PARTITIONING OF IRON, COBALT AND NICKEL B ETWEEN MINERALS OF THE EARTHS LOWER MANTLE AT HIGH-PRESSURE AND HIGH-TEMPERATURE

Cobalt- and nickel-bearing olivines (Mg0.78Fe0.20Ni0.02)(2)SiO4 and (M g0.71Fe0.20Co0.09)(2)SiO4 have been transformed at high pressure up to 40 GPa and high temperature in a laser-heated diamond-anvil cell. The reaction products have then been examined by analytical transmission electron microscopy. In cobalt-bearing samples, gamma-spinel and perov skite-magnesiowustite assemblages have been characterized; like iron, cobalt partitions preferentially into the magnesiowustite phase. The p artition coefficients K-mw . pv(M . M9) of iron and cobalt (M=Fe, Co) relative to Mg between perovskite and magnesiowustite mw-pv are 3+/-0. 5 and 4.7+/-1 respectively. This trend is qualitatively understood in terms of crystal-field theory. In all the nickel-bearing samples, iron -nickel precipitates are present. At about 15 GPa, the metallic phase coexisting with (Mg0.78Fe0.20Ni0.02)(2)SiO4 beta-phase, has a composit ion of approximately (Fe0.71Ni0.29), whereas at higher pressure (40 GP a), the triphasic assemblage consists of magnesiowustite, an amorphous silicate phase, and (Fe0.8Ni0.2) metal. The distribution coefficients of nickel between metal and oxides and silicates have much lower valu es than at atmospheric pressure. This suggests a decrease in the sider ophile character of nickel al high pressure and high temperature.