The effect of dissolved water on the oxidation state of silicic melts

Abstract

Experiments have been conducted to study the effect of dissolved water on the ferric-ferrous ratio in natural rhyolite melts of metaluminous and peralkaline composition. The experiments were conducted at temperatures of 725–1150°C, pressures of 100–2000 bars, and oxygen fugacities from NiNiO (NNO) to MnOMn 3O 4 (MNO). The final water contents of the rhyolites ranged from 0.4–8 wt% H 2O as measured by difference on the electron microprobe. Ferrous iron contents of the quenched glasses were measured by micro-titration. Results suggest that addition of dissolved water does change the ferric-ferrous ratio over part of P-T-X-- f O2 space. At NNO and high temperatures, addition of water greatly increases the ferric-ferrous ratio in peraluminous rhyolites. The increase is linear with increasing melt hydroxyl content. At MNO, or at low temperatures, water has no effect on the ferric-ferrous ratio in metaluminous rhyolites. In peralkaline rhyolites, addition of water increases the ferric-ferrous ratio at both the MNO and NNO buffers. The observed effect of water on redox ratio at some conditions may be due to interaction between ferric iron and dissolved hydroxyl ions. It is possible to add a dissolved water term to expressions which relate ferric-ferrous ratio to melt conditions; this term is similar to those for the alkali content of the melt.