Abstract

The distribution of H2O and alkali aluminosilicate components between coexisting H2O-saturated melt and aluminosilicate-saturated aqueous fluids has been determined in the pressure and temperature range 0.8-2.0 GPa and 700°-1300°C, respectively. The solubility of aluminosilicate materials in aqueous fluid is between ~1 and ~12 mol % with the solubility being a positive function of both pressure and temperature. The aluminosilicate solubility increases with increasing alkali content and diminishes as the system becomes more aluminous. The H2O-solibility in coexisting H2O-saturated, peralkaline aluminosilicate melt ranges between 10 and 50 mol%. The solubility is a positive and near linear function of increasing pressure and of decreasing temperature. The H2O saturation values are marginally sensitive to aluminum content. The Si/K and Al/K in the fluid are less than those of the coexisting silicate melt.

The partial molar volume of H2O in the melts is < 13 cm3/mol and decreases slightly with increasing temperature. At the same temperature, this volume decreases as the melts become more aluminous.

The partial molar volume of H2O in the aluminosilicate-saturated aqueous fluids (16.7-22 cm3/mol), generally is less than that of pure H2O at the same pressure and temperature. The is insensitive to pressure in the 0.8-2.0 pressure range.

The partial molar volumes of H2O in silicate melts and silicate-saturated aqueous solutions were combined with published data to estimate the energy release during exsolution of H2O from H2O-saturated magma in shallow magma chambers such as those feeding explosive dacitic eruptions (~0.2 GPa and 800°-1000°C). About 2.8x109 ergs/g H2O were obtained. This energy aided in initiating the eruptions. Once an eruption is underway, from independent published estimates of total energy release during an explosive eruption, the pressure on top of the magma chamber was estimated to drop to near 10 MPa.