The oxidation state of europium in silicate melts as a function of oxygen fugacity, composition and temperature

Abstract

Europium LIII-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of synthetic glasses and melts equilibrated over a range of oxygen fugacities (fO2s, from -14 to +6 logarithmic units relative to the quartz-fayalite-magnetite, QFM, buffer) and temperatures (1250-1500°C). Eu3+/σEu (where σEu=Eu2++Eu3+) values were determined from the spectra with a precision of ±0.015. Eu3+/σEu varies systematically with fO2 from 0 to 1 over the range studied, increases with decreasing melt polymerisation and temperature, and can be described by the empirical equation: Eu3+/σEu=11+10-0.25logfO2-6410T-14.2Λ-10.1, where Λ is the optical basicity of the melt and T is the temperature in K. Eu3+/σEu in glasses and melts equilibrated at the same conditions are in excellent agreement for Fe-free systems. For Fe-bearing compositions the reaction Eu2++Fe3+=Eu3++Fe2+ occurs during quenching to a glass and the high temperature value of Eu3+/σEu is not preserved on cooling; in situ measurements are essential for determining Eu3+/σEu in natural melts. Show more