Rates of redox reactions in basaltic melts determined by Mössbauer spectroscopy

Mössbauer spectroscopy has been widely used for determining the ferric/ferrous ratio in amorphous rock samples to reveal the oxygen pressure in the melt. In the present investigation, Mössbauer spectroscopy in conjunction with melting experiments at controlled oxygen pressures was used to determine the rates of redox reactions in basaltic melts at 1300°C. The samples were kept at a fixed oxygen pressure long enough to reach equilibrium at a well established ferric/ferrous ratio. Then, the oxygen fugacity in the furnace was changed abruptly and the samples were kept for different lengths of time, from 15 min, to 4 hrs, at the new condition. At the end of each run the samples were quenched and the ferric/ferrous ratio analyzed by Mössbauer spectroscopy. A geological corollary of our results is that natural volcanic glasses, representing quenched melts, retain and reflect the oxidation state in the melt immediately prior to eruption, and hence the oxygen fugacity in the magma.