Study of oxygen fugacity influence on redox state of iron in granitoidic melts

A series of experiments has been carried out on two granitoid melt compositions under the wide range of oxygen fugacities. Room temperature 57Fe Mössbauer spectroscopy has been used to study the structural and oxidation state of Fe in granitoid composition glasses. Samples were produced in a series of melting experiments at temperatures from 1120 to 1420 °C and oxygen fugacities from 10−0.7 (air) to 10−12 (CCO buffer) bars. The analysis has shown that Fe3+/Fe2+ ratio in the melt considerably decreases with fO2 decreasing and/or temperature increasing. Dependence of Fe3+/Fe2+ ratio on fO2 is linear in logarithmic scale with slope 0.18–0.24. Ferrous iron occupies octahedral and probably five-fold positions and is regarded as a network-modifier. Ferric iron coordination depends on iron redox state. Its structural role changes from network-modifier to network-former.