Pressure dependence of metal-silicate partitioning explains the mantle phosphorus abundance.

Previous experiments performed below 20 GPa suggested that the metal/silicate partition coefficient of phosphorus (P), D P , extrapolated to typical high-pressure and -temperature conditions of the Earth's core formation gives too high P concentration in the core unless a large amount of silicon was included in metals. Here we examined D P between liquid metal and coexisting molten silicate at 27-61 GPa and 3820-4760 K, corresponding to conditions of core-forming metal segregation from silicate, by measuring recovered samples using a high-resolution imaging technique coupled with secondary ion mass spectrometry. The results demonstrate that the pressure dependence of D P changes from positive to negative above 15 GPa, likely because of an increase in the coordination number of P 5+ in silicate melt. With the present new partitioning data, the observed mantle P abundance may indicate ~ 0.2 wt% P in the core, consistent with the cosmo-/geochemical estimates, based on both single-stage and multi-stage core formation models without involving high amounts of silicon in metals.