The influence of pressure and temperature on the metal-silicate partition coefficients of nickel and cobalt in a model C1 chondrite and implications for metal segregation in a deep magma ocean

Abstract

The metal-silicate partition coefficients of Ni and Co in a model C1 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 D M∗ met/sil (M = Ni or Co) and log f O 2 (ΔIW) 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 K DM/Fe met/sil = ( X M met/X MO sil)( X FeO sil/ X Fe met), which are essentially independent of oxygen fugacity. In the pressure-temperature interval investigated, increasing temperature and pressure both result in lowering K DNi/Fe met/sil and K D Co/Fe met¡l . In the context of reasonable geothermal gradients, the pressure effect is significantly more important, 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 experimental 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 experiments at higher pressures (> 12.0 GPa) are necessary to get a more critical evaluation of high-pressure/high-temperature core-mantle equilibrium models.