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

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.