Abstract
The effect of Al 2 O 3 , MgO and total iron on the ferric/ferrous ratio in silicate melts was investigated in model silicate melts in the temperature range 1400- 1550 °C at 1 atm total pressure. The experiments were done mostly in air and partially in pure CO 2 . It is demonstrated that an increase in Al2O3 concentration in a basaltic melt results in a moderate decrease of Fe 3+ /Fe 2+ ratio. In con- trast, the increase in Al2O3 in more silicic melts results in a much more pronounced decrease of Fe 3+ /Fe 2+ ratio. The increase of MgO concentration in a basaltic melt results in a moderate increase of Fe 3+ /Fe 2+ ratio but has a negligi- ble effects in more silicic melts. The different behavior of Al 2 O 3 and MgO in basaltic and more silicic melts indicates that at constant T-fO2 conditions, the effects of melt com- position on ferric/ferrous ratio cannot be predicted accu- rately as a function of ΣdiXi where di 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 SiO 2 , TiO 2 , Al 2 O 3 , total iron, MgO and P 2 O 5 is proposed.
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