Metal-silicate partitioning of Ni and Co in a deep magma ocean
Résumé
The pattern of siderophile (iron-loving) element abundance in the silicate portion of the Earth is a consequence of metal separation during core formation. The apparent excess of nickel and cobalt in mantlederived rocks has been attributed to metal-silicate equilibration in a deep terrestrial magma ocean. Based on the extrapolation of phase equilibria and metal-silicate partitioning results obtained at lower pressure (P) and temperature (T), previous estimates of the P-T of equilibration are all greater than 25 GPa and 3000 K. Using the laser-heated diamond anvil cell, we have extended metal-silicate partitioning measurements for Ni and Co to 75 GPa and 4400 K, exceeding the liquidus temperatures for both metal and silicate (basalt or peridotite) and, therefore, achieving thermodynamic conditions directly comparable to those of the magma ocean. The metal-silicate partition coefficients of nickel and cobalt decrease with increasing pressure and reach the values required to yield present mantle concentrations at ~50 GPa. At these conditions, silicon and oxygen concentrations measured in the metallic liquid allow to solve the seismically constrained core density deficit. Above 60 GPa, the partition coefficients become too low, resulting in an overabundance of Ni and Co in the silicate mantle. Our data therefore support the paradigm of core formation in a deep mama ocean, providing an upper bound for the depth at which Earth's core may have formed, and explaining the main geophysical (density) and geochemical (excess siderophile elements) observables.