Water in H2O-saturated magma-fluid systems: Solubility behavior in K2O-Al2O3-SiO2-H2O to 2.0 GPa and 1300 degrees C

The distribution of H2O, alkali silicate, and alkali aluminosilicate compon ents between coexisting H2O-saturated melt and silicate- and aluminosilicat e-saturated aqueous fluids has been determined in the pressure and temperat ure range 0.8-2.0 GPa and 700 degrees-1300 degrees C, respectively. The sol ubility of silicate and aluminosilicate materials in aqueous fluid is betwe en similar to 1 and similar to 12 mol%. This solubility is a positive funct ion of both pressure and temperature. The solubility increases with increas ing alkali content and diminishes as the system becomes more aluminous. The H2O-solubility in coexisting H2O-saturated, peralkaline silicate and alumi nosilicate melt ranges between 10 and 50 mol%. The H2O solubility is a posi tive and near linear function increasing pressure and of decreasing tempera ture. The H2O saturation values are marginally sensitive to aluminum conten t. The Si/K and Al/K ratios in the aqueous fluid an less than those of the coexisting aluminosilicate melt. The partial molar volume of H2O in the mel ts, V-H2O(melt), is less than or equal to 14 cm(3)/mol and decreases slight ly with increasing temperature. At the same temperature, V-H2O(melt) decrea ses as the melts become more aluminous. The partial molar volume of H2O in the silicate- and aluminosilicate-saturated aqueous fluids, V-H2O(melt) (16 .7-22 cm(3)/mol), generally is less than that of pure H2O at the same press ure and temperature. The V-H2O(melt) is insensitive to pressure in the 0.8 to 2.0 pressure range. The (partial derivative V-H2O(melt)/partial derivati ve P)(T) is much smaller than that of pure H2O in the 0.8-2.0 GPa pressure range. The partial molar volumes df H2O in silicate melts and silicate-satu rated aqueous solutions were combined with published data to estimate the e nergy release during exsolution of H2O from H2O-saturated magma in shallow magma chambers such as those feeding explosive dacitic eruptions (similar t o 0.2 GPa and 800 degrees-1000 degrees C). About 1.9.10(9) ergs/g H2O were obtained at 800 degrees C and 0.2 GPa. This value decreases by about 10% be tween 0.2 GPa and ambient pressure. Thus, the exact pressure during such er uptions is not needed to evaluate the effect of exsolved H2O on the energy budget. The energy contribution from exsolved H2O appears to be 2 to 4% of the total energy budget of such volcanic eruptions; Copyright (C) 1999 Else vier Science Ltd.