The effect of oxygen fugacity, melt composition, temperature and pressure on the oxidation state of cerium in silicate melts

Abstract

Ce LIII-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of synthetic glasses prepared over a range of oxygen fugacities (fO2s, from −10 to +11 in logarithmic units relative to the quartz–fayalite–magnetite, QFM, buffer), temperatures (1300–1500°C), and pressures (1atm and 1GPa). The oxidation state ratio of Ce, Ce4+/∑Ce (where ∑Ce=Ce3++Ce4+), was determined from the spectra allowing the relationships between Ce4+/∑Ce and fO2, temperature, pressure and melt composition to be determined. Ce4+/∑Ce varied systematically with fO2 from 0 to ~0.8 over the range of conditions studied. Ce4+ is stabilised relative to Ce3+ by less polymerised compositions and lower temperatures, while pressure appears to have almost no effect (possibly stabilising Ce4+). Ce4+ in an Fe2+-bearing melt is not preserved on cooling to a glass due to the reaction Ce4++Fe2+=Ce3++Fe3+. Ce4+/∑Ce in natural melts is exceedingly small but may be recorded in the mineral zircon as an increased abundance of Ce relative to the other rare earth elements, which occur exclusively in the trivalent state. The magnitude of this Ce anomaly has considerable potential as an oxy-barometer.