Water in H 2O-saturated magma–fluid systems: solubility behavior in K 2O–Al 2O 3–SiO 2–H 2O to 2.0 GPa and 1300°C

Abstract

The distribution of H 2O, alkali silicate, and alkali aluminosilicate components between coexisting H 2O-saturated melt and silicate- 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 silicate and aluminosilicate materials in aqueous fluid is between ∼1 and ∼12 mol%. This solubility is a positive function of both pressure and temperature. The solubility increases with increasing alkali content and diminishes as the system becomes more aluminous. The H 2O-solubility in coexisting H 2O-saturated, peralkaline silicate and aluminosilicate melt ranges between 10 and 50 mol%. The H 2O solubility is a positive and near linear function increasing pressure and of decreasing temperature. The H 2O saturation values are marginally sensitive to aluminum content. The Si/K and Al/K ratios in the aqueous fluid are less than those of the coexisting aluminosilicate melt. The partial molar volume of H 2O in the melts, V̄ H 2 O melt, is ≤14 cm 3/mol and decreases slightly with increasing temperature. At the same temperature, V̄ H 2 O melt decreases as the melts become more aluminous. The partial molar volume of H 2O in the silicate- and aluminosilicate-saturated aqueous fluids, V̄ H 2 O fluid (16.7–22 cm 3/mol), generally is less than that of pure H 2O at the same pressure and temperature. The V̄ H 2 O fluid is insensitive to pressure in the 0.8 to 2.0 pressure range. The (∂ V̄ H 2 O fluid/∂P) T is much smaller than that of pure H 2O in the 0.8–2.0 GPa pressure range. The partial molar volumes of H 2O in silicate melts and silicate-saturated aqueous solutions were combined with published data to estimate the energy release during exsolution of H 2O from H 2O-saturated magma in shallow magma chambers such as those feeding explosive dacitic eruptions (∼0.2 GPa and 800°–1000°C). About 1.9 · 10 9 ergs/g H 2O were obtained at 800°C and 0.2 GPa. This value decreases by about 10% between 0.2 GPa and ambient pressure. Thus, the exact pressure during such eruptions is not needed to evaluate the effect of exsolved H 2O on the energy budget. The energy contribution from exsolved H 2O appears to be 2 to 4% of the total energy budget of such volcanic eruptions.