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

Stability of carbon phases in the upper mantle.
TitleCarbon storage in Fe-Ni-S liquids in the deep upper mantle and its relation to diamond and Fe-Ni alloy precipitation
Publication TypeJournal Article
Year of Publication2019
AuthorsZhang Z., Qin T., Pommier A., Hirschmann M.M
Date Published2019/08
Type of ArticleArticle
ISBN Number0012-821X
Accession NumberWOS:000474502200017
Keywordsalloy; constraints; Deep carbon; diamonds; Geochemistry & Geophysics; kaapvaal craton; mantle; melting relations; metal; metal saturation; Olivine; oxygen fugacity; pressure; sulfide inclusions; sulfide melt; sulfur; trace-elements

To better understand the role of sulfide in C storage in the upper mantle, we construct a thermodynamic model for Fe-Ni-S-C sulfide melts and consider equilibrium between sulfide melts, mantle silicates, Fe-Ni alloy, and diamond. The sulfide melt model is based upon previous parameterization of Fe-Ni-S melts calibrated at 100 kPa, which we have extended to high pressure based on volumetric properties of end-member components. We calculate the behavior of C in the sulfide melt from empirical parameterization of experimental C solubility data. We calculate the continuous compositional evolution of Fe-Ni sulfide liquid and associated effects on carbon storage at pressure and redox conditions corresponding to mantle depths of 60 to 410 km. Equilibrium and mass balance conditions were solved for coexisting Fe-Ni-S melt and silicate minerals (olivine [(Mg,Fe,Ni)(2)SiO4], pyroxene [(Mg,Fe)SiO3]) in a mantle with 200 ppmw S. With increasing depth and decreasing oxygen fugacity ( f(02)), the calculated melt (Fe+Ni)/S atomic ratio increases from 0.8-1.5 in the shallow oxidized mantle to 2.0-10.5 in the reduced deep upper mantle (>8 GPa), with Fe-Ni alloy saturation occurring at >10 GPa. Compared to previous calculations for the reduced deep upper mantle, alloy saturation occurs at greater depth owing to the capacity of sulfide melt to dissolve metal species, thereby attenuating the rise of Fe and Ni metal activities. The corresponding carbon storage capacity in the metal-rich sulfide liquid rises from negligible below 6 GPa to 8-20 ppmw at 9 GPa, and thence increases sharply to 90-110 ppmw at the point of alloy saturation at 10-12 GPa. The combined C storage capacity of liquid and solid alloy reaches 110-170 ppmw at 14 GPa. Thus, in the deep upper mantle, all carbon in depleted sources (10-30 ppmw C) can be stored in the sulfide liquid, and alloy and sulfide liquids host a significant fraction of the C in enriched sources (30-500 ppmw C). Application of these results to the occurrences of inferred metal-rich sulfide melts in the Fe-Ni-S-C system and inclusions in diamonds from the mantle transition zone suggests that oxidization of a reduced metal-rich sulfide melt is an efficient mechanism for deep-mantle diamond precipitation, owing to the strong effect of (Fe+Ni)/S ratio on carbon solubility in Fe-Ni-S melts. This redox reaction likely occurs near the boundary between oxidized subducted slabs and the reduced ambient peridotitic mantle. (C) 2019 Elsevier B.V. All rights reserved.


Thermodynamic calculations quantify bulk carbon storage capacity in the Earth’s upper mantle to a depth of 410 km. Thermodynamic modeling of the Fe-Ni-S-C system and comparison with experimental data suggest that elemental Fe and Ni species con-trol carbon solubility in sulfide melts. The reaction of Fe-Ni-S melts with silicate minerals along the convecting mantle P-T-fO2profile suggests that olivine transfers Fe and Ni to the equilibrated sulfide melt, resulting in an increase of the (Fe+Ni)/S ratio and a decrease in the Ni/(Ni+Fe) ratio in the sulfide melt with increasing depth. C storage in sulfide melt is small in the shallow upper mantle but becomes significant for pressures above 8 GPa. At >10GPa, alloy saturates from the Fe-Ni-S liquid because of high metal activity. The alloy precipitation depth depends on the mantle P-T-fO2pro-file and the carbon present in the mantle source. Fe-Ni-S liquid is the principal host of carbon in the reduced deep upper mantle, with maximum bulk carbon storage capacity at 110-150 ppmw. As a result, all carbon in the depleted mantle (estimated to be 10-30 ppmw C) is stored in sulfide, as will much of the carbon in C-enriched domains with 30-500 ppmw C, with the excess be-ing in diamond. We suggest that redox reactions at the boundary of subducted slabs represent a likely mechanism for changing the (Fe+Ni)/S composition of metal-rich sulfide liquids and precipitat-ing deep diamonds.

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