CALCULATION OF THE FERRIC-FERROUS RATIO I N MAGMATIC MELTS - TESTING AND ADDITIONAL CALIBRATION OF EMPIRICAL EQUATIONS FOR VARIOUS MAGMATICSERIES

The existing empirical equations for calculating the redox state of ir on in natural silicate melts were tested. The best results were obtain ed with the Borisov-Shapkin equation (1989). None of the equations ade quately describes the ferric-ferrous ratio in intermediate and acid me lts. All the equations tested consistently underestimate the Fe3+/Fe2 value in the petrologically important region of low oxygen fugacities . An alternative approach is proposed that improves the accuracy in ca lculating the redox state of iron.