An experimental study of the solubility and partitioning of iridium, osmium and gold between olivine and silicate melt

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 (10(5) Pa), 1260-1350 degrees C and fO(2) of FMQ+0.6 to +5.4 were analysed by laser ablation ICPMS. Olivine-melt partition coefficients (Ds) for Ir increase from similar to 0.4 at FMQ+5.4 to similar 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 D-Ir with decreasing fO(2) 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 x 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 An, 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. (c) 2005 Elsevier B.V. All rights reserved.