Iron-Nickel-Sulfur-Carbon System Under High Pressure, With Implications To Earth’S Mantle

Abstract

Fe-Ni-S-C phases are accessory phases in the Earth’s mantle, but carry important geochemical and geophysical implications. According to their chemical behavior, Fe-Ni-S-C phases preferentially store siderophile and chalcophile elements (and potentially noble gases). Physically, Fe-Ni-S-C phases have distinctly higher densities, surface tensions, and electrical conductivities, and lower melting points than mantle silicates. Understanding the geochemical and geophysical impacts caused by Fe-Ni-S-C phases requires accurate quantification of the basic properties of Fe-Ni-S-C phases under mantle conditions. This PhD thesis uses both high-pressure experiments and thermodynamic calculations to constrain the melting temperatures and compositions of Fe-Ni-S-C phases in the Earth’s upper mantle mantle, and their potential for deep carbon storage. This study suggests that monosulfides in the upper mantle are mostly molten, even in significant portions of cratonic roots under continental geotherms. Incorporation of carbon depresses the monosulfide solidus by 50-100˚C. Experiments and calculations of reactions between Fe-Ni-S melts and silicates at mantle conditions suggest that Fe-Ni-S melts are Ni-rich (Ni/(Ni+Fe)~0.6) monosulfides ((Fe+Ni)/S~1 or Xs~0.5) under oxidized (FMQ -2 to FMQ) conditions at 2 GPa. With increasing depth in the mantle (thus decreasing fO2), Fe-Ni-S melts become increasingly Ni- and S-poor, characterized by Ni/(Ni+Fe)~0.4, (Fe+Ni)/S~3, and Xs~0。4 at 8 GPa, and Ni/(Ni+Fe)~0.2, Xs~0.05 and (Fe+Ni)/S~10 at 12 GPa. Carbon solubility in Fe-Ni-S melts determined by high-pressure experiments suggests that carbon solubility decreases exponentially with increasing Xs. Based on mantle Fe-Ni-S melt compositions, C-S relations in carbon-saturated melts, and the typical mantle P-T-fO2 profile and sulfur abundance (200 ppm), it is suggested that significant amounts (40-100%) of deep carbon could potentially be stored in Fe-Ni-S melts in the Earth’s reduced deep upper mantle.