The speciation of dissolved water in rhyolitic melt

Abstract

Concentrations of water molecules and hydroxyl groups have been measured in rhyolitic glasses with 0.5% to 5.0% H_2O_t using infrared spectroscopy at room temperature. The glasses were cooled at ~10^2 C/s after having been held at 400 to 600C, for sufficient time for the equilibrium distribution of species to have been reached. The speciation of water in samples with greater than 2.5 wt.% total dissolved water and quenched rapidly from temperatures ≥600C were shown to reequilibrate during quench. However, samples with less than 2.0% water and quenched rapidly from ≤600C, and those with less than 5.5% water and quenched rapidly from ≤500C, did not undergo changes on quench and record the equilibrium species concentrations of the experimental run conditions. Knowledge of the equilibrium speciation of water in samples at lower temperatures can be used to predict the species concentrations at magmatic temperatures and allow us to explore the effect of melt structure on the physical properties of natural hydrous magmas. Ideal mixing models can be used as a rough approximation for modeling the solution of water in rhyolitic melts with less than 2.5 wt.% total water: ln[(X^(melt)_(OH))^2/(X^(melt)_(H_2O_m) X^(melt)_(O))] = ln K = 1.89 ± 0.05 - (3120 ± 40)/T, where X^(melt)_i is the mole fraction of species i on a single oxygen basis, H_2O_m = water molecules, O = anhydrous oxygens, and T is temperature in Kelvin. This fit provides a standard state enthalpy and entropy of ΔH° = 25.9 ± 0.4 kJ/mol and ΔS° = 15.7 ± 0.4 J/mol • K for the mixing of water molecules in rhyolitic melt. At high water contents, either a modification to the infrared calibration or more complex models (such as a regular solution model) are required to fit the data. Our measurements differ with recent studies using in situ measurement techniques that show lesser concentrations of molecular species at magmatic temperatures, and we address concerns associated with the in situ method. Our study on quenched glasses can be applied to natural rhyolites; using measured species concentrations, the “apparent” equilibration temperature can be calculated to within 12°C (2σ uncertainty) which can be used to determine the cooling rate of a naturally quenched rhyolitic glass.