THE SPECIATION OF DISSOLVED WATER IN RHYOLITIC MELT - PRELIMINARY RESULTS

Abstract

Concentrations of water molecules and hydroxyl groups have been measured in rhyolitic glasses with 0.5% to 5.0% H2Ot using infrared spectroscopy at room temperature. The glasses were cooled at ~102C/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, those 5.5% did not undergo changes on quench record the equilibrium species concentrations of experimental run conditions. knowledge speciation in at lower temperatures can be used predict magmatic allow us explore effect melt structure physical properties natural hydrous magmas. ideal mixing models as a rough approximation for modeling solution rhyolitic melts 2.5 wt.% total water: ln[(xOHmelt)2/(XH2OmmeltXOmelt)]=ln K=1.89+/-0.05-(3120+/-40)/T, where Ximelt is the mole fraction of species i on a single oxygen basis, H2Om = 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.