The effect of composition, temperature and pressure on the oxidation state and coordination environment of copper in silicate melts

Abstract

Copper is a redox variable element that may occur as Cu0, Cu+, and Cu2+ in the Earth’s crust. The oxidation state will affect its partitioning between coexisting minerals, melts and fluids and hence its behaviour in magmatic processes. Copper bearing silicate glasses were quenched from melts with 23 synthetic compositions (19 CaO-Na2O-MgO-Al2O3-SiO2 (CNMAS), two “granites” containing K2O ± H2O, and Fe-bearing “MORB” and “andesite”) equilibrated at oxygen fugacities (fO2), expressed in log units relative to the fayalite-magnetite-quartz (FMQ) buffer, ranging from −0.7 to 14, temperatures from 900 to 1500 °C and pressures from 0 to 2.5 GPa. Cu K-edge X-ray absorption near edge structure (XANES) spectra were recorded from the glasses and a pre-edge feature in the XANES spectra was found to scale with the proportion of Cu+. Cu+/ΣCu (where ΣCu = Cu+ + Cu2+) was quantified by fitting the intensity of the pre-edge feature as a function of fO2 to the thermodynamically expected relationship. Cu+/ΣCu was found to only weakly depend on melt composition, with more basic melts (e.g., basalts rather than granites) preferentially stabilising Cu2+. Increasing temperature stabilises Cu+, while increasing pressure had little effect on Cu+/ΣCu in CNMAS melts but preferentially stabilised Cu2+ in granite melts. Cu+/ΣCu can be predicted in silicate melts by the empirical equation:log(Cu2+/Cu+) = 0.25(ΔFMQ + 8.58 − 25050/T + 940P/T – 0.02P) − 4.73 + 5400/T + 1.99Λ + (280P – 90P2)where T is temperature in K, P is pressure in GPa and Λ is the optical basicity of the composition. The effects of fO2, melt composition, temperature and pressure on Cu+/ΣCu indicate that Cu+ will be the dominant oxidation state in terrestrial silicate melts (e.g., Cu+/ΣCu = 99% in an andesitic melt at 900 °C, 1 GPa and ΔFMQ = 1). The electron exchange reaction Cu2+ + Fe2+→ Cu+ + Fe3+ occurs on cooling and, given the abundance of Fe in natural melts, the oxidation state of Cu in natural glasses is unlikely to correspond to that in the original melt.