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

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)₂SiO₄], pyroxene [(Mg,Fe)SiO₃]) in a mantle with 200 ppmw S. With increasing depth and decreasing oxygen fugacity (f_O2), 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.