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

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 60GPa (corresponding to a depth of ~2000km), >2000K 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 (<25GPa) partition coefficient data that cannot be extrapolated to higher pressures. In this work, metal-silicate partitioning experiments were performed for pressures up to ~52GPa and ~3500K 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 30GPa to the maximum pressure reached. In the range 30-52GPa 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 30GPa and for temperatures between 3000 and 3500K.