The speciation of dissolved H2O in dacitic melt

We report the first systematic study of H2O speciation in dacitic melt with total dissolved H2O ranging from 1.4 to 2.6 wt%. Samples were heated in a one-atmosphere tube furnace at 480–590 °C for certain durations and then quenched in water or air. Species concentrations were measured with Fourier transform infrared (FTIR) spectroscopy. At a given temperature, the equilibrium constant for the reaction, H2Om (melt) + O (melt) = 2OH (melt), is roughly constant. At the same temperature and water content, the equilibrium constant for the hydrous species interconversion reaction in dacitic melt appears to be slightly greater than in hydrous rhyolitic melt, reflecting a larger proportion of dissolved H2O as hydroxyl groups in dacitic melt. Our results can be applied to model species equilibrium in hydrous dacitic melt, to estimate the apparent equilibrium temperature of natural dacitic glasses, and to investigate viscosity of hydrous dacitic melts. more than 2.5 wt% (Ihinger et al. 1999). Whether K depends on water content at H2Ot > 2.5 wt% remains a topic of debate (Ihinger et al. 1999; Nowak and Behrens 2001). Speciation in basaltic melt also has received considerable attention (Dixon et al. 1995; Ohlhorst et al. 2001). Experiments were carried out at 1200 °C in internally heated pressure vessels. Dixon et al. (1995) showed that the quenched speciation is the same for normal (8 K/s) and rapid quench (calculated as 500 K/s, Holloway et al. 1992). However, the kinetics of the reaction has not been investigated and the quench effect has not been quantified. Other melt compositions (synthetic Ca-Al-silicate glasses, orthoclase, anorthite, jadeite: Silver et al. 1990; Fe-free andesite: Richet et al. 1996; andesite and dacite: Ohlhorst et al. 2001; andesite and Fe-free andesite: Mandeville et al. 2002) also have been investigated, but these preliminary data also are based on samples quenched from high-temperature experiments (≥850 °C) and probably reflect speciation at the apparent equilibrium temperature (Tae). Here we report the first experimental study of H2O speciation in dacitic melt at 480–590 °C and 1 bar using the heat-and-quench technique. It has been shown that reaction of hydrous species during quench from 400–600 °C is negligible in rhyolitic glasses (Withers et al. 1999). Behrens and Nowak (2003) also showed that water speciation data obtained by the quench technique, by in situ IR measurements and by controlled cooling from superliquidus temperature, are in excellent agreement for rhyolitic and albitic compositions. EXPERIMENTAL AND ANALYTICAL METHODS