Oxide-metal equilibria to 2500°C and 25 GPa: Implications for core formation and the light component in the Earth's core

Authors

  • Hugh St. C. O'Neill,

  • Dante Canil,

  • David C. Rubie


Abstract

The solubility of O in Fe-rich liquid metal in equilibrium with magnesiowüstite (Mg,Fe)O was investigated at pressures to 25 GPa and temperatures to 2500°C in a 6–8 type multianvil apparatus. The experiments were designed to produce near millimetre sized pools of quenched liquid metal free of other phases, so that the equilibrium composition of the liquid metal could be recovered, taking into account the effects of quench modification. The amounts of O found in the metal are relatively small (<1 wt %) and decrease with increasing pressure at constant temperature and Mg/(Mg+Fe) ratio of the coexisting magnesiowüstite. The solubility of Si varies inversely with O and does not increase substantially with pressure when in equilibrium with mantle compositions. Experiments at 25 GPa in which silica activity is buffered by coexisting MgSiC>3-perovskite produce only small amounts of Si (∼1 wt %) dissolved in Fe-rich metal. These results, in conjunction with cosmochemical constraints on the bulk composition of the Earth (the depletion of the Earth in cosmochemically volatile elements), leave the identity of the putative “light component” in the Earth's core as an enigma. It is difficult to account for the light component if it constitutes more than just a few percent of the core. The experiments also measure the partitioning of Fe, Ni, Co, Cr, and Ti between the Fe-rich liquid metal and magnesiowüstite phases. Ti is not significantly siderophile under the investigated conditions. When normalized to a constant metal/oxide partition coefficient for Fe, both Ni and Co become less siderophile with increasing pressure and Cr becomes more siderophile. The effect of temperature on Ni and Co partitioning is small at high temperatures, but important for Cr. The presently observed mantle abundances of Fe, Ni, Co, and Cr cannot be explained by equilibrium partitioning into the metal of the Earth's core under the pressure temperature conditions covered by the present experiments.

Ancillary