The effect of bulk carbon on its core-mantle partitioning behavior

Abstract:

Segregation of carbon (C) into the cores is a primary process that may explain its depletion in the bulk silicate reservoirs of rocky bodies in the Solar System (e.g., 1–3). Most of the experimental work to determine DC-alloy/silicate has been conducted in graphite capsules, i.e., both alloy and silicate melts being saturated with graphite. As core forming alloy melts and silicate magma oceans (MOs) were not necessarily graphite saturated in all stages of accretion, it is not known whether graphite-saturated DC-alloy/silicate values are directly applicable to differentiation of rocky bodies which for most part probably occurred in graphite-undersaturated conditions, i.e., where carbon was a trace element and did not form an accessory phase. We experimentally determined graphite-undersaturated DC-alloy/silicate values in MgO capsules with variable amount of bulk C content between IW–6.35 and IW–2.59 at a fixed P (3 GPa)-T (1700 °C). An ultramafic (NBO/T = 1.23-1.72) and mildly hydrous (bulk H = 44-161 ppm) nature of the silicate melts causes anhydrous C species (CO3(2-) + CO) to dominate over a wide range (≥IW–4.5). This results in an increase in DC-alloy/silicate values with a decrease in fO₂ to IW–4.5 followed by a drop at more reducing conditions due to the formation of hydrated C species. Importantly, DC-alloy/silicate increases with increase in bulk C content of the system at a given fO₂. C dissolution in alloy melts shows higher deviations from Henry’s Law with increase in bulk C content of the system such that Henry’s Law is not applicable for C dissolution in alloy melts except for extremely low bulk C contents. Therefore, on one hand in rocky bodies where chondritic amount of C was available for core-mantle differentiation, almost all of the accreted C would be segregated into the cores (due to high DC-alloy/silicate values) resulting in C poor MOs. On another other hand, accretion of C-poor rocky bodies would result in lower DC-alloy/silicate values and significantly lower proportion of C being segregated into the cores.

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