Siderophile geochemistry of Ga, Ge, and Sn: Cationic oxidation states in silicate melts and the effect of composition in iron-nickel alloys

We report a series of metal-silicate partitioning experiments for Ga, Ge, a nd Sn to characterize the dependence of the partition coefficient, D, on ox ygen fugacity, f(O2). These were isothermal (1260 degrees C) and isobaric ( 1 bar) experiments using a silicate composition that approximates a eucriti c meteorite. It is well known that elements such as Ni, which exist in only one valence state under redox conditions of planetary interest, produce li near trends on log D vs log f(O2) diagrams. For our experiments on Ga, Ge, and Sn, however, large deviations from linearity were evident and seemed to suggest unusual changes in the oxidation states for these elements in the silicate melt. But such an inference would have been mistaken because the m etallic phase of these experiments, Ni-Fe alloys, was not of constant compo sition; instead, the Ni/Fe ratio was varied systematically to control oxyge n fugacity. Although Ni and Fe form alloys that are not far from ideal, bin ary interactions between the trace elements and either major metallic compo nent, Ni or Fe, are not necessarily similar. Using only information obtained from the literature on binary mixing proper ties among the metallic components, Fe, Ni, Ga, Sn, and Ge, a simple thermo dynamic solution model was formulated to calculate the activity coefficient s for the metallic components in our experimental system. It was found that , despite the slight deviations from ideality for Ni-Fe alloys, large diffe rences exist between the way Ni interacts with trace elements and the way F e does. Activity coefficients calculated from the thermodynamics of the met allic solution rationalized the experimentally derived log D vs log f(O2) p lots. When the independently derived activity coefficients for the trace el ements in the alloys (gamma) are used to plot log gamma K-D vs log fO(2), t he unusual oxygen fugacity dependencies can be fully reconciled with the ex pected valences of 3+ for Ga, 3+ for Ge, and predominantly 4+ at high oxyge n fugacities changing to predominantly 2+ at low oxygen fugacities for Sn. If a planetary metal-silicate system evolves with increasing oxygen fugacit y, the increasing Ni content of the metal will maintain the siderophile cha racter of Ga, Ge, and Sn because these elements are more stable dissolved i n Ni compared to Fe. Copyright (C) 1999 Elsevier Science Ltd.