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
2011
Bouhifd, M.A. | Jephcoat, A.P.
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 (DSᵢ) together with DNᵢ and DCₒ all match the theoretical partition coefficients required for an equilibrium core–mantle differentiation at pressures above 30GPa and for temperatures between 3000 and 3500K.
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