Effects of melt composition on Fe3+/Fe2+ in silicate melts: a step to model ferric/ferrous ratio in multicomponent systems

The effect of Al2O3, MgO and total iron on the ferric/ferrous ratio in silicate melts was investigated in model silicate melts in the temperature range 1400-1550 degrees C at 1 atm total pressure. The experiments were done mostly in air and partially in pure CO2. It is demonstrated that an increase in Al2O3 concentration in a basaltic melt results in a moderate decrease of Fe3+/Fe2+ ratio. In contrast, the increase in Al2O3 in more silicic melts results in a much more pronounced decrease of Fe3+/Fe2+ ratio. The increase of MgO concentration in a basaltic melt results in a moderate increase of Fe3+/Fe2+ ratio but has a negligible effects in more silicic melts. The different behavior of Al2O3 and MgO in basaltic and more silicic melts indicates that at constant T-fO(2) conditions, the effects of melt composition on ferric/ferrous ratio cannot be predicted accurately as a function of Sigma d(i)X(i) where d(i) are fixed empirical coefficients and X-i are mole fractions of the main oxide component in silicate melts. We suggest an alternative approach which accounts for the interaction of cations in complex silicate melts. Based on the data obtained in this study, an equation predicting the ferric/ferrous ratio of ultramafic to silicic melts at air conditions with changing SiO2, TiO2, Al2O3, total iron, MgO and P2O5 is proposed.