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3.2 c. The effect of silicate liquid composition on the metal-silicate partitioning of siderophile elements at high pressures and temperatures (D.C. Rubie, in collaboration with V. Hillgren/Houston and M. Drake/Tucson)

Metal/silicate partition coeffcients for Ni, Co, Mo, W, P, Ga, and Ge in a liquid silicate/ liquid iron system have been determined at 10 GPa and 2000 °C using a multianvil press. The samples were contained in capsules consisting of either MgO or Al2O3. These two different capsule materials impose different oxygen fugacities (redox states) on the samples as well as producing very different silicate liquid compositions by reacting with the samples. The samples run in Al2O3 capsules were subjected to a higher oxygen fugacity than those run in MgO capsules. Depending on the capsule material used, the silicate liquid was either enriched in Al2O3 or MgO during the experiments, relative to the composition of the basaltic starting material. Although the partition coefficients determined for Ni, Co, Mo, and W agree well with our previous work, we have now obtained some important new results. Normally as the oxygen fugacity decreases, the metal-silicate partition coefficients should increase (see Sect. 3.2b). However if the variability of the silicate liquid composition is ignored we find that the partition coefficients for W, P, and Ge apparently decrease with decreasing oxygen fugacity, which is the opposite to what is expected. We also find that the partition coefficients for Mo, Ga, Ni and Co apparently do not increase as much as expected with decreasing oxygen fugacity. Analysis of the structures of the two different silicate melts indicates that the MgO-rich melt is considerably more depolymerized than the Al2O3-rich melt. The partitioning behavior of all the elements indicates that the cations are stabilized in the more depolymerized MgO-rich melt. This is in accord with previous work on the effects of composition on forsterite/liquid partitioning and the partitioning of metal cations between immiscible silicate liquids. Previously it was believed that oxygen fugacity is the most important parameter controlling metal/silicate partitioning and that the silicate liquid composition would have, at most, a minor influence. Our results show, on the contrary, that to fully understand the metal/silicate partitioning behaviour of elements, the effects of all variables must be carefully investigated. In addition, the results suggest that possible changes in mantle composition envisioned in some accretion scenarios for the Earth, such as heterogeneous accretion, may play an important role in geochemical evolution during core formation.

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