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

摘要

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 (10~5 Pa) and log fO_2 of -2.6, -4.9 and -7.4 (FMQ +4.3 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. Olivinemelt partition coefficients (Ds) for Re and the PGEs follow the order: D_(Rh) > D_(Ru) D_(Pd) ～ D_(Re) ～ D_(Pt). With decreasing fO_2, Rh and Ru become more compatible, with maximum partition coefficients of ～2.6 and ～2, respectively, at log fO_2 of -4.9. In contrast, D values for Pd become smaller with decreasing fO_2, to a value of ～0.006 at log fO_2 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 fO_2 dependence on partitioning implies that at higher fO_2 some portion of PGEs exist in higher valence states than predicted from their solubility.