Convergence of Ni and Co metal–silicate partition coefficients in the deep magma-ocean and coupled silicon–oxygen solubility in iron melts at high pressures

Abstract

Models for a deep magma ocean have gained wide acceptance although with variations in the specific conditions at which core formation may have taken place. Preliminary high-pressure studies produced results consistent with metal–silicate equilibration at the base of a magma ocean that would have extended to as much as 60 GPa (corresponding to a depth of ~ 2000 km), > 2000 K and an oxygen fugacity two orders of magnitude below iron–wüstite (IW) buffer. However, up to now the magma models are based on extrapolations of low pressure (< 25 GPa) partition coefficient data that cannot be extrapolated to higher pressures. In this work, metal–silicate partitioning experiments were performed for pressures up to ~ 52 GPa and ~ 3500 K to investigate the behaviour of Ni and Co during terrestrial core formation using Laser-Heated Diamond-Anvil Cell (LHDAC) techniques. Our experimental results show that Ni and Co partitioning coefficients converge and remain similar above 30 GPa to the maximum pressure reached. In the range 30–52 GPa the data account for the relative depletions of Ni and Co ( e.g., the chondritic Ni/Co ratio) confirming evidence for a deep-magma ocean. The present results suggest a wide interval of pressure where the siderophile elements can match their mantle concentrations. We also show that both the solubilities of oxygen and silicon in molten Fe-rich alloy increase with increasing pressure. The experimental partition coefficient of Si ( D Si) together with D Ni and D Co all match the theoretical partition coefficients required for an equilibrium core–mantle differentiation at pressures above 30 GPa and for temperatures between 3000 and 3500 K.