Metal-silicate equilibrium in a homogeneously accreting earth: New results for Re

The accretion and early differentiation of the Earth is the starting point of earth history. The abundances of metal-seeking (siderophile) elements in the mantle are a powerful probe of those events. It has long been known that siderophile element abundances in the Earth's mantle are too high to have resulted from metal-silicate equilibrium at near surface conditions. This mismatch provided support for the idea that the Earth accreted heterogeneously. We report new experimental results for the highly siderophile element Re and show that metal-silicate partition coefficients decrease with increasing temperature (at fixed pressure and relative oxygen fugacity). Calculations using these and previously published experimental results indicate that the abundances of the moderately siderophile elements Fe, Ni, Co, Mo, W, P and, most importantly, the highly siderophile element Re, in Earth's upper mantle are consistent with early equilibration between metal and silicate liquid at the base of a deep (800-1000 km) magma ocean. Reconciliation of mantle abundances of moderately and highly siderophile elements with high temperature and pressure metal-silicate equilibrium would obviate the need for heterogeneous accretion. These new results indicate that the Earth accreted homogeneously, rather than heterogeneously, or that evidence for heterogeneous accretion was erased by early high temperature and pressure melting events.