Experimental study of the partitioning of some platinum group elements (Pd and Ir) between orthopyroxene and silicate melt

Abstract

Past experiments and observations on natural samples have largely focused on the roles of olivine and chromite in controlling the behaviour of the platinum-group elements (PGE) during melting and solidification, whereas other phases, such as pyroxene, have gone largely uncharacterized. To address this, experiments have been done to measure the partitioning of Pd (with a subset of results for Ir), between orthopyroxene and silicate melt at 1340 °C, 0.1 MPa and log fO2 of FMQ − 1 to FMQ + 6 (FMQ = Fayalite-Magnetite-Quartz). The X-ray Absorption Near-Edge Structure (XANES) was measured in a subset of experiment glasses. Glass concentrations of Pd (corrected to unit Pd activity) increase from ∼6 to ∼650 ug/g over the fO2 range of experiments. The slope of the solubility-fO2 relation is consistent with Pd1+ as the dominant oxidation state, with evidence for Pd0 and Pd2+ at the lowest and highest experiment fO2, respectively. Consistent with this result, the XANES reveal spectral features similar to Pd0 and Pd2+ spectral reference materials (specRM) at the most reduced and oxidized synthesis conditions, respectively. Other lines of evidence require the presence of a third melt species, here interpreted to be Pd1+. Values of orthopyroxene/melt partition coefficients for Pd (DPdOpx/melt) are 0.0051 (+/−0.006) at log fO2 < ΔFMQ + 3, increasing with fO2 to a maximum of 0.013 at ∼FMQ + 6. Sodium partition coefficients, expected to be similar to Pd, range from 0.0061 (+/−0.00061) at FMQ + 3, increase to 0.007–0.009 at higher fO2, but with no clear systematic trend. A value for DIrOpx-melt of ∼0.6 was measured at ∼FMQ + 4, indicating significantly more compatible behaviour for Ir relative to Pd. Partitioning results are interpreted in the context of the Blundy-Wood elastic strain model in which the variation in partitioning is related to ionic radius mismatch to an optimal crystallographic site size. Based on the trend in ionic radius with oxidation state, the estimated ionic radius of Pd1+ in octahedral coordination is similar to Na1+, and comparison to previous orthopyroxene-melt partitioning experiments suggests DPd1+opx/melt and DNaopx/melt should be nearly identical, consistent with the results of this study. The ionic radius of VI-fold Pd2+ is close to the optimal M2 site size, so an increased proportion of this species with fO2 accounts for the larger values of DPdopx/melt at the highest fO2 investigated. The much larger partition coefficient for Ir is consistent with the presence of Ir2+, whose estimated ionic radius is close to Fe2+ and Mg2+, as well as predictions for the optimal M1 site size. With the assumption that DPdopx/melt = DNaopx/melt, combined with a revised value for the Pd content of the primitive mantle, a melting model is presented that better reproduces the Pd concentration of high degree melts from sulfide-free mantle sources.