Partitioning and Solubility of PGEs in Oxides and Silicates

Platinum group elements (PGEs) are among the most siderophile in the cosmoehemical inventory. By analogy with iron meteorites, the bulk of Earth's chondritic endowment of PGEs is thought to be ensconced in its metallic core. In spite of their siderophilic nature, however, PGEs in the mantle are at nearly 1% of their chondritic concentrations. This relative enrichment has provided for a significant, and as yet poorly understood igneous geochemistry for the PGEs. However, recent experimental efforts have increased our understanding of their magmatic chemistry (e.g. Fleet et al. 1991, Borisov et al., 1994 or Peach et al., in press). After metals, the next most favored phases for PGEs are sulfides. Stabilization of PGEs in metals and sulfides accounts for much of their geochemistry. However, not all magmatic systems are metalor sulfide-saturated. What governs PGE geochemistry in the absence of these important phases must be their solubility and partitioning behavior in oxide and silicate phases. Moreover, the strength of PGE chalcophility and siderophility is in part bound to their silicate melt chemistry and can be weakened by compatibilities among lithophile phases. We report here some experimental results on PGEs in oxides and silicates and discuss factors relevant to their lithophile geochemistry. Experiments on the lithophile geochemistry of PGEs are complicated by their low solubilities in crystalline and molten silicates. To measure PGE concentrat ions in experimentally produced charges using microprobe techniques, such as the electron microprobe, ion microprobe and PIXE microprobe, we have conducted experiments under conditions (high oxygen fugacity) that produce PGE concentrations (ppm range) significantly above natural levels (ppb range). I f the chemical species of dissolved PGEs does not change between our experimental conditions and those relevant to natural systems then our partition coefficients will be directly applicable, otherwise, their applicability depends on special considerations discussed below. Partitioning and solubility experiments for Ru, Rh and Pd have been conducted in 1 arm. furnaces between 1250~ and 1510~ Bulk compositions in simple systems containing subsets of the following components: CaO, MgO, A1203, SiO2, FeO, TiO2 have been investigated in equilibrations lasting between 1 and 16 days, a typical run is 3 -4 days. Ru, Pd and Rh, the light PGEs, were selected for study primarily because their solubilities are generally greater than the heavy PGEs: Os, Ir and Pt, and so they are more easily analyzed. However, we are also attempting solubility measurements of Ir in Fe oxides using piston