Metal/silicate partitioning of Mn, Co, and Ni at high-pressures and high temperatures and implications for core formation in a deep magma ocean

Abstract

On melting of natural peridotite at about 30 GPa, magnesiowustite is the first liquidus phase and perovskite appears at slightly lower temperature than the liquidus, implying that these phases would be potential solid phases in a deep magma ocean. Thus, in order to assess the chemical equilibrium of core formation, partitionings of Mn, Ni and Co between molten iron (MI), silicate liquid (SL), and magnesiowustite (Mw) were investigated in the olivine-Fe and peridotite-Fe alloy systems within a MgO capsule at 20, 24, and 26 GPa at about 2600°C. With increasing pressure, Ni and Co become less siderophile and Mn becomes less lithophile. The exchange partition coefficient between molten iron and silicate liquid, K'(MI/SL), is definitely larger than that between molten iron and magnesiowustite, K'(MI/Mw), for both Ni and Co. Pressure decreases the latter more conspicuously than the former. Convergence of partition coefficents of Ni and Co between MI and SL suggested by Li and Agee [1996] was not observed in the present study. Comparing the K' values extrapolated to higher pressure with the partition coefficients of Ni and Co between the core and the mantle derived from mass balance calculation, it is implied that the core material would have been equilibrated with magnesiowustite and silicate liquid at the bottom of a magma ocean with a depth greater than 900 km. It is also suggested that the fraction of silicate liquid increases for the core segregation in the deeper magma ocean. As K's of Ni and Co between molten iron and perovskite have remarkably larger values than those of K'(MI/Mw) and K'(MI/SL), possible coexistence of this phase requires a significantly greater depth for the core seggregation.