrasting melts - metal-sulfide with dissolved carbon (Fe-S-C) and sulfide-oxide (Fe-S-O). In the temperature range of 1400–1600 °C spontaneous diamond nucleation was found to occur via redox interactions of carbide or iron with carbonate. It was established, that interactions of Fe-S-C and Fe-S-O melts as well as of Fe-S-C melt and magnesiowüstite, were С ⁰ -forming processes, accompanied by disproportionation of Fe. These resulted in the crystallization of Fe ³⁺ -magnesiowüstite+graphite assemblage and growth of diamond. We show that a participation of sulfur in subduction-related elemental carbon-forming processes results in sharp decrease of partial melting temperatures (~300 °C), reducting the reactivity of the Fe-S-C melt relatively to Fe–C melt with respect to graphite and diamond crystallization and decrease of diamond growth rate.

Large gem diamonds from metallic liquid in Earth’s deep mantle

Article

Full-text available

Dec 2016
SCIENCE

Diamonds rock their metal roots Massive diamonds are rare, expensive, and captivating. These diamonds now appear to be distinctive not only in their size but also in their origin. Smith et al. probed mineral inclusions from these very large diamonds and found abundant slivers of iron metal surrounded by reducing gases. This suggests that the large diamonds grew from liquid metal in Earth's mantle. The inclusions also provide direct evidence of a long-suspected metal precipitation reaction that requires a more reducing mantle. Science , this issue p. 1403

Carbon and sulfur budget of the silicate Earth explained by accretion of differentiated planetary embryos

Article

Full-text available

Sep 2016

The abundances of volatile elements in the Earth's mantle have been attributed to the delivery of volatile-rich material after the main phase of accretion. However, no known meteorites could deliver the volatile elements, such as carbon, nitrogen, hydrogen and sulfur, at the relative abundances observed for the silicate Earth. Alternatively, Earth could have acquired its volatile inventory during accretion and differentiation, but the fate of volatile elements during core formation is known only for a limited set of conditions. Here we present constraints from laboratory experiments on the partitioning of carbon and sulfur between metallic cores and silicate mantles under conditions relevant for rocky planetary bodies. We find that carbon remains more siderophile than sulfur over a range of oxygen fugacities; however, our experiments suggest that in reduced or sulfur-rich bodies, carbon is expelled from the segregating core. Combined with previous constraints, we propose that the ratio of carbon to sulfur in the silicate Earth could have been established by differentiation of a planetary embryo that was then accreted to the proto-Earth. We suggest that the accretion of a Mercury-like (reduced) or a sulfur-rich (oxidized) differentiated body - in which carbon has been preferentially partitioned into the mantle - may explain the Earth's carbon and sulfur budgets.

Experimental constraints on mantle sulfide melting up to 8 GPa

Article

Full-text available

Jan 2016

We present high-pressure experiments up to 8 GPa that constrain the solidus and liquidus of a composition, Fe0.69Ni0.23Cu0.01S1.00, typical of upper mantle sulfide. Solidus and liquidus brackets of this monosulfide are parameterized according to a relation similar to the Simon-Glatzel equation, yielding, respectively, T (°C) = 1015.1 [P(GPa)/1.88 + 1]0.206 and T (°C) = 1067.3 [P(GPa)/1.19 + 1]0.149 (1 ≤ P ≤ 8). The solidus fit is accurate within ±15 °C over the pressure intervals 1–3.5 GPa and within ±30 °C over the pressure intervals 3.5–8.0 GPa. The solidus of the material examined is cooler than the geotherm for convecting mantle, but hotter than typical continental geotherms, suggesting that sulfide is molten or partially molten through much of the convecting upper mantle, but potentially solid in the continental mantle. However, the material examined is one of the more refractory among the spectrum of natural mantle sulfide compositions. This, together with the solidus-lowering effects of O and C not constrained by the present experiments, indicates that the experimentally derived melting curves are upper bounds on sulfide melting in the Earth’s upper mantle and that the regions where sulfide is molten are likely extensive in both the convecting upper mantle and, potentially, the deeper parts of the oceanic and continental lithosphere, including common source regions of many diamonds.

Carbon-saturated monosulfide melting in the shallow mantle: solubility and effect on solidus

Article

Full-text available

Nov 2015
CONTRIB MINERAL PETR

We present high-pressure experiments from 0.8 to 7.95 GPa to determine the effect of carbon on the solidus of mantle monosulfide. The graphite-saturated solidus of monosulfide (Fe0.69Ni0.23Cu0.01S1.00) is described by a Simon and Glatzel (Z Anorg Allg Chem 178:309–316, 1929) equation T (°C) = 969.0[P (GPa)/5.92 + 1]0.39 (1 ≤ P ≤ 8) and is ~80 ± 25 °C below the melting temperature found for carbon-free conditions. A series of comparison experiments using different capsule configurations and preparations document that the observed solidus-lowering is owing to graphite saturation and not an artifact of different capsules or hydrogen contamination. Concentrations of carbon in quenched graphite-saturated monosulfide melt measured by electron microprobe are 0.1–0.3 wt% in monosulfide melt and below the detection limit (<0.2 wt%) in crystalline monosulfide solid solution. Although there is only a small amount of carbon dissolved in monosulfide melts, the substantial effect on monosulfide solidus temperature means that the carbon-saturated monosulfide (Fe0.69Ni0.23Cu0.01S1.00) solidus intersects continental mantle geotherms inferred from diamond inclusion geobarometry at 6–7 GPa (~200 km), whereas carbon-free monosulfide (Fe0.69Ni0.23Cu0.01S1.00) solidus does not. The composition investigated (Fe0.69Ni0.23Cu0.01S1.00) has a comparatively low metal/sulfur (M/S) ratio and low Ni/(Fe + Ni), but sulfides with higher (M/S) and with greater Ni/(Fe + Ni) should melt at lower temperatures and these should have a broader melt stability field in the diamond formation environment and in the continental lithosphere. Low carbon solubility in monosulfide melt excludes the possibility that diamonds are crystallized from sulfide melt. Although monosulfide melt can store no more than 2 ppm C in a bulk mantle with 225 ppm S, melts with higher M/S could be a primary host of carbon in the deeper part of the upper mantle. For example, the storage capacity of C in sulfide melts in the deep upper mantle (~400 km) for a depleted mantle domain (MORB source, 120 ± 30 ppm S) is estimated to be $$57 \pm_{30}^{63}$$57±3063 ppm, and so all the C could be in a sulfide melt. In an enriched (OIB source, 225 ± 25 ppm S) mantle domain, the C stored in sulfide melt in the deep upper mantle is estimated to be $$86 \pm_{44}^{92}$$86±4492 ppm, which would amount to about half the available carbon.

Roebling Medal lecture. Trace element partitioning into sulfide: How lithophile elements become chalcophile and vice versa

Article

Full-text available

Nov 2015
AM MINERAL

Sulfides, although modally of low abundance in most igneous rocks, have strong influences on the geochemical behavior of many elements including Pb, Cu, Ni, and the PGEs. In a recent paper, we demonstrated a simple relationship between the sulfide-silicate partition coefficients DMsulf/sil of such elements and the FeO content of the coexisting silicate melt (Kiseeva and Wood 2013). log DMsulf/sil = A-n/2log[FeO] where n is the valency of element M, [FeO] is the concentration of FeO in the silicate in wt%, and A is a constant that depends on temperature and pressure. Elements that closely obey the simple model are Pb, In, Sb, Cd, Co, Zn, and Cr. We show here, however, that the fitted slope n depends not only on the valency of element M, but also on how M interacts with oxygen, the dissolution of which as FeO in the sulfide increases as the FeO content of the silicate melt increases. To take account of interactions of trace element M with FeO dissolved in the sulfide we intro- duce an additional parameter εFeOsulf (Wagner 1962), which represents the difference between the lithophile and chalcophile properties of M and those of Fe. If εFeOsulf is positive, then element M is more chalcophile than Fe and if negative more lithophile. We performed experiments to investigate partitioning of lithophile Nb, Ta, Ce, and Ti between sulfide and simplified basaltic melt and find that they all exhibit, as expected, concave upward behavior on a plot of logD vs. log[FeO]. New experiments on Cu at low FeO contents confirm that it is more chalcophile than Fe, yielding a concave-downward curve of logD vs. log[FeO]. The combined results mean that nominally lithophile elements may partition more strongly into sulfide than nominally chalcophile elements at either very low or very high FeO contents of the silicate melt. For example, as the FeO content of the silicate melt declines below about 1 wt%, the partition coefficient of Cu, DCsulf/sil declines to an unusually low value (DCsulf/sil ~80), whereas those for Nb (DNsublf/sil ~600), and rare earths (REE’s) increase strongly. Under these conditions, Nb is, therefore, substantially more “chalcophile” than Cu in that it partitions much more strongly into sulfide. The implications of these observations for the Earth are that under a wide range of conditions one would expect significant partitioning of REE, Nb, Ta, Ti, and other lithophile elements into sulfides. Wohlers and Wood (2015) have, for example, shown that the partitioning of U and the REE into sulfides at low FeO content of the silicate is sufficiently pronounced that addition of a reduced sulfur-rich body to the accreting Earth could generate observable fractionation of Nd from Sm and, together with S, transfer sufficient U to the core to provide a significant energy source for the geodynamo.

Nonstoichiometry and growth of some Fe carbides

Article

Full-text available

Sep 2013

Iron carbides containing from 31 to 17 atomic % carbon, with cohenite XRD structure and optical properties, were grown in experiments in Fe-Ni-S-C, Fe-Ni-C, and in Fe-C at 1, 6, and 7 GPa. X-ray cell volumes increase with C content. Compositions listed above vary considerably outside the nominal (Fe,Ni)3C stoichiometry of cohenite/cementite. Cohenites coexisting with Fe-C liquid are carbon poor. The Eckstrom-Adcock carbide, nominally Fe7C3, was found to show compositions from 29 to 36 atomic % C at 7 GPa in Fe-C. Both these materials are better regarded as solutions than as stoichiometric compounds, and their properties such as volume have compositional dependencies, as do the iron oxides, sulfides, silicides, and hydrides. The fraction of C dissolved in cohenite-saturated alloy is found to become smaller between 1 and 7 GPa. If this trend continues at higher pressures, the deep mantle should be easier to saturate with carbide than the shallow mantle, whether or not carbide is metastable as at ambient pressure. At temperatures below the cohenite-graphite peritectic, cohenite may grow as a compositionally zoned layer between Fe and graphite. The Eckstrom-Adcock carbide joins the assemblage at 7 GPa. Phases appear between Fe and C in an order consistent with metasomatic interface growth between chemically incompatible feed stocks. Diffusion across the carbide layer is not the growth rate limiting step. Carbon transport along the grain boundaries of solid Fe source stock at 1 GPa, to form C-saturated Fe alloy, is observed to be orders of magnitude faster than the cohenite layer growth. Growth stagnates too rapidly to be consistent with diffusion control. Furthermore, lateral variations in carbide layer thickness, convoluted inert marker horizons, and variable compositional profiles within the layers suggest that there are local transport complexities not covered by one-dimensional diffusive metasomatic growth. In contrast to many transport phenomena which slow with pressure, at 7 GPa and 1,162 °C, carbide growth without open grain boundaries is faster than at 1 GPa with fast grain boundary channels, again suggesting C transport is less of a constraint on growth than C supply. C supply at 7 GPa is enhanced by graphite metastability and the absence of fast grain boundary channels to divert C into the Fe instead of growing carbide. At both 1 and 7 GPa, the growth rate of carbide is found to systematically vary depending on which of two stock pieces of graphite are used to form the growth couple, suggesting that some property of each specific graphite, like C release rate, possibly from amorphous binder material, may influence the cohenite growth process. At temperatures near and above the cohenite-graphite peritectic at 1-1.5 GPa, complex intergrowths involving Fe-C liquids and extensive thermal migration transport were encountered, eroding the organized spatial resolution, and the range of cohenite compositions found grown below this peritectic from growth couples of crystalline Fe and graphite. The migration of graphite to a position in the metasomatic sequence between liquid and cohenite demonstrates that the solubility of graphite in liquid increases with temperature above the peritectic, whereas the solubility of graphite in cohenite below the peritectic decreases with temperature. The variable solubility of graphite in cohenite, shown by thermal migration, emphasizes that cohenite does have compositional variations.

Platinum-group element systematics and petrogenetic processing of the continental upper mantle: A review

Article

Full-text available

Apr 2013
LITHOS

The platinum-group element (PGE) systematics of continental mantle peridotites show large variability, reflecting petrogenetic processing of the upper mantle during partial melting and melt/fluid percolation inside the lithosphere. By removing Pd-Cu-Ni rich sulfides, partial melting events that have stabilized the sub-continental mantle lithosphere fractionated PPGEs (Palladium-group PGE; Pt, Pd) relative to IPGEs (Iridium-group PGE; Os, Ir, Ru, Rh). Residual base-metal sulfides (BMS) survive as enclosed IPGE-enriched Monosulfide Solid Solutions (Mss), which otherwise decompose into Ru-Os-Ir-rich refractory platinum-group minerals (PGMs) once the partial melts become S-undersaturated. The small-scale heterogeneous distribution of these microphases may cause extreme nugget effects, as seen in the huge variations in absolute PGE concentrations documented in cratonic peridotites. Magmas fluxing through the lithospheric mantle may change the initial PGE budgets inherited from the melting events, resulting in the great diversity of PGE systematics seen in peridotites from the sub-continental lithosphere. For instance, melt-rock reactions at increasing melt/rock ratios operate as open-system melting processes removing residual BMS/PGMs. Highly percolated peridotites are characterized by extreme PGE depletion, coupled with PGE patterns and Os-isotope compositions that gradually evolve toward that of the percolating melt. Reactions at decreasing melt-rock ratios (usually referred to as «mantle metasomatism») precipitate PPGE-enriched BMS that yield suprachondritic Pd/Ir and occasionally affect Pt/Ir and Rh/Ir ratios as well. Moreover, volatile-rich, small volume melts fractionate Os relative to Ir and S relative to Se, thereby producing rocks with supra-chondritic Os/Ir and S/Se coupled with supra-chondritic Pd/Ir and Pt/Ir. Major magmatic inputs at the lithosphere-asthenosphere boundary may rejuvenate the PGE systematics of the depleted mantle. Integrated studies of «refertilized» peridotites with worldwide provenance provide evidence for mixing between old PGM-rich harzburgitic protoliths and newly-precipitated BMS. Long-lived PGMs carry the Os-isotope compositions of ancient melt-depletion events into seemingly undepleted fertile lherzolites. Another diagnostic feature of major refertilization processes is the increasing modal abundance of Pt-Pd-Te-Bi or Pt-As-S microphases. Due to regional-scale refertilization processes, sizeable (> 100 km) domains of the upper lithospheric mantle are now significantly enriched in Pd, Au, Cu, Se, and other incompatible chalcophile elements that are of considerable importance in PGE-ore forming events.

Ratios of S, Se and Te in the silicate Earth require a volatile-rich late veneer

Article

Full-text available

Jul 2013
NATURE

The excess of highly siderophile (iron-loving) elements (HSEs) and the chondritic ratios of most HSEs in the bulk silicate Earth (BSE) may reflect the accretion of a chondritic 'late veneer' of about 0.5 per cent of Earth's mass after core formation. The amount of volatiles contained in the late veneer is a key constraint on the budget and the origin of the volatiles in Earth. At high pressures and temperatures, the moderately volatile chalcogen elements sulphur (S), selenium (Se) and tellurium (Te) are moderately to highly siderophile; thus, if depleted by core formation their mantle abundances should reflect the volatile composition of the late veneer. Here we report ratios and abundances of S, Se and Te in the mantle determined from new isotope dilution data for post-Archaean mantle peridotites. The mean S/Se and Se/Te ratios of mantle lherzolites overlap with CI (Ivuna-type) carbonaceous chondrite values. The Se/Te ratios of ordinary and enstatite chondrites are significantly different. The chalcogen/HSE ratio of the BSE is similar to that of CM (Mighei-type) carbonaceous chondrites, consistent with the view that the HSE signature of the BSE reflects a predominance of slightly volatile-depleted, carbonaceous-chondrite-like material, possibly with a minor proportion of non-chondritic material. Depending on the estimates for the abundances of water and carbon in the BSE, the late veneer may have supplied 20 to 100 per cent of the budget of hydrogen and carbon in the BSE.

High-pressure melting relations in Fe–C–S systems: Implications for formation, evolution, and structure of metallic cores in planetary bodies

Article

Full-text available

Nov 2009
GEOCHIM COSMOCHIM AC

We present new high-pressure temperature experiments on melting phase relations of Fe–C–S systems with applications to metallic core formation in planetary interiors. Experiments were performed on Fe–5wt% C–5wt% S and Fe–5wt% C–15wt% S at 2–6GPa and 1050–2000°C in MgO capsules and on Fe–13wt% S, Fe–5wt% S, and Fe–1.4wt% S at 2GPa and 1600°C in graphite capsules. Our experiments show that: (a) At a given P–T, the solubility of carbon in iron-rich metallic melt decreases modestly with increasing sulfur content and at sufficiently high concentration, the interaction between carbon and sulfur can cause formation of two immiscible melts, one rich in Fe-carbide and the other rich in Fe-sulfide. (b) The mutual solubility of carbon and sulfur increases with increasing pressure and no super-liquidus immiscibility in Fe-rich compositions is likely expected at pressures greater than 5–6GPa even for bulk compositions that are volatile-rich. (c) The liquidus temperature in the Fe–C–S ternary is significantly different compared to the binary liquidus in the Fe–C and Fe–S systems. At 6GPa, the liquidus of Fe–5wt% C–5wt% S is 150–200°C lower than the Fe–5wt% S. (d) For Fe–C–S bulk compositions with modest concentration of carbon, the sole liquidus phase is iron carbide, Fe3C at 2GPa and Fe7C3 at 6GPa and metallic iron crystallizes only with further cooling as sulfur is concentrated in the late crystallizing liquid. Our results suggest that for carbon and sulfur-rich core compositions, immiscibility induced core stratification can be expected for planets with core pressure less than ∼6GPa. Thus planetary bodies in the outer solar system such as Ganymede, Europa, and Io with present day core–mantle boundary (CMB) pressures of ∼8, ∼5, and 7GPa, respectively, if sufficiently volatile-rich, may either have a stratified core or may have experienced core stratification owing to liquid immiscibility at some stage of their accretion. A similar argument can be made for terrestrial planetary bodies such as Mercury and Earth’s Moon, but no such stratification is predicted for cores of terrestrial planets such as Earth, Venus, and Mars with the present day core pressure in the order ⩾136GPa, ⩾100GPa, and ⩾23GPa. (e) Owing to different expected densities of Fe-rich (and carbon-bearing) and sulfur-rich metallic melts, their settling velocities are likely different; thus core formation in terrestrial planets may involve rain of more than one metallic melt through silicate magma ocean. (f) For small planetary bodies that have core pressures

Carbon solubility in mantle minerals

Article

Full-text available

May 2006
EARTH PLANET SC LETT

The solubility of carbon in olivine, enstatite, diopside, pyrope, MgAl2O4 spinel, wadsleyite, ringwoodite, MgSiO3–ilmenite and MgSiO3–perovskite has been quantified. Carbon-saturated crystals were grown from carbonatite melts at 900–1400 °C and 1.5 to ∼ 26 GPa in piston cylinder or multi-anvil presses using carbon enriched to >99% in the 13C isotope. In upper mantle silicates, carbon solubility increases as a function of pressure to a maximum of ∼ 12 ppm by weight in olivine at 11 GPa. No clear dependence of carbon solubility on temperature, oxygen fugacity or iron content was observed. The observation that carbon solubility in olivine is insensitive to oxygen fugacity implies that the oxidation state of carbon in the carbonatite melt and in olivine is the same, i.e., carbon dissolves as C4+ in olivine. Carbon solubility in spinel MgAl2O4, transition zone minerals (wadsleyite and ringwoodite), MgSiO3–ilmenite and MgSiO3–perovskite are below the limit of detection of our SIMS-based analytical technique (i.e., below 30–200 ppb by weight). The differences in carbon solubilities between the various minerals studied appear to correlate with the polyhedral volume of the Si4+ site, consistent with a direct substitution of C4+ for Si4+. These results show that other, minor carbon-rich phases, rather than major, nominally volatile-free minerals, dominate the carbon budget within the bulk Earth's mantle. A significant fraction of total carbon could only be stored in silicates in a thin zone in the lowermost upper mantle, just above the transition zone, and only if the bulk carbon content is at the lower limit of published estimates. The carbon budget of the remaining mantle is dominated by carbonates and possibly diamond. The low melting point of carbonates and the high mobility of carbonate melts suggest that carbon distribution in the mantle may be highly heterogeneous, including the possibility of massive carbon enrichments on a local scale, particularly in the shallow subcontinental mantle.

Experimental Evidence for a Reduced Metal-saturated Upper Mantle

Article

Full-text available

Mar 2011

The uppermost mantle as sampled by xenoliths, peridotite massifs and primitive basaltic melts appears to be relatively oxidized, with oxygen fugacities between the magnetite-wustite and fayalite-ferrosilite-magnetite equilibria. Whether this range in oxygen fugacity is a shallow mantle signature or representative of the entire upper mantle still is unclear and a matter of debate because mantle regions deeper than 200 km are not well sampled. To constrain the redox state of the deeper upper mantle, we performed experiments from 1 to 14 GPa and 1220 to 1650 degrees C on a model peridotite composition, encompassing the convecting asthenospheric mantle down to the Transition Zone at 410 km depth. The experiments were run in iron metal capsules to buffer fO(2) close to an oxygen fugacity about 0.5 log units below the iron-wustite equilibrium. Analysis of the experimental phases for ferric iron using electron energy loss spectroscopy reveals that at pressures higher than 7 GPa, subcalcic pyroxene and majoritic garnet incorporate appreciable amounts of ferric iron, even though at the experimental conditions they were in redox equilibrium with metallic iron. The major ferric iron carrier in the upper mantle is majoritic garnet, followed by subcalcic pyroxene. At around 8 +/- 1GPa, corresponding to similar to 250 +/- 30 km depth in the upper mantle, sufficient quantities of subcalcic pyroxene and majoritic garnet are stabilized that all the ferric iron thought to be present in fertile upper mantle (i.e. similar to 2000 ppm) can be accommodated in solid solution in these phases, even though they were synthesized in redox equilibrium with metallic Fe. Based on the results of the experiments, it can be stated that, on a global scale, an oxidized upper mantle near the fayalite-ferrosilite-magnetite equilibrium is the exception rather than the rule. More than 75 vol. % of the Earth's present-day mantle is likely to be saturated with metallic iron.

The oxidation state of the mantle and the extraction of carbon from Earth’s interior

Article

Full-text available

Jan 2013
NATURE

Determining the oxygen fugacity of Earth's silicate mantle is of prime importance because it affects the speciation and mobility of volatile elements in the interior and has controlled the character of degassing species from the Earth since the planet's formation. Oxygen fugacities recorded by garnet-bearing peridotite xenoliths from Archaean lithosphere are of particular interest, because they provide constraints on the nature of volatile-bearing metasomatic fluids and melts active in the oldest mantle samples, including those in which diamonds are found. Here we report the results of experiments to test garnet oxythermobarometry equilibria under high-pressure conditions relevant to the deepest mantle xenoliths. We present a formulation for the most successful equilibrium and use it to determine an accurate picture of the oxygen fugacity through cratonic lithosphere. The oxygen fugacity of the deepest rocks is found to be at least one order of magnitude more oxidized than previously estimated. At depths where diamonds can form, the oxygen fugacity is not compatible with the stability of either carbonate- or methane-rich liquid but is instead compatible with a metasomatic liquid poor in carbonate and dominated by either water or silicate melt. The equilibrium also indicates that the relative oxygen fugacity of garnet-bearing rocks will increase with decreasing depth during adiabatic decompression. This implies that carbon in the asthenospheric mantle will be hosted as graphite or diamond but will be oxidized to produce carbonate melt through the reduction of Fe(3+) in silicate minerals during upwelling. The depth of carbonate melt formation will depend on the ratio of Fe(3+) to total iron in the bulk rock. This 'redox melting' relationship has important implications for the onset of geophysically detectable incipient melting and for the extraction of carbon dioxide from the mantle through decompressive melting.

The composition of the Earth

Article

Full-text available

Mar 1995
CHEM GEOL

Compositional models of the Earth are critically dependent on three main sources of information: the seismic profile of the Earth and its interpretation, comparisons between primitive meteorites and the solar nebula composition, and chemical and petrological models of peridotite-basalt melting relationships. Whereas a family of compositional models for the Earth are permissible based on these methods, the model that is most consistent with the seismological and geodynamic structure of the Earth comprises an upper and lower mantle of similar composition, an FeNi core having between 5% and 15% of a low-atomic-weight element, and a mantle which, when compared to CI carbonaceous chondrites, is depleted in Mg and Si relative to the refractory lithophile elements.The absolute and relative abundances of the refractory elements in carbonaceous, ordinary, and enstatite chondritic meteorites are compared. The bulk composition of an average CI carbonaceous chondrite is defined from previous compilations and from the refractory element compositions of different groups of chondrites. The absolute uncertainties in their refractory element compositions are evaluated by comparing ratios of these elements. These data are then used to evaluate existing models of the composition of the Silicate Earth.The systematic behavior of major and trace elements during differentiation of the mantle is used to constrain the Silicate Earth composition. Seemingly fertile peridotites have experienced a previous melting event that must be accounted for when developing these models. The approach taken here avoids unnecessary assumptions inherent in several existing models, and results in an internally consistent Silicate Earth composition having chondritic proportions of the refractory lithophile elements at ∼ 2.75 times that in CI carbonaceous chondrites. Element ratios in peridotites, komatiites, basalts and various crustal rocks are used to assess the abundances of both non-lithophile and non-refractory elements in the Silicate Earth. These data provide insights into the accretion processes of the Earth, the chemical evolution of the Earth's mantle, the effect of core formation, and indicate negligible exchange between the core and mantle throughout the geologic record (the last 3.5 Ga).The composition of the Earth's core is poorly constrained beyond its major constituents (i.e. an FeNi alloy). Density contrasts between the inner and outer core boundary are used to suggest the presence (∼ 10 ± 5%) of a light element or a combination of elements (e.g., O, S, Si) in the outer core. The core is the dominant repository of siderophile elements in the Earth. The limits of our understanding of the core's composition (including the light-element component) depend on models of core formation and the class of chondritic meteorites we have chosen when constructing models of the bulk Earth's composition.The Earth has a bulk of ∼ 20 ± 2, established by assuming that the Earth's budget of Al is stored entirely within the Silicate Earth and Fe is partitioned between the Silicate Earth (∼ 14%) and the core (∼ 86%). Chondritic meteorites display a range of ratios, with many having a value close to 20. A comparison of the bulk composition of the Earth and chondritic meteorites reveals both similarities and differences, with the Earth being more strongly depleted in the more volatile elements. There is no group of meteorites that has a bulk composition matching that of the Earth's.

The deep carbon cycle and melting in Earth's interior

Article

Full-text available

Sep 2010
EARTH PLANET SC LETT

Carbon geochemistry of mantle-derived samples suggests that the fluxes and reservoir sizes associated with deep cycle are in the order of 1012–13 g C/yr and 1022–23 g C, respectively. This deep cycle is responsible for the billion year-scale evolution of the terrestrial carbon reservoirs. The petrology of deep storage modulates the long-term evolution and distribution of terrestrial carbon. Unlike water, which in most of the Earth's mantle is held in nominally anhydrous silicates, carbon is stored in accessory phases. The accessory phase of interest, with increasing depth, typically changes from fluids/melts → calcite/dolomite → magnesite → diamond/Fe-rich alloy/Fe-metal carbide, assuming that the mass balance and oxidation state are buffered solely by silicates. If, however, carbon is sufficiently abundant, it may reside as carbonate even in the deep mantle. If Earth's deep mantle is Fe-metal saturated, carbon storage in metal alloy and as metal carbide cannot be avoided for depleted and enriched domains, respectively. Carbon ingassing to the interior is aided by modern subduction of the carbonated oceanic lithosphere, whereas outgassing from the mantle is controlled by decompression melting of carbonated mantle. Carbonated melting at > 300 km depth or redox melting of diamond-bearing or metal-bearing mantle at somewhat shallower depth generates carbonatitic and carbonated silicate melts and are the chief agents for liberating carbon from the solid Earth to the exosphere. Petrology allows net ingassing of carbon into the mantle in the modern Earth, but in the hotter subduction zones that prevailed during the Hadean, Archean, and Paleoproterozoic, carbonate likely was released at shallow depths and may have returned to the exosphere. Inefficient ingassing, along with efficient outgassing, may have kept the ancient mantle carbon-poor. The influence of carbon on deep Earth dynamics is through inducing melting and mobilization of structurally bound mineral water. Extraction of carbonated melt on one hand can dehydrate the mantle and enhance viscosity; the presence of trace carbonated melt on other may generate seismic low-velocity zones and amplify attenuation.

Fe-Ni exchange between olivine and sulphide liquid: Implications for oxygen barometry in sulphide-saturated magmas

Article

Full-text available

Jan 2000
GEOCHIM COSMOCHIM AC

In order to better understand the behaviour of nickel in magmatic processes, we have measured the apparent equilibrium constant (KD) for the exchange of Fe and Ni between coexisting olivine and sulphide liquid at controlled oxygen and sulphur fugacities (fO2 = 10−8–10−10 and fS2 = 10−2–10−4) over the temperature range 1200 to 1400°C and with 5 to 50 wt.% nickel in the sulphide liquid. Measured values of KD are independent of temperature and sulphur fugacity, but increase linearly with the nickel content of the sulphide liquid, and follow a power-law increase with oxygen fugacity; behaviour that is consistent with previous measurements of KD under controlled conditions of fO2 and fS2. The variation of KD with melt nickel content and fO2 is most likely the result of nonideal mixing in the sulphide liquid, which results in a decrease in γNiS/γFeS with melt metal/sulphur ratio. As a consequence of the systematic dependence of KD on fO2, a new oxygen barometer is proposed for estimating oxygen fugacity in igneous rocks that were cosaturated in olivine and sulphide liquid.

Deep global cycling of carbon constrained by the solidus of anhydrous, carbonated eclogite under upper mantle conditions

Article

Full-text available

Oct 2004
EARTH PLANET SC LETT

We present partial melting experiments that constrain the near solidus phase relations of carbonated eclogite from 2 to 8.5 GPa. The starting material was prepared by adding 5 wt.% CO2 in the form of a mixture of Fe–Mg–Ca–Na–K carbonates to an eclogite from Salt Lake crater, Oahu, Hawaii and is a reasonable approximation of carbonated oceanic crust from which siliceous hydrous fluids have been extracted during subduction. Melt-present versus melt-absent conditions are distinguished based on textural criteria. Garnet and clinopyroxene appear in all the experiments. Between 2 and 3 GPa, the subsolidus assemblage also includes ilmenite±calcio-dolomitess±CO2, whereas above the solidus (1050–1075 °C at 3 GPa) calcio-dolomitic liquid appears. From 3 to 4.5 GPa, dolomitess is stable at the solidus and the near-solidus melt becomes increasingly dolomitic. The appearance of dolomite above 3 GPa is accompanied by a negative Clapeyron slope of the solidus, with a minimum located between 995 and 1025 °C at ca. 4 GPa. Above 4 GPa, the solidus rises with increasing pressure to 1245±35 °C at 8.5 GPa and magnesite becomes the subsolidus carbonate. Dolomitic melt coexists with magnesite+garnet+cpx+rutile along the solidus from 5 to 8.5 GPa.

Carbon solubility and core melts in a shallow magma ocean environment and distribution of carbon between the Earth’s core and mantle

Article

Full-text available

Sep 2008
GEOCHIM COSMOCHIM AC

The solubility of carbon in Fe and Fe–5.2 wt.% Ni melts, saturated with graphite, determined by electron microprobe analysis of quenched metal melts was 5.8 ± 0.1 wt.% at 2000 °C, 6.7 ± 0.2 wt.% at 2200 °C, and 7.4 ± 0.2 wt.% at 2410 °C at 2 GPa, conditions relevant for core/mantle differentiation in a shallow magma ocean. These solubilities are slightly lower than low-pressure literature values and significantly beneath calculated values for even higher pressures [e.g., Wood B. J. (1993) Carbon in the core. Earth Planet. Sci. Lett.117, 593–607]. The trend of C solubility versus temperature for Fe–5.2 wt.% Ni melt, within analytical uncertainties, is similar to or slightly lower (∼0.2–0.4 wt.%) than that of pure Fe. Carbon content of core melts and residual mantle silicates derived from equilibrium batch or fractional segregation of core liquids and their comparison with our solubility data and carbon content estimate of the present day mantle, respectively, constrain the partition coefficient of carbon between silicate and metallic melts, in a magma ocean. For the entire range of possible bulk Earth carbon content from chondritic to subchondritic values, of 10−4 to 1 is derived. But for ∼1000 ppm bulk Earth carbon, is between 10−2 and 1. Using the complete range of possible for a magma ocean at ∼2200 °C, we predict maximum carbon content of the Earth’s core to be ∼6–7 wt.% and a preferred value of 0.25 ± 0.15 wt.% for a bulk Earth carbon concentration of ∼1000 ppm.

Fe-Ni-S SYSTEM: II. A THERMODYNAMIC MODEL FOR THE TERNARY MONOSULFIDE PHASE WITH THE NICKEL ARSENIDE STRUCTURE.

Article

Feb 1987

A statistical thermodynamic model for ternary phases with the NiAs (B8//1) structure is presented. The model is formulated assuming the presence of metal vacancies and metal interstitials in the lattice. The model is used to describe the thermodynamic properties of solid solutions of iron sulfide and nickel sulfide. Good agreement is obtained between the calculated values and the numerous experimental data over wide ranges of composition and temperature.

CALCZAF, TRYZAF and CITZAF: The Use of Multi-Correction-Algorithm Programs for Estimating Uncertainties and Improving Quantitative X-ray Analysis of Difficult Specimens

Article

Aug 2013

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Two-component mantle melting-mixing model for the generation of mid-ocean ridge basalts: Implications for the volatile content of the Pacific upper mantle

Article

Nov 2015
GEOCHIM COSMOCHIM AC

We report major, trace, and volatile element (CO2, H2O, F, Cl, S) contents and Sr, Nd, and Pb isotopes of mid-ocean ridge basalt (MORB) glasses from the Northern East Pacific Rise (NEPR) off-axis seamounts, the Quebrada-Discovery-GoFar (QDG) transform fault system, and the Macquarie Island. The incompatible trace element (ITE) contents of the samples range from highly depleted (DMORB, Th/La ⩽ 0.035) to enriched (EMORB, Th/La ⩾ 0.07), and the isotopic composition spans the entire range observed in EPR MORB. Our data suggest that at the time of melt generation, the source that generated the EMORB was essentially peridotitic, and that the composition of NMORB might not represent melting of a single upper mantle source (DMM), but rather mixing of melts from a two-component mantle (depleted and enriched DMM or D-DMM and E-DMM, respectively). After filtering the volatile element data for secondary processes (degassing, sulfide saturation, assimilation of seawater-derived component, and fractional crystallization), we use the volatiles to ITE ratios of our samples and a two-component mantle melting-mixing model to estimate the volatile content of the D-DMM (CO2 = 22 ppm, H2O = 59 ppm, F = 8 ppm, Cl = 0.4 ppm, and S = 100 ppm) and the E-DMM (CO2 = 990 ppm, H2O = 660 ppm, F = 31 ppm, Cl = 22 ppm, and S = 165 ppm). Our two-component mantle melting-mixing model reproduces the kernel density estimates (KDE) of Th/La and 143Nd/144Nd ratios for our samples and for EPR axial MORB compiled from the literature. This model suggests that: (1) 78% of the Pacific upper mantle is highly depleted (D-DMM) while 22% is enriched (E-DMM) in volatile and refractory ITE, (2) the melts produced during variable degrees of melting of the E-DMM controls most of the MORB geochemical variation, and (3) a fraction (∼65 to 80%) of the low degree EMORB melts (produced by ∼1.3% melting) may escape melt aggregation by freezing at the base of the oceanic lithosphere, significantly enriching it in volatile and trace element contents. Our results are consistent with previously proposed geodynamical processes acting at mid-ocean ridges and with the generation of the E-DMM. Our observations indicate that the D-DMM and E-DMM have (1) a relatively constant CO2/Cl ratio of ∼57±8, and (2) volatile and ITE element abundance patterns that can be related by a simple melting event, supporting the hypothesis that the E-DMM is a recycled oceanic lithosphere mantle metasomatized by low degree melts. Our calculation and model give rise to a Pacific upper mantle with volatile content of CO2 = 235 ppm, H2O = 191 ppm, F = 13 ppm, Cl = 5 ppm, and S = 114 ppm.

The Behavior and Concentration of CO2 in the Suboceanic Mantle: Inferences from Undegassed Ocean Ridge and Ocean Island Basalts

Article

Sep 2015
LITHOS

In order to better determine the behavior of CO2 relative to incompatible elements, and improve the accuracy of mantle CO2 concentration and flux estimates, we determined CO2 glass and vesicle concentrations, plus trace element contents for fifty-one ultradepleted mid-ocean ridge basalt (MORB) glasses from the global mid-ocean ridge system. Sixteen contained no vesicles and were volatile undersaturated for their depth of eruption. Thirty-five contained vesicles and were slightly oversaturated, and so may not have retained all of their CO2. If this latter group lost some bubbles during emplacement, then CO2/Ba calculated for the undersaturated group alone is the most reliable and uniform ratio at 98 ± 10, and CO2/Nb is 283 ± 32. If the oversaturated MORBs did not lose bubbles, then CO2/Nb is the most uniform ratio within the entire suite of ultradepleted MORBs at 291 ± 132, while CO2/Ba decreases with increasing incompatible element enrichment.

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

Article

Feb 2015
EARTH PLANET SC LETT

Experimental determination of C, F, and H partitioning between mantle minerals and carbonated basalt, CO2/Ba and CO2/Nb systematics of partial melting, and the CO2 contents of basaltic source regions

Article

Feb 2015
EARTH PLANET SC LETT

A simple model for chalcophile element partitioning between sulphide and silicate liquids with geochemical applications

Article

Dec 2013
EARTH PLANET SC LETT

The stability of Fe-Ni carbides in the Earth's mantle: Evidence for a low Fe-Ni-C melt fraction in the deep mantle

Article

Feb 2014
EARTH PLANET SC LETT

Carbon solution and partitioning between metallic and silicate melts in a shallow magma ocean: Implications for the origin and distribution of terrestrial carbon

Article

Feb 2013
GEOCHIM COSMOCHIM AC

The origin of bulk silicate Earth carbon inventory is unknown and the fate of carbon during the early Earth differentiation and core formation is a missing link in the evolution of the terrestrial carbon cycle. Here we present high pressure (P)–temperature (T) experiments that offer new constraints upon the partitioning of carbon between metallic and silicate melt in a shallow magma ocean. Experiments were performed at 1–5 GPa, 1600–2100 °C on mixtures of synthetic or natural silicates (tholeiitic basalt/alkali basalt/komatiite/fertile peridotite) and Fe–Ni–C ± Co ± S contained in graphite or MgO capsules. All the experiments produced immiscible Fe-rich metallic and silicate melts at oxygen fugacity (fO2) between ∼IW-1.5 and IW-1.9. Carbon and hydrogen concentrations of basaltic glasses and non-glassy quenched silicate melts were determined using secondary ionization mass spectrometry (SIMS) and speciation of dissolved C–O–H volatiles in silicate glasses was studied using Raman spectroscopy. Carbon contents of metallic melts were determined using both electron microprobe and SIMS. Our experiments indicate that at core-forming, reduced conditions, carbon in deep mafic–ultramafic magmas may dissolve primarily as various hydrogenated species but the total carbon storage capacity, although is significantly higher than solubility of CO2 under similar conditions, remains low (<500 ppm). The total carbon content in our reduced melts at graphite saturation increases with increasing melt depolymerization (NBO/T), consistent with recent spectroscopic studies, and modestly with increasing hydration. Carbon behaves as a metal-loving element during core-mantle separation and our experimental varies between ∼4750 and ⩾150 and increases with increasing pressure and decreases with increasing temperature and melt NBO/T.

Effect of carbon, sulfur and silicon on iron melting at high pressure: Implications for composition and evolution of the planetary terrestrial cores

Article

Aug 2013
GEOCHIM COSMOCHIM AC

High-pressure melting experiments in the Fe–S–C ternary and Fe–S–Si–C quaternary systems have been conducted in the range of 3.5–20 GPa and 920–1700 °C in the multi-anvil press. The mutual solubility, melting relations, and crystallization sequences were systematically investigated with changes of pressure, temperature and bulk composition. Five starting materials of Fe(84.69 wt%)–C(4.35 wt%)–S(7.85 wt%), Fe(84.87 wt%)–C(2.08 wt%)–S(11.41 wt%), Fe(86.36 wt%)–C(0.96 wt%)–S(10.31 wt%), Fe(85.71 wt%)–C(0.33 wt%)–S(11.86 wt%) and Fe(82.95 wt%)–C(0.66 wt%)–S(13.7 wt%)–Si(2.89 wt%) were employed. For Fe(84.69 wt%)–C(4.35 wt%)–S(7.85 wt%), the first crystallized phase is Fe3C at 5 GPa and Fe7C3 at 10–20 GPa. For Fe(84.87 wt%)–C(2.08 wt%)–S(11.41 wt%), Fe3C is the stable carbide at subsolidus temperature at 5–15 GPa. For Fe(86.36 wt%)–C(0.96 wt%)–S(10.31 wt%) and Fe(85.71 wt%)–C(0.33 wt%)–S(11.86 wt%), the first crystallized phase is metallic Fe instead of iron carbide at 5–10 GPa. The cotectic curves in Fe–S–C ternary system indicate only a small amount of C is needed to form an iron carbide solid inner core with the presence of S. Experiments on Fe(82.95 wt%)–C(0.66 wt%)–S(13.7 wt%)–Si(2.89 wt%) showed that a small amount of C does not significantly change the closure pressure of miscibility gap compared with that in Fe–S–Si system. It is observed that S preferentially partitions into molten iron while a significant amount of Si enters the solid phase with temperature decrease. Meanwhile, the C concentration in the liquid and solid iron metal changes little with temperature variations. If S, C and Si partitioning behavior between molten iron and solid iron metal with temperature remains the same under Earth’s present core pressure conditions, the solid inner core should be iron dominated with dissolved Si. On the other hand, the liquid outer core will be S rich and Si poor. Moderate carbon will be evenly present in both solid and liquid cores. Based on our melting data in a multi-component system, no layered liquid core should exist in the Earth, Mars and Mercury.

The Fe–C system at 5 GPa and implications for Earth’s core

Article

Aug 2008
GEOCHIM COSMOCHIM AC

Earth’s core may contain C, and it has been suggested that C in the core could stabilize the formation of a solid inner core composed of Fe3C. We experimentally examined the Fe–C system at a pressure of 5GPa and determined the Fe–C phase diagram at this pressure. In addition, we measured solid metal/liquid metal partition coefficients for 17 trace elements and examined the partitioning behavior between Fe3C and liquid metal for 14 trace elements. Solid metal/liquid metal partition coefficients are similar to those found in one atmosphere studies, indicating that the effect of pressure to 5GPa is negligible. All measured Fe3C/liquid metal partition coefficients investigated are less than one, such that all trace elements prefer the C-rich liquid to Fe3C. Fe3C/liquid metal partition coefficients tend to decrease with decreasing atomic radii within a given period. Of particular interest, our 5GPa Fe–C phase diagram does not show any evidence that the Fe-Fe3C eutectic composition shifts to lower C contents with increasing pressure, which is central to the previous reasoning that the inner core may be composed of Fe3C.

The melting relation of the system, iron and carbon at high pressure and its bearing on the early stage of the Earth

Article

Oct 1993
GEOPHYS RES LETT

The melting relation of iron‐carbon system is studied to see how iron takes in or disgorges carbon at high pressure (up to 12GPa). The understanding on these points is closely related to the problem of the reservoir of carbon in the earth's interior. The results are: (1) Iron‐carbon system shows eutectic melting till at least 12GPa. (2) The eutectic composition is about 3 or 4 wt% of carbon, which does not vary very much with pressure. (3) The eutectic temperature slightly goes up as pressure increases at a rate of 7°C/GPa. This gradient is fairly lower than that of the melting temperature of silicates. Based on these results and other facts, the following scenario is inferred on the core formation during the early stage of the earth. Because carbon melts into iron forming eutectic system at low temperature, carbon within the accreted chondritic materials might be absorbed into iron melt near the surface of the magma ocean. The observation that the melting temperature of silicate goes up more rapidly as pressure increases than the eutectic temperature of iron‐carbon system, indicates that the temperature within the magma ocean is maintained higher than the melting temperature of iron‐carbon system. Therefore, the carbon‐bearing iron melt may sink into the deep interior of the earth without solidifying and disgorging carbon. After all, it is strongly suggested that carbon may settle with liquid iron forming core.

Solubility of Carbon in Sulfide Melts of the System Fe-Ni-S

Article

Jun 2001

The solubility of carbon in iron-nickel sulfide melts at 1673 K was studied in the entire range of compositions of the Fe-FeS-Ni3S2-Ni tetragon. The boundaries of the stratification area were determined in this system upon its being saturated with carbon. Lines of carbon isosolubility are plotted in the tetragon field outside the stratification area.

Phase Equilibria of Liquid Fe-S-C Ternary System

Article

Jan 1991

Phase equilibria of Fe-S-C ternary melt has been studied to establish fundamental knowledge on the copper removal from iron melt by sulfide fluxes. Measurements were made to clarify the solubility of carbon and the miscibility gap between iron and FeS melts in Fe-S-C system at the temperature range from 1 473 to 1 873 K. Thermodynamic analysis was tried by applying interstitial solution model. Activity of iron in liquid Fe-C binary alloy was determined by distributing iron between liquid iron and silver phases to determine the interaction parameter between iron and carbon in Fe-C melt. It was concluded that interstitial solution model was applicable to express thermodynamic relation in this system. Phase diagram and activity contours of constituents in Fe-S-C ternary melt were calculated by the model.

Mantle sulfide geobarometry

Article

May 1993
GEOCHIM COSMOCHIM AC

A method of oxygen and sulfur barometry is based on the equilibrium, 2 Fe2SiO4 + S2 = 2 FeS + Fe2Si2O6 + O2, as contained in the peridotite mineral assemblage olivine-orthopyroxene-MSS (monosulfide solid solution {Fe,Ni,Co,Cu} S-S2). The method was applied to spinel peridotite xenoliths in basalts from the Massif Central and Montferrier and to the Ariege (Pyrenees) orogenic peridotite massifs. Monosulfide solid solution compositions were taken exclusively from sulfide inclusions within silicates and are interpreted to represent subsolidus MSS formed during high P-T recrystallization and deformation of the peridotites. Calculated 's are consistent with spinel oxybarometry of shallow subcontinental mantle lithosphere but are more scattered. Scatter is due, at least in part, to nonrandom sectioning of multiphase sulfide grains. The 's, which are coupled to through the equilibrium above, average 1–2 log units above the quartz-fayalite-magnetite-pyrrhotite reference buffer. At such conditions, in primitive mantle containing 200–250 ppm S, at least 95% of that S would be present in MSS and 5% or less (as minor H2S) in CO2-H2O fluids.

Ultramafic Xenoliths: Clues to Earth's Late Accretionary History

Article

Nov 1986

John W. Morgan

Fourteen spinel lherzolites, for which extensive trace-element data are available, may be divided into three groups depending upon the percentage loss of basaltic partial melt; averages are slightly depleted, -7 percent melt (Ca/Sa greater than 0.09); moderately depleted, -13 percent (0.09 greater than Ca/Si greater than 0.06); and strongly depleted, -20 percent (0.06 greater than Ca/Si). Re abundances of the groups are correlated with percent depletion, and the intercept of 0.0071 x Cl chondrite corresponding to undepleted mantle is identical to that independently derived from (Os-187)/(Os-186) in osmiridium of known age. The distribution of Re in mafic and ultramafic rocks is apparently closely related to S and Se abundances. Lherzolite data suggest that the abundances of highly siderophile elements (Os, Re, Ir, Pd, Au) and chalcogenic elements (S, Se, Te) in a primary basaltic melt are significantly higher than those of average oceanic ridge basalts, but may be similar to those of trace-element-rich Indian Ocean ridge basalts. The estimated S content (1000 ppm) of a primary basaltic melt is compatible with experimentally measured S solubilities at high temperature and pressure. On the basis of lherzolite depletion and using published trace-element data for spinel lherzolites and ocean-ridge basalts, estimated abundances in ppb in pristine upper mantle are Os, 3.1; Re, 0.26; Ir, 3.4; Pd, 4.5; Au, 1.01; S, 200,000; Se, 57. The trace-element pattern closely resembles that of the CM2 chondrites, but not of the CO3, CV3 or H chondrites. The pattern is a reasonable match, except for Au, with Apollo-17 lunar breccias that contain a 'group 2' ancient meteoritic component, suggesting that rather similar objects bombarded earth and moon during the first 600 Myr of their history.

Quantitative Electron Probe Microanalysis of Carbon in Binary Carbides

Article

Jan 1984

Among the many problems connected with quantitative electron probe microanalysis of light elements (Z less than 10), there is one problem that has been neglected. That is the fact that intensity measurements for these elements can no longer be performed at the maximum of the emission peak. Instead, intensities have to be measured in an integral fashion - a direct consequence of the fact that in the K-ionization process of (for example) carbon the bonding electrons are involved, which leads to serious alterations in the shape of the C-K alpha peak, leading in turn to an absolute necessity for integral measurements. If this factor is neglected errors of 30-50% are made, depending on the type of carbon standard. In the present case of binary carbides the problem can be overcome by the introduction of so-called Area/Peak (A/P) factors for each carbide, which represent the ratio between the (true) Area k-ratio and the Peak k-ratio. The next major problem is the choice of a correction procedure for matrix effects. An effort has therefore been made to produce a new set of consistent co-efficients using the thin-film approximation of Duncumb and Melford. This correction program, based on the use of modified Gaussian phi ( rho z) curves, was finally used to convert the measured k-ratios into concentrations.

Sulfur in basaltic magmas

Article

May 1992
GEOCHIM COSMOCHIM AC

The concentration of S in basaltic magmas at 1 atm pressure is strongly dependent on temperature, the fugacities of oxygen ( f O 2 ) and sulfur ( f S 2 ), and bulk composition. Microprobe analyses of total S in rapidly quenched, submarine basalt glasses, used in conjunction with wet chemical analyses of Fe 2 O 3 / FeO and its relationship to f o 2 , allow direct calculation of f s 2 using an expression which relates dissolved sulfur content to sulfur fugacity. The relationship between S fugacity and dissolved S in a silicate liquid at 1 bar total pressure can be represented by the expression ln X s = a ln f s 2 - b ln f o 2 + c ln X FeO + d / T + e + f i X i , where X s is the mole fraction of dissolved S, a through f i are experimentally calibrated regression coefficients, and the summation is over melt components, i . Back calculation of the input data yields a standard error of 0.026 wt% S for a magma with 0.1 wt% of S. Prediction of the immiscible Fe-S-O liquid saturation surface for basaltic liquids in T - X - f o 2 - f s 2 space is made through consideration of the heterogeneous equilibrium 1/2 S 2( gas ) + FeO ( silicate melt ) = FeS ( sulfide melt ) + 1/2 O 2( gas ) , using standard state thermodynamic data. Data from natural basalt glasses demonstrate that during the differentiation and Fe enrichment of basaltic magmas, the increases in S content which are observed require the ratio f o 2 / f s 2 to decrease by 3 log 10 units over the temperature range 1260 to 1050°C. This decrease is equivalent to the enthalpy change of the above reaction. Application to basalts and gases from Kilauea volcano demonstrates that during ascent, S is depleted in the magma as a result of shallow effervescence, and the gases which are evolved are in equilibrium with the magma during fire fountaining.

Composition of the depleted mantle

Article

May 2004
GEOCHEM GEOPHY GEOSY

We present an estimate for the composition of the depleted mantle (DM), the source for mid-ocean ridge basalts (MORBs). A combination of approaches is required to estimate the major and trace element abundances in DM. Absolute concentrations of few elements can be estimated directly, and the bulk of the estimates is derived using elemental ratios. The isotopic composition of MORB allows calculation of parent-daughter ratios. These estimates form the ``backbone'' of the abundances of the trace elements that make up the Coryell-Masuda diagram (spider diagram). The remaining elements of the Coryell-Masuda diagram are estimated through the composition of MORB. A third group of estimates is derived from the elemental and isotopic composition of peridotites. The major element composition is obtained by subtraction of a low-degree melt from a bulk silicate Earth (BSE) composition. The continental crust (CC) is thought to be complementary to the DM, and ratios that are chondritic in the CC are expected to also be chondritic in the DM. Thus some of the remaining elements are estimated using the composition of CC and chondrites. Volatile element and noble gas concentrations are estimated using constraints from the composition of MORBs and ocean island basalts (OIBs). Mass balance with BSE, CC, and DM indicates that CC and this estimate of the DM are not complementary reservoirs.

The effect of Fe on olivine H2O storage capacity: Consequences for H2O in the Martian mantle

Article

Jul 2011

To investigate the influence of chemical composition on the behavior of H2O in Fe-rich nominally anhydrous minerals, and to determine the difference between H2O behavior in the martian and terrestrial mantles, we conducted high-pressure H2O storage capacity experiments employing a wide range of olivine compositions. Experiments were conducted with bulk compositions in the system FeO-MgO-SiO2-H2O with Mg no. [Mg no. = 100 x molar Mg/(Mg+Fe)] ranging between 50 and 100 at 3 GPa in a piston-cylinder and at 6 GPa in a multi-anvil apparatus. Experiments at 3 GPa were conducted at 1200 degrees C, with f(O2) buffered by the coexistence of Fe and FeO, and at 1300-1500 degrees C in unbuffered assemblies. Experiments at 6 GPa were conducted at 1200 degrees C without buffers. Experiments at 1200 degrees C produced olivine+orthopyroxene+hydrous liquid (liq), and higher T experiments produced olivine+liq. Additionally, we synthesized a suite of 7 olivine standards (Mg no. = 90) for low blank secondary ion mass spectrometry (SIMS) analysis of H in multi-anvil experiments at 3-10 GPa and 1250 degrees C, resulting in large (200-400 mu m) homogeneous crystals with 0.037 to 0.30 wt% H2O. Polarized Fourier transform infrared (FTIR) measurements on randomly oriented grains from the synthesis experiments were used to determine principal axis spectra through least-squares regression, and H contents were calculated from the total absorbance in the OH stretching region. Using these olivines as calibrants for SIMS analyses, the H contents of olivines and pyroxenes from the variable Mg no. experiments were measured by counting (OH)-O-16 ions. Ignoring any matrix effects owing to variation in Mg no., H contents of olivine and pyroxene increase linearly with decreasing Mg no. At 6 GPa and 1200 degrees C, olivine H contents increase from 0.05 to 0.13 wt% H2O (8360 to 23 900 H/10(6) Si) as olivine Mg no. decreases from 100 to 68, and at 3 GPa and 1200 degrees C olivine H contents increase from 0.017 to 0.054 wt% (278 to 10000 H/106 Si) as Mg no. decreases from 100 to 55. The partition coefficient for H between pyroxene and olivine, D-H(opx/ol), decreases from 1.05 at 3 GPa and 1200 degrees C to 0.61 at 6 GPa and 1200 degrees C. The storage capacity of Fe-rich olivines with compositions expected in the martian mantle is similar to 1.5 times greater than those in the terrestrial mantle, suggesting that the geochemical behavior of H2O in the mantles of the two planets are quite similar. If 50% of the K2O on Mars remains in its mantle (Taylor et al. 2006), then a similar or greater proportion of the H2O is also in the mantle. Given accretionary models of the total martian H2O budget (Lunine et al. 2003), this suggests concentrations of 100-500 ppm H2O in the martian mantle and 0.1-1.9 wt% H2O in primary martian basalts.

The Redox State of Earth's Mantle

Article

Apr 2008

Oxygen thermobarometry measurements on spinel peridotite rocks indicate that the oxygen fugacity at the top of the upper mantle falls within ±2 log units of the fayalite-magnetite-quartz (FMQ) oxygen buffer. Measurements on garnet peridotites from cratonic lithosphere reveal a general decrease in fo2 with depth, which appears to result principally from the effect of pressure on the controlling Fe3+/Fe2+ equilibria. Modeling of experimental data indicates that at approximately 8 GPa, mantle fo2 will be 5 log units below FMQ and at a level where Ni-Fe metal becomes stable. Fe-Ni alloy and an Fe2O3-garnet component will be formed as a result of the disproportionation of FeO, which is experimentally demonstrated through observations of high Fe3+/ΣFe ratios in minerals in equilibrium with metallic Fe. In the lower mantle, the favorable coupled substitution of Al and Fe3+ into (Fe,Mg)SiO3 perovskite results in very high perovskite Fe3+/ΣFe ratios in equilibrium with metallic Fe. As a result, the lower mantle sh...

Thermochemical Description of the Ternary Iron-Nickel-Sulfur System

Article

Oct 1987

A general approach combining experimental measurements of Gibbs energy and phase equilibria, thermodynamic modelling of phases and calculation of phase diagrams is used to study the ternary Fe–Ni–S system. A quasi-subregular model is used to describe the thermodynamic properties of the metal alloy phases at high temperatures, a statistical thermodynamic model for the monosulfide phase, an associated solution model for the metal-sulfur liquid and the disulfide phase is treated as a pseudo binary solid solution. Values of the solution parameters were obtained using thermodynamic and phase equilibrium data obtained in our own laboratory at Wisconsin as well as those reported in the literature. Agreement between the calculated and experimental values are quite good. Using the thermodynamic values for the phases, phase diagrams of the Fe–Ni–S system were calculated from the liquid phase down to 1173 K. A set of phase diagrams is presented as well as stability diagrams. Phase diagrams at temperatures lower than 1173 K were not calculated since there are not sufficient data to describe the thermodynamic properties of the (Fe, Ni)4S3, or (Fe, Ni)3S2, phase.

Effects of fO2, fS2, temperature, and melt composition on Fe-Ni exchange between olivine and sulfide liquid: Implications for natural olivine-sulfide assemblages

Article

Jul 2003
GEOCHIM COSMOCHIM AC

James M. Brenan

The apparent equilibrium constant for the exchange of Fe and Ni between coexisting olivine and sulfide liquid (KD = (XNiS/XFeS)liquid/(XNiSi12O2/XFeSi12O2)olivine; Xi = mole fraction) has been measured at controlled oxygen and sulfur fugacities (fO2 = 10−8.1 to 10−10 and fS2 = 10−0.9 to 10−1.7) over the temperature range 1200 to 1385°C, with 5 to 37 wt% Ni and 7 to 18 wt% Cu in the sulfide liquid. At log fO2 of −8.7 ± 0.1, and log fS2 of −0.9 to −1.7, KD is relatively insensitive to sulfur fugacity, but comparison with previous results shows that KD increases at very low sulfur fugacities. KD values show an increase with the nickel content of the sulfide liquid, but this effect is more complex than found previously, and is greatest at log fO2 of −8.1, lessens with decreasing fO2, and KD becomes independent of melt Ni content at log fO2 ≤ −9.5. The origin of this variation in KD with fO2 and fS2 is most likely the result of nonideal mixing of Fe and Ni species in the sulfide liquid. Such behavior causes activity coefficients to change with either melt oxygen content or metal/sulfur ratio, effects that are well documented for metal-rich sulfide melts.

Investigation of various SiC materials by means of EPMA/WDS techniques

Article

Dec 1994

The microstructure and compositions of SiC materials from different sources and processing routes were investigated by means of EPMA/WDS and image analysis techniques. The influence of various sources of errors like carbon contamination and spectrometers defocusing on the analysis has been assessed. The presence of dissolved sintering aids, or impurities, and their distributions were investigated by EPMA/WDS. In addition, inhomogeneities, porosity agglomerations and heterogeneous inclusions were found in almost all the SiC materials, which are known to influence the corrosion and mechanical behavior of the material. Quantification of secondary phase contents was performed by means of image analysis, EPMA and, when possible, by density measurement. All methods are affected by errors of difficult assessment. In particular, the EPMA/WDS technique has to handle the problem of non homogeneous volumes of analysis. Two quantitative approaches were attempted, both based on the averaging of many points. In the first, the beam was highly focused. In the second approach, large areas (from 10 to 50 μm in diameter) were illuminated. The errors and limits of these methods are discussed and the results compared.

Trace elements in sulfide inclusion from Yakutian diamonds

Article

Jul 1996

Sulfide inclusions in diamonds may provide the only pristine samples of mantle sulfides, and they carry important information on the distribution and abundances of chalcophile elements in the deep lithosphere. Trace-element abundances were measured by proton microprobe in >50 sulfide inclusions (SDI) from Yakutian diamonds; about half of these were measured in situ in polished plates of diamonds, providing information on the spatial distribution of compositional variations. Many of the diamonds were identified as peridotitic or eclogitic from the nature of coexisting silicate or oxide inclusions. Known peridotitic diamonds contain SDIs with Ni contents of 22–36%, consistent with equilibration between olivine, monosulfide solid solution (MSS) and sulfide melt, whereas SDIs in eclogitic diamonds contain 0–12% Ni. A group of diamonds without silicate or oxide inclusions has SDIs with 11–18% Ni, and may be derived from pyroxenitic parageneses. Eclogitic SDIs have lower Ni, Cu and Te than peridotitic SDIs; the ranges of the two parageneses overlap for Se, As and Mo. The Mo and Se contents range up to 700 and 300 ppm, respectively; the highest levels are found in peridotitic diamonds. Among the in-situ SDIs, significant Zn and Pb levels are found in those connected by cracks to diamond surfaces, and these elements reflect interaction with kimberlitic melt. Significant levels of Ru (30–1300 ppm) and Rh (10–170 ppm) are found in many peridotitic SDIs; SDIs in one diamond with wustite and olivine inclusions and complex internal structures have high levels of other platinum-group elements (PGEs) as well, and high chondrite-normalized Ir/Pd. Comparison with experimental data on element partitioning between crystals of monosulfide solid solution (MSS) and sulfide melts suggests that most of the inclusions in both parageneses were trapped as MSS, while some high-Cu SDIs with high Pd±Rh may represent fractionated sulfide melts. Spatial variations of SDI composition within single diamonds are consistent with growth histories shown by cathodoluminescence images, in which several stages of growth and resorption have occurred within magmatic environments that evolved during diamond formation.

The carbon isotope geochemistry of mantle xenoliths

Article

Oct 2002
EARTH-SCI REV

Peter Deines

Carbon occurs in mantle samples in several chemical, mineralogical and morphological forms. It has been observed as CO2, CH4 and CO in fluid inclusions, as carbonate, graphite, diamond, moissanite, solid solution in silicates, and organic compounds. The total carbon concentration reported for mantle xenoliths varies by four orders of magnitude from below 1 ppm to close to 10 000 ppm. About 40% of these samples contain less than 50 ppm, 70% less than 100 ppm and 95% less than 500 ppm C. Carbon with δ13C of about −5‰ has been identified as a major isotopic composition signature for the mantle (carbonatite and kimberlite carbonates, diamonds and volcanic CO2 exhalations); it is also observed in mantle xenoliths. However, there may also be a minor signature of C depleted in 13C (δ13C=−22‰ to −26‰). Such light carbon has been observed in the dissolution residue of mantle minerals (olivine, pyroxene) and rocks, C fractions that have been interpreted as C dissolved in silicates, in diamonds, graphite, carbide, and hydrocarbons which are thought to be indigenous to the mantle. The data on xenoliths from basalts indicate that their δ13C distribution is essentially bimodal with peaks at −5‰ and −25‰ although the geologic occurrence of this light carbon has not yet been clearly delineated. Xenoliths from both hotspot and non-hotspot volcanoes cover the whole C isotopic composition range observed in mantle xenoliths; however, on average, xenoliths from non-hotspot volcanoes contain isotopically lighter carbon. Xenoliths from kimberlites cover the whole isotopic composition range as well, but, on average, probably show the lowest degree of 13C depletion. In addition, the second, low δ13C, mode may occur just above −20‰, coincident with the low 13C mode of southern African diamonds. Differences in the C concentration and isotopic composition have been observed between mantle minerals. The data are too few, however, to support firm conclusions on their size, or on how systematic these differences might be. Chemically more fractionated xenoliths tend to have higher δ13C values than less chemically fractionated ones. Processes that have been considered to be responsible for the considerable δ13C range in mantle C include the subduction of organic material and degassing. The observations on mantle xenoliths do not provide support for either, but indicate that as yet unexplored thermodynamic isotope effects, probably involving dissolved C in minerals and SiC bonds, may be responsible for the observed mantle carbon isotope distribution. The occurrence of such isotope effects would help to understand a number of observations on the carbon isotope geochemistry of diamonds. In so far as mantle-degassing models have been based, in part, on the carbon isotopic composition and C/3He ratios, an understanding of the mantle carbon isotope geochemistry is essential to support or refute their validity. The xenolith data do not support degassing models based on the assumption of limited indigenous carbon isotope variability within the mantle, nor the supposition that all 13C depleted carbon is of surface origin. The relative proportions of mantle C's of differing isotope signature are not known; they will have to be established for well-founded C cycle models to be developed.

“Carbon in the core” revisited

Article

May 2009
PHYS EARTH PLANET IN

In order to investigate the existence of carbon in the core, we performed high-pressure melting experiments using a Kawai-type multi-anvil apparatus by two methods: (1) quench experiments up to 14 GPa and 2200 °C and (2) in situ X-ray diffraction experiments up to 29 GPa. From the quench experiments, carbon solubility in molten iron and liquidus phase relations in the Fe–C system was investigated. The carbon solubility in molten iron is 8.5 wt% at 5 GPa and 2000 °C, where graphite is the liquidus phase. At 10–14 GPa, the carbon solubility in molten iron coexisting with diamond is about 7 wt% at 2000 °C and is pressure insensitive. At 5 GPa, Fe3C (6.7 wt% C) is the liquidus phase below 1400 °C. Above 10 GPa, Fe7C3 (8.4 wt% C) appears as the liquidus phase below 1700 °C and Fe3C melts incongruently to liquid iron and Fe7C3 at temperatures below 1500 °C. From in situ X-ray diffraction observations, the incongruent melting of Fe3C was found to occur at least up to 29 GPa, and liquidus temperatures of the Fe3C composition were found to be 1825 and 1925 °C at 21.0 and 29.2 GPa, respectively. Based on our results, it is concluded that large amount of carbon could be incorporated into the Earth's core if the Earth forming material was rich in carbon. Our result also shows that Fe7C3 would be the first crystallizing phase from the liquid of the outer core, implying that Fe7C3 could be a potential constituent of the solid inner core.

Sulfide and whole rock Re–Os systematics of eclogite and pyroxenite xenoliths from the Slave Craton, Canada

Article

Jun 2009
EARTH PLANET SC LETT

We characterized single sulfides in eclogite and pyroxenite xenoliths from the Diavik kimberlites (central Slave Craton, Canada) with regard to their petrography, major-element composition and Re–Os isotope systematics. Together with trace-element and Re–Os isotope compositions of whole rocks, these data allow identification of the major Re–Os host phases and provide constraints on the origin(s) of sulfides in these samples.The majority of sulfide minerals contain 8 to 28 at.% Ni, with intragranular sulfides having on average significantly lower contents (~ 6 at.%) than intergranular sulfides (~ 12 at.%). These high Ni-sulfides are not in equilibrium with an eclogitic assemblage and were likely introduced from a peridotitic source subsequent to eclogite formation. In contrast, their Re–Os abundances and Re/Os ratios (average ~ 825 ppb, 190 ppb and 10, respectively) overlap those of primary eclogitic sulfides. These conflicting compositional characteristics may document open-system disequilibrium processes accompanying the introduction of sulfides into eclogites. The general association of high 187Os/188Os with high 187Re/188Os of sulfides in three low-temperature eclogite xenoliths suggests that the addition is not young. In contrast, sulfides in a high-temperature eclogite plot on a ~ 90 Ma errorchron with radiogenic initial 187Os/188Os, perhaps indicative of young introduction of sulfides from a deep enriched source.Sulfides in a single pyroxenite xenolith have Ni, Re and Os contents intermediate between pristine eclogitic and peridotitic sulfides, and correlated Re–Os isotope systematics defining an age of 1.84 ± 0.14 Ga with a radiogenic 187Os/188Osi (0.16 ± 0.01). The age and 187Os/188Osi are identical to those obtained for eclogitic sulfide inclusions in diamonds from Diavik, thus supporting a link between eclogite and pyroxenite formation.Several eclogite and pyroxenite whole rocks show evidence for addition of secondary sulfides, but many plot on Paleoproterozoic Re–Os age arrays – particularly so at low Re/Os – coincident with previously determined ages using Lu–Hf and Pb–Pb techniques. They may represent sulfide-poor varieties that did not suffer secondary sulfide addition and that may be best suited to yielding meaningful Re–Os ages.

Platinum-group element systematics in Mid-Oceanic Ridge Basaltic glasses from the Pacific, Atlantic and Indian Oceans

Article

May 2005
GEOCHIM COSMOCHIM AC

The concentrations of Ir, Ru, Pt and Pd have been determined in 29 Mid-Oceanic Ridge basaltic (MORB) glasses from the Pacific (N = 7), the Atlantic (N = 10) and the Indian (N = 11) oceanic ridges and the Red Sea (N = 1) spreading centers. The effect of sulfide segregation during magmatic differentiation has been discussed with sample suites deriving from parental melts produced by high (16%) and low (6%) degrees of partial melting, respectively. Both sample suites define positive and distinct covariation trends in platinum-group elements (PGE) vs. Ni binary plots. The high-degree melting suite displays, for a given Ni content, systematically higher PGE contents relative to the low-degree melting suite. The mass fraction of sulfide segregated during crystallization (Xsulf), the achievement of equilibrium between sulfide melt and silicate melts (Reff), and the respective proportions between fractional and batch crystallization processes (Sb) are key parameters for modeling the PGE partitioning behavior during S-saturated MORB differentiation. Regardless of the model chosen, similar sulfide melt/silicate melt partition coefficients for Ir, Ru, Pt and Pd are needed to model the sulfide segregation process, in agreement with experimental data. When corrected for the effect of magmatic differentiation, the PGE data display coherent variations with partial melting degrees. Iridium, Ru and Pt are found to be compatible in nonsulfide minerals whereas the Pd behaves as a purely chalcophile element. The calculated partition coefficients between mantle sulfides and silicate melts (assuming a PGE concentration in the oceanic mantle at ∼0.007 × CI-chondritic abundances) increase from Pd (∼103) to Ir (∼105). This contrasting behavior of PGE during S-saturated magmatic differentiation and mantle melting processes can be accounted for by assuming that Monosufide Solid Solution (Mss) controls the PGE budget in MORB melting residues whereas MORB differentiation processes involve Cu-Ni-rich sulfide melt segregation.

The effects of variable sources, processes and contaminants on the composition of northern EPR MORB (8–10°N and 12–14°N): Evidence from volatiles (H2O, CO2, S) and halogens (F, Cl)

Article

Nov 2006
EARTH PLANET SC LETT

New volatile (H2O, CO2, S), halogen (F, Cl) and trace-element data for selected fresh MORB glasses are reported from two geologically and geophysically well-studied regions on the East Pacific Rise (8–10°N and 12–14°N) with distinct differences in spreading rate and magma supply. Sample locations include on-axis and young off-axis eruptions, as well as off-axis fissures, abyssal hills and pillow mounds. H2O, F, S and trace-element concentrations increase with decreasing MgO content, displaying over-enriched liquid lines of descent consistent with combined fractional and in-situ crystallization. A negative correlation between CO2/Nb and MgO indicates simultaneous degassing and magma crystallization, while broadening of this correlation to lower CO2/Nb at constant MgO indicates shallow degassing and CO2 loss during magma transport to the seafloor.

The effect of titanium on the silica content and on mineral-liquid partitioning of mantle-equilibrated melts

Article

Jul 2001
GEOCHIM COSMOCHIM AC

High pressure, high temperature phase equilibria experiments were performed to investigate the effect of TiO2 on mineral-liquid equilibria for near natural olivine + orthopyroxene-saturated mafic liquids. At 1.2 GPa and 1360°C, and at 2.8 GPa and 1530°C, near-natural picritic basaltic liquids with TiO2 concentrations ranging from 0.4 to 22 wt.% were forced to be in equilibrium with olivine (ol) + orthopyroxene (opx) by adding appropriate quantities of San Carlos olivine with increasing TiO2. Titanium enrichment of the ol + opx saturated mafic liquids results in nearly pressure-independent linear decreases in SiO2 amounting to 0.60 ± 0.04 and 0.56 ± 0.05 wt.% SiO2 per wt.% TiO2 at 1.2 and 2.8 GPa, respectively. Olivine-liquid Fe-Mg KD decreases at 1.2 GPa from 0.33 at 0.4 wt.% TiO2 down to 0.22 at 19 wt.% TiO2. A similar trend, shifted to slightly higher KD values, is observed at 2.8 GPa.

Metal–silicate partitioning and constraints on core composition and oxygen fugacity during Earth accretion

Article

Jan 2008
GEOCHIM COSMOCHIM AC

We present the results of new partitioning experiments between metal and silicate melts for a series of elements normally regarded as refractory lithophile and moderately siderophile and volatile. These include Si, Ti, Ni, Cr, Mn, Ga, Nb, Ta, Cu and Zn. Our new data obtained at 3.6 and 7.7 GPa and between 2123 and 2473 K are combined with literature data to parameterize the individual effects of oxygen fugacity, temperature, pressure and composition on partitioning. We find that Ni, Cu and Zn become less siderophile with increasing temperature. In contrast, Mn, Cr, Si, Ta, Nb, Ga and Ti become more siderophile with increasing temperature, with the highly charged cations (Nb, Ta, Si and Ti) being the most sensitive to variations of temperature. We also find that Ni, Cr, Nb, Ta and Ga become less siderophile with increasing pressure, while Mn becomes more siderophile with increasing pressure. Pressure effects on the partitioning of Si, Ti, Cu and Zn appear to be negligible, as are the effects of silicate melt composition on the partitioning of divalent cations. From the derived parameterization, we predict that the silicate Earth abundances of the elements mentioned above are best explained if core formation in a magma ocean took place under increasing conditions of oxygen fugacity, starting from moderately reduced conditions and finishing at the current mantle–core equilibrium value.

Melting in the Fe–C system to 70 GPa

Article

Jun 2009
EARTH PLANET SC LETT

We determined high-pressure melting curves for Fe3C, Fe7C3 and the Fe–Fe3C eutectic using laser-heated diamond anvil cell techniques. The principal criterion for melting is the observation of plateaus in the temperature vs. laser power function, which is an expected behavior at isobaric invariant points (e.g. congruent, eutectic, or peritectic melting) as increased power provides the latent heat of melting. We verified this technique by reproducing the melting curves of well-studied congruently melting compounds at high pressure (Fe, Pt, FeS, Pb), and by comparison with melting determinations made using thermocouple-based large-volume press techniques. The incongruent melting curve of Fe3C measured to 70 GPa has an apparent change in slope at ~ 8 GPa, which we attribute to stabilization of Fe7C3 at the solidus and the creation of a P–T invariant point. We observe that Fe7C3 melts at higher temperatures than Fe3C between 14 and 52 GPa and has a steep P–T slope, and on this basis predicts an expanding field of Fe7C3 + liquid with pressure. The Fe–Fe3C eutectic melting curve measured to 70 GPa agrees closely with multi-anvil data and thermodynamic calculations. We also measured the eutectic composition as a function of pressure using an in situ X-radiographic imaging technique, and find a rapid drop in carbon in the eutectic composition above about 20 GPa, generally consistent with previous thermodynamic calculations, and predict that the eutectic lies close to pure iron by ~ 50 GPa. We use these observations to extrapolate phase relations to core-relevant pressures. Convergence of the Fe3C and Fe–Fe3C eutectic melting curves indicate that Fe3C is replaced at the solidus by Fe7C3 at ~ 120 GPa, forming another P–T invariant point and a new eutectic between Fe and Fe7C3. Thus, Fe3C is unlikely to be an important crystallizing phase at core conditions, whereas Fe7C3 could become an important crystallizing phase.

Experimental determination of carbon solubility in Fe-Ni-S melts

Abstract

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