Abstract
The effect of MgO and total FeO on ferric/ferrous ratio in model multicomponent silicate melts was investigated experimentally in the temperature range 1300–1500 °C at 1 atm total pressure in air. We demonstrate that the addition of these weak network modifier cations results in an increase of Fe3+/Fe2+ ratio in both mafic and silicic melts. Based on present and published experimental data, a new empirical equation is proposed to predict the ferric/ferrous ratio as a function of oxygen fugacity, temperature and melt composition. In contrast to previous equations, the compositional effect of melts on the Fe3+/Fe2+ ratio is not only modeled by the sum of the molar fraction of the individual oxide components. Additional interactions terms have also been incorporated. The main advantage of the proposed model is its applicability for a wide compositional range. However, its application to felsic melts (> 68 wt% SiO2) is not recommended. Other advantages of this equation and differences when compared with previous models are discussed.
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