• , 1 ELSEVIER Physics of the Earth and Planetary Interiors 96 (1996) 209-220 PHYSICS OFTHE EARTH ANDPLANETARY INTERIORS Pressure dependence of Ni, Co and Mn partitioning between iron hydride and olivine, magnesiowiistite and pyroxene Toshihiro Suzuki a, *, Masaki Akaogi a, Takehiko Yagi b a Department of Chemistry, Gakushuin University, Mejiro. Toshima-ku. Tokyo 171. Japan b lnstituteJbr Solid State Physics. University of Tokyo. Minato-ku, Tokyo 106, Japan Received 17 April 1995; revised 28 September 1995; accepted 8 November t995 Abstract The partitioning of Mn, Co and Ni between iron hydride and mantle minerals (olivine, pyroxene and magnesiowi~stite) was experimentally investigated at 1400°C and 3-I1GPa using a cubic-anvil and 6-8 multi-anvil high-pressure apparatus. At atmospheric pressure (0. ! 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 = (XM//XFe)Metal//( XM//XFe)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. 1. Introduction Core formation of the Earth, which must have taken place in the earliest history of the Earth, has been discussed from geochemical evidence found in the upper mantle. The abundances of siderophile and chalcophile elements give important clues for under- standing the core formation process, because these elements are partitioned differently between metallic and silicate phases. * Corresponding author. The geochemical approach to the Earth's core formation starts generally from an assumption that the core and upper mantle are in chemical equilib- rium. A discrepancy from this 'equilibrium Earth' was first found by Duke (1965) and independently by Ringwood (1966). They pointed out that Ni abun- dance in the upper mantle is too large if the composi- tion of the bulk Earth is assumed to be chondritic. Such discrepancies from equilibrium consideration are also found in many other elements: the abun- dance pattern of siderophile and chalcophile ele- ments in the upper mantle looks nearly 'chondritic'. 0031-9201/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0031 9201(96)03152-4