Experimental constraints on the partitioning of rhenium and some platinum-group elements between olivine and silicate melt

We have performed partitioning experiments to assess the role of olivine in controlling the behavior of rhenium and the platinum group elements (PGEs) during basalt petrogenesis. Olivines were crystallized from an iron-bearing basalt at 1 bar (10⁵ Pa) and log fO₂ of -2.6, -4.9 and -7.4 (FMQ +4.3, +2 and -0.5, respectively). In situ analyses of olivine and glass by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) reveal a homogeneous distribution of Ru, Rh, Pd, Re, and Pt, but significant Os heterogeneity at the μm scale. This latter behavior arises from the presence of undissolved Os micronuggets suspended in the melt, and included in olivine crystals. Olivine-melt partition coefficients (Ds) for Re and the PGEs follow the order: D_Rh > D_Ru ≫ D_Pd ∼ D_Re ∼ D_Pt. With decreasing fO₂, Rh and Ru become more compatible, with maximum partition coefficients of ∼2.6 and ∼2, respectively, at log fO₂ of -4.9. In contrast, D values for Pd become smaller with decreasing fO₂, to a value of ∼0.006 at log fO₂ of -7.4. Olivine-melt partitioning of Rh, Ru, Pd, Re and Pt derived from our experiments is confirmed by the behavior of these elements in lavas that have evolved by olivine fractionation. An elastic strain model predicts the olivine-melt partitioning of these elements, excepting our measured value of D_Pt, which is much lower. The fO₂ dependence on partitioning implies that at higher fO₂ some portion of PGEs exist in higher valence states than predicted from their solubility.