Rhodium Solubility in Silicate Melts

Abstract

All published experimental data on Rh solubility in silicate melts were combined to derive an equation relating Rh solubility to temperature, oxygen fugacity, and a melt composition. It is demonstrated that Rh is dissolved in a melt dominantly as Rh²⁺ in the entire experimental fO₂ range, from pure oxygen to QFM + 2 (QFM is the quartz–magnetite–fayalite buffer). The temperature dependence of Rh solubility is anomalous. Similar to the solubilities of other noble metals, Rh solubility at a constant fO₂ increases with increasing temperature. The Rh metal/silicate partition coefficient was calculated (\(D_{{{\text{Me/Sil}}}}^{{{\text{Rh}}}}\) ≈ 3.5 × 10⁷) for the expected conditions of Earth differentiation into a core and mantle. It is demonstrated that the late chondritic veneer model is able to best explain high Rh contents in upper mantle rocks. The suggested equation makes it possible to discard experimental glasses contaminated with metallic Rh micronuggets and thus to get rid of at least the most gross errors in the determination of Rh partition coefficients between rock-forming minerals and melt.