Pressure dependence of Ni, Co and Mn partitioning between iron hydride and olivine, magnesiowustite and pyroxene

The partitioning of Mn, Co and Ni between iron hydride and mantle minerals (olivine, pyroxene and magnesiowustite) was experimentally investigated at 1400 degrees C and 3-11GPa using a cubic-anvil and 6-8 multi-anvil high-pressure apparatus. At atmospheric pressure (0.1 MPa), it is well known that Co and Ni concentrate in the metallic phase and Mn concentrates in oxide phases. However, the present experimental results show that the siderophile nature of Co and Ni, and the lithophile nature of Mn decrease at high pressures, The observed exchange partition coefficient, K-D = (X(M)/X(Fe))(Metal)/(X(M)/X(Fe))(Mineral) for the pure-metal-mineral system without hydrogen are a little lower than those for the metal-hydride-mineral system, but the difference in K-D between these systems is very small. It has been pointed out that the abundance of siderophile and chalcophile elements in the Earth's upper mantle cannot be explained by metal-silicate equilibrium when partition coefficients determined at atmospheric pressure are used. From the present experimental results, however, it is expected that partition coefficients of Mn increase, and those of Co and Ni decrease in the Earth's deep interior, and that observed upper-mantle elemental abundances of these elements may be explained by equilibrium partitioning between metal and silicates at extremely high pressure. When we compare the geochemical characters of Earth, Moon, Shergottite parent body and Eucrite parent body, it is found that the apparent partition coefficients of these terrestrial bodies systematically change with change in size of the planet. This observation suggests that the pressure effect on K-D values played an important role in the chemical evolution of the terrestrial planets.