Abstract:
We have performed metal solubility and olivine–melt partitioning experiments to understand the behaviour of iridium, osmium and gold during crust–mantle differentiation. All experiments were performed metal-saturated, with molten gold added to suppress nugget formation. Coexisting olivine and iron-bearing basalt glass produced in experiments at 1 bar (105 Pa), 1260–1350 °C and fO2 of FMQ + 0.6 to + 5.4 were analysed by laser ablation ICPMS. Olivine–melt partition coefficients (Ds) for Ir increase from ∼0.4 at FMQ + 5.4 to ∼2 at FMQ + 2.5, with the latter value being consistent with the behaviour of Ir in lavas that have evolved by olivine fractionation. The increase in DIr with decreasing fO2 is consistent with an increase in the relative proportion of Ir2+ in the melt, as it has an estimated ionic radius close to the “strain-free” value for octahedral coordination in olivine. Gold is highly incompatible in olivine (D = 1 − 2 × 10− 3 or less), consistent with its relatively large ionic radius in the Au1+ oxidation state. This, together with our previous measurements of olivine–melt partitioning for Pt, Pd, Ru and Rh, indicates that olivine-equilibrated melts will have depletions in Ir, Rh and Ru relative to Pt, Pd and Au, consistent with that measured in primitive, sulfur-poor magmas. Our data provide an upper bound on Os solubility in silicate melt of 10 ppb at FMQ + 0.6, even though Os partition coefficients could not be measured. The solubility of Os, Ru and Ir in silicate melt is comparable to the concentrations of these elements in primitive, sulfur-poor magmas, suggesting they could be metal-saturated.
