The solubility of platinum in silicate melt under reducing conditions: Results from experiments without metal inclusions

Abstract

The solubility of Pt in silicate melt was investigated at conditions of 2073–2573K, 2GPa and ∼IW −1.5 to +3.5. These are the first measurements of Pt solubility under conditions more reducing than the iron-wüstite buffer (IW) which are demonstrably free from contamination by metal-inclusions. Pt solubility increases with increasing temperature and decreasing oxygen fugacity. The ability of carbon to enhance Pt solubility under reducing conditions (<IW +2) was tested by performing a set of isobaric, isothermal experiments which utilised graphite and PtIr alloy capsules. Analyses of the run-product glasses from these experiments show no correlation between dissolved C and Pt contents. In light of this result, the dissolution of Pt as a neutral species is suggested as being responsible for the observed relationship between solubility and fO2. A compilation of data from this study, Ertel et al. (2006) and Mann et al. (2012), suggests that pressure has no significant effect on solubility between 2 and 18GPa. Metal–silicate partition coefficients calculated from our Pt solubility data show that, for core–mantle equilibrium at IW −2, Pt concentrations in the primitive upper mantle (PUM) can be satisfied if the temperature of equilibration is >3500K. Under these conditions however, the estimated Pt/Os ratio is ∼40,000 times higher than that estimated for the PUM (Brandon et al., 2006). Instead, the PUM composition is generated most readily by metal–silicate equilibrium at more modest temperatures (∼3100K), followed by a late accretion of chondritic material subsequent to core formation.