Ferric/ferrous ratio in silicate melts: a model calibrated for felsic compositions

There are several models to account for the compositional effect of mafic silicate melts on ferric/ferrous ratio. However, felsic melts have been poorly investigated. In this study, both synthetic and natural silicic melts with a large compositional range (eighteen experimental series) were equilibrated at oxidizing (atmospheric conditions) and more reducing conditions in the temperature range 1401–1533 °C at 1 bar total pressure. The bulk glass composition was determined with electron probe microanalysis (EPMA) and the ferric/ferrous ratio in experimental glasses was analyzed using a wet-chemical technique. The experimental data were combined with previous data in order to formulate an empirical equation describing ferric/ferrous ratio in silicic melts as a function of oxygen fugacity, temperature and melt composition. The proposed equation predicts the log(X_FeO1.5/X_FeO) ratio of melts with a standard error of 0.077. The mean and standard deviation of the residuals for back-calculations of logfO₂ are 0.00 and 0.36 and are − 1 and 39 °C for the temperature.