The partitioning of Pt–Re–Os between solid and liquid metal in the Fe–Ni–S system at high pressure: Implications for inner core fractionation

Coupled 186Os/188Os and 187Os/188Os enrichments of plume-derived lavas have been suggested to reflect contributions of materials from the outer core (Brandon et al., 1998). This hypothesis is based on the assumption that the Earth’s liquid outer core has high Pt/Os and slightly high Re/Os ratios as a result of the crystallization of the solid inner core, and shows coupled enrichments in the 186Os/188Os and 187Os/188Os ratios, reflecting the decay of 190Pt and 187Re to 186Os and 187Os, respectively. Partitioning experiments of Pt–Re–Os between solid and liquid metal were performed at 5–20 GPa and 1250–1400 °C, to examine the effects of pressure in the Fe–Ni–S system. The ratios (DOs/DPt, DOs/DRe) of measured partition coefficients of Pt, Re and Os are almost constant with increasing pressure. DOs/DPt increases significantly, whereas DOs/DRe decreases, with increasing sulphur content in the liquid metal. On the basis of the present experimental results, it is unlikely that the required Pt–Re–Os fractionation is generated during inner core crystallization, assuming that the light element in the Earth’s core is sulphur.