Partitioning of Ni between magnesiowüstite and metal at high pressure: implications for core-mantle equilibrium

Abstract

The partitioning of Ni between metal and magnesiowüstite is experimentally investigated up to 17 GPa. From the thermodynamic analysis of the experimentally determined distribution coefficient, K D(M/MW) = ( X Ni/ X Fe) metal/ X Ni/ X Fe) oxide, we obtain seven thermodynamic parameters relevant to its dependency on pressure, temperature and chemical composition. K D(M/MW) is strongly dependent on pressure, temperature and MgO content in magnesiowüstite. The pressure effect is most effective and K D(M/MW) significantly decreases with increasing pressure (i.e., chemical affinity of Ni with magnesiowüstite is enhanced by high pressure). K D between metal and bulk mantle silicates, K D(M/BMS), is estimated on the basis of compiled data of K D(M/Silicates) and the extrapolated value of K D(M/MW). K D(M/BMS) also decreases with pressure. The chemical reaction between sinking Fe alloy and mantle material at high pressure leads that Ni transfer into the mantle material from Fe alloy. From the comparison K D(M/BMS) and K D(core/mantle), the present high Ni concentration in the earth's uppermost mantle is well explained by the equilibrium partitioning of Ni during core-mantle separation combined with a subsequent mantle differentiation.