Authors: Damanveer S. Grewal, Rajdeep Dasgupta, Sanath Aithala
Partitioning of carbon (C) into the cores of rocky protoplanets and planets is one of the primary causes of its depletion in their bulk silicate reservoirs. Most of the experimental studies that determined the alloy to silicate melt partition coefficient of carbon (DCalloy/silicate) have been conducted in graphite-saturated conditions. Because carbon is a minor element in all known protoplanetary and planetary cores, it is not known whether graphite-saturated DCalloy/silicate values are applicable to core-mantle differentiation in rocky bodies which likely occurred in C-poor conditions. In this study we experimentally determined DCalloy/silicate in MgO capsules with variable bulk C contents between oxygen fugacity (fO2) of IW–6.35 and IW–2.59 at a fixed P (3 GPa)-T (1700 °C). A mafic-ultramafic (NBO/T = 1.23-1.72) and mildly hydrous (bulk H = 44-161 ppm) nature of the silicate melts caused anhydrous C species (CO32− + CO) to dominate over a wider fO2 range (>IW–4.2) in comparison to previous studies. This resulted in an increase in DCalloy/silicate with decreasing fO2 from IW–2.6 to IW–4.2 followed by a drop at more reduced conditions due to the formation of C-H species. Importantly, DCalloy/silicate increases with increasing bulk C content of the system at a given fO2. Partitioning of C between alloy and silicate melts follows non-Henrian behavior (i.e., it depends on bulk C content) because the activity coefficient of C in the alloy melt (γCalloymelt) varies with C content in the alloy. Therefore, in addition to other intensive (P, T, fO2) and extensive variables (alloy and silicate melt compositions), DCalloy/silicate also depends on the bulk C content available during core-mantle differentiation. Consequently, previously determined DCalloy/silicate for C-rich alloys are not directly applicable for core-mantle differentiation in relatively C-poor magma oceans (MOs). Because the experiments from the present study more realistically simulate C-poor cores and mildly hydrous, mafic-ultramafic silicate MOs, our data can be used to more accurately predict C fractionation between MOs and cores in inner Solar System rocky bodies. Our study suggests that closed system MO-core equilibration should have led to less severe depletion of C in the silicate reservoirs of inner Solar System rocky bodies than previously predicted.
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