Pergamon Geochimica et Cosmochimica Acta, Vol. 59, No. 5, pp. 99 I - 1002. 1995 Copyright 0 1995 Elsevier Science Ltd Printed in the USA. All rights reserved 0016.7037/95 $9.50 + .I0 0016-7037(95)00017-8 The influence of pressure and temperature on the metal-silicate partition coefficients of nickel and cobalt in a model Cl chondrite and implications for metal segregation in a deep magma ocean YVES THIBAULT”* and MICHAEL J. WALTERS ‘CM. Scarfe Laboratory of Experimental Petrology, Department of Geology, University of Alberta, Edmonton, Alberta T6G 2E3, Canada *Geophysical Laboratory and Center for High Pressure Research, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, DC 20015. USA zyxwvutsrqponmlkjihgfedcbaZYXWVUTS (Received April 22, 1994; accepted, in revised form November 11, 1994) Abstract-The metal-silicate partition coefficients of Ni and Co in a model C 1 chondrite were determined at pressures ranging from 1.2 to 12.0 GPa and temperatures between 2123 and 2750 K. At 5.0 GPa and 2500 K, the effect of variable oxygen contents on the partitioning of Ni and Co was also investigated. Graphite was chosen as the sample container. Carbon is an integral part of the system because about 5 wt% C dissolved in the metal liquid. The slopes obtained for the relation between log DFl;“r”” (M = Ni or Co) and log f0, (AIW) are compatible with Ni and Co, having valencies of 2 in the silicate melt. This allows the partitioning of Ni and Co as a function of pressure and temperature to be expressed in terms of exchange partition coefficients Kr$& = (XFt;“‘lX f,!&) (X &JX~~t), which are essentially independent of oxygen fugacity. In the pressure- temperature interval investigated, increasing temperature and pressure both result in lowering KF?:“ & and K;;ed&. In the context of reasonable geothermal gradients, the pressure effect is significantly more im- portant, especially for Ni. The results obtained agree with recent studies on the partitioning of Ni and Co at high pressure and high temperature in that they all record much lower metal-silicate partition coefficients for these elements than those obtained at lower temperature and atmospheric pressure. The significant differences in the experi- mental conditions between these recent high-pressure/high-temperature studies can probably explain the small but significant discrepancies observed in the results. The abundance of Ni and Co that would be observed in the primitive mantle for pressures of equilibration up to 12.0 GPa were calculated assuming simple core-mantle equilibrium in a magma ocean. The resulting abundances of Ni and Co do not reach the values estimated for Earth’s primitive mantle. Nevertheless, the significant decrease in the partition coefficients of Ni and Co with increasing pressure indicates that ex- oeriments at higher uressures ( > 12.0 GPa) are necessary to get a more critical evaluation of high-pressure/ high-tempera&e core-mantle‘equilibrium’ models. INTRODUCTION Siderophile elements (e.g. Fe, W, Co, Ni, MO, Re, OS) pref- erentially partition into metallic phases. If segregation of the Earth’s metallic core was an equilibrium process, either before (within parent bodies), during, or after accretion, then the abundance of siderophile elements left behind in the silicate portion of the Earth should reveal the conditions of equilib- rium (Ringwood, 1966, 1979). Therefore, it is critical in eval- uating models of core formation to know the partitioning be- havior of siderophile elements between metal and silicate over a wide range of conditions of pressure, temperature, and com- position. At low pressure (atmospheric and 0.5 GPa), the partition- ing behavior of many siderophile elements has been deter- mined as a function of oxygen fugacity (f zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA O,), temperature ( = 1200- 16OO” C), and composition (e.g., Rammensee and Wanke, 1977; Newsom and Drake, 1982, 1983; Drake et al., 1984; Jones and Drake, 1986; Seifert et al., 1988; Schmitt et al., 1989; Lodders and Palme, 1991; Capobianco and Amelin, * Present address: Department of Earth Sciences, Biology and Ge- ology Building, University of Western Ontario, London, Ontario N6A 5B7, Canada. 991 1994; Borisov et al., 1994; Dingwell et al., 1994; Holzheid et al., 1994). On the basis of the atmospheric partition coeffi- cients (Dr” erla” ) obtained, the abundances of siderophile ele- ments in the Earth’s silicate upper mantle are too high to be explained by low-pressure equilibrium processes, at least in the temperature interval investigated (e.g., Jones and Drake, 1986; Schmitt et al., 1989; Newsom, 1990). Murthy ( 1991) made the suggestion that, with increasing temperature, sidero- phile elements may become progressively more lithophile as DE”s” values tend towards unity. This prediction has partic- ular relevance to core segregation if the Earth was entirely molten as a consequence of the accretion process, with tem- peratures reaching values as high as 3,000 to > 10,000 K (e.g., Benz and Cameron, 1990; Melosh, 1990). The prediction of Murthy ( 1991) has fueled a great deal of controversy and has inspired attempts to measure siderophile element partition co- efficients at extreme temperatures. Ohtani et al. ( 1991) investigated the partition coefficients for V, Cr, Mn, Fe, Co, and Ni between Mg-perovskite, mag- nesiowtistite, and solid metallic iron at very high pressures (25-27 GPa) and temperatures of 1700 and 1900°C. They concluded that the abundance of Mn, Co, and Ni in the upper mantle cannot be explained by single core-mantle equilib- rium, even accompanied with olivine accumulation. Walker