EXPERIMENTALLY DETERMINED SULFIDE MELT-SILICATE MELT PARTITION-COEFFICIENTS FOR IRIDIUM AND PALLADIUM

Sulfide melt-silicate melt partition coefficients for Ir and Pd have been determined from experiments run in a piston-cylinder apparatus at 1450 degrees C, 8 kbar and under f(O2)/f(S2) conditions appropriate for mafic magmas. Preferred values of D-Ir and D-Pd [D=(wt% sulfide melt)/(wt% silicate melt)] are 3.5.10(4) and 3.4.10(4), respectively. This validates the common assumption that sulfide melt dominates the geochemistry of the platinum-group elements in igneous processes. It also indicates that in magmatic differentiation processes in which sulfide alone controls precious metal abundance, Pd and Ir should not significantly fractionate from each other. The Pd results are consistent with those deduced from analyses of coexisting MORB sulfide and glass and with other published experiments. No dependence of D-Pd on f(O2)/f(S2) was observed. For Ir, disparate results were obtained depending on the nature of the starting material. Experiments using natural silicate glass as a starting material yielded consistent values of D-Ir in the range 2.5.10(4)-5.4.10(4). The experiments included a compositional convergence, in which the Ir-bearing sulfide produced in one experiment was used as the starting material for the second experiment. The results using natural starting compositions are similar to the value of D-Ir deduced from analyses of MORB. In contrast, all experiments in which synthetic silicate starting compositions were used yielded erratic values of D-Ir from 13.10(4) to 152.10(4). There are several possibilities to account for these high and variable values: ( I) an unidentified trace or minor element present in the natural compositions but absent in the synthetic ones might complex with Ir in the silicate melt and enhance its solubility, (2) this element might enhance reaction rate, or (3) an Ir-rich quench phase might have formed in the experiments using synthetic compositions but not in the natural ones and been removed during preparation of the charges for analysis. All of these possibilities suggest that the experiments using synthetic compositions are not appropriate for determining the behavior of Ir in nature. The preferred, experimentally-determined values of D-Ir and D-Pd do not explain the fractionation of these elements observed in natural systems. The observed relative and absolute abundances of Ir and Pd reflect either concentration of Ir or Pd in a phase other than sulfide or redistribution of the metals subsequent to their initial concentration.