Pressure dependence of the speciation of dissolved water in rhyolitic melts

Abstract

Water speciation in rhyolitic melts with dissolved water ranging from 0.8 to 4 wt% under high pressure was investigated. Samples were heated in a piston-cylinder apparatus at 624–1027 K and 0.94–2.83 GPa for sufficient time to equilibrate hydrous species (molecular H 2O and hydroxyl group, H 2O m + O ⇌ 2OH) in the melts and then quenched roughly isobarically. The concentrations of both hydrous species in the quenched glasses were measured with Fourier transform infrared (FTIR) spectroscopy. For the samples with total water content less than 2.7 wt%, the equilibrium constant ( K) is independent of total H 2O concentration. Incorporating samples with higher water contents, the equilibrium constant depends on total H 2O content, and a regular solution model is used to describe the dependence. K changes with pressure nonmonotonically for samples with a given water content at a given temperature. The equilibrium constant does not change much from ambient pressure to 1 GPa, but it increases significantly from 1 to 3 GPa. In other words, more molecular H 2O reacts to form hydroxyl groups as pressure increases from 1 GPa, which is consistent with breakage of tetrahedral aluminosilicate units due to compression of the melt induced by high pressure. The effect of 1.9 GPa (from 0.94 to 2.83 GPa) on the equilibrium constant at 873 K is equivalent to a temperature effect of 49 K (from 873 K to 922 K) at 0.94 GPa. The results can be used to evaluate the role of speciation in water diffusion, to estimate the apparent equilibrium temperature, and to infer viscosity of hydrous rhyolitic melts under high pressure.