Geochemistry of mantle-core differentiation at high pressure

THE apparent excess of siderophile (iron-Ioving) elements in the Earth's mantle has been a long-standing enigma in the geochemistry of mantle-core differentiation^1,2. Although current models have proved successful in explaining some aspects of this problem^3-7, important questions remain. In particular, the mantle's near-chondritic ratio of nickel to cobalt (close to that expected for the material from which the Earth formed) is hard to explain, given the markedly different ambient-pressure partitioning behaviour of these elements between iron-alloy and silicate melts^3-8. Here we report experimental results which show that both elements become less siderophile with pressure, but the effect is much more pronounced for Ni, so that the partition coefficients of the two elements become essentially equivalent at an extrapolated pressure of ~28 GPa. The absolute and relative abundances of Ni and Co in the mantle are therefore consistent with alloy-silicate chemical equilibrium at high pressure, indicating that core formation may have taken place in a magma ocean with a depth of 750-1,100 km. We also find that, unlike Ni and Co, sulphur becomes more siderophile with pressure. Sulphur's increased affinity for iron with depth could make it the dominant light element in the Earth's core.