Siderophile Element Partitioning between Cohenite and Liquid in the Fe-Ni-S-C System and Implications for Geochemistry of Planetary Cores and Mantles
We experimentally investigated the effects of pressure and S content on partition coefficients (D) between crystalline cohenite and liquid in the Fe–Ni–S–C system. Compositions with S contents of 0, 4.72, and 14.15 wt.%, in an Fe-rich mix containing a constant C (4.72 wt.%), Ni (5.23 wt.%), and W, Re, Os, Pt, and Co (totaling 0.43 wt.%) were equilibrated at 1150 °C and 3 and 6 GPa. Our cohenite–melt D data are compared to literature Fe–Ni–S and Fe–Ni–C experiments involving a crystalline phase of Fe. There is a change in D when the solid is cohenite rather than crystalline iron. Compared to solid-Fe/melt Ds, cohenite/melt Ds are lower for all elements except W.
The light element (S + C) content of the liquid influences partitioning between cohenite and liquid as it also does between crystalline Fe and liquid. The controls are similar but not identical. In the cohenite-bearing experiments, DNi decreases as S + C increases. Ni is excluded from the crystallizing solid if the solid is cohenite. And yet, in the Fe–Ni–S–C system, cohenite is stable to higher P than in the Fe–S–C system. As in the Fe-metallic liquid systems the non-metal avoidance principle of Jones and Malvin (1990) is applicable to the Fe3C-metallic liquid system studied here.
Carbon in the mantle is expected to combine with neutral Fe to replace metal with cohenite. In which case, the budget of highly siderophile elements is likely to be accommodated by Fe–Ni–S–C liquids once they form upon incipient melting because most Ds for cohenite/liquid are <1 in this system. The crystallization of cohenite from such liquids in Earth’s core is unlikely to improve the case for core–mantle interaction based on Pt–Re–Os systematics.
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- Geochimica et Cosmochimica Acta