Solubility behavior of water in haploandesitic melts at high pressure and high temperature

The solubility of H2O in three melt compositions along the haploandesite jo in Na2Si4O9- Na-2(NaAl)(4)O-9 (0, 3, and 6 mol% Al2O3) was determined as a function of pressure and temperature from 0.8 to 2.0 GPa and 1000 to 1300 O C. Water solubility is a linear (or near-linear) positive function of press ure (16-18 mol% H2O/GPa) at constant temperature, and a negative near-linea r function of temperature (1-2 mol% H2O/100 degrees C) at constant pressure . The solubility is correlated negatively with Al2O3 content of the melts. Partial molar volume of H2O in the melt, (V) over bar(H2O)(melt), was deriv ed from solubility isotherms (1000, 1100, 1200, and 1300 degrees C) at 0.8, 1.05, 1.3, 1.65, and 2.0 GPa pressure. Values range between 7.8 and 12.8 c m(3)/mol, and decrease with increasing Al2O3 content. In the pressure-tempe rature range studied, (partial derivative (V) over bar(H2O)(melt)/partial d erivative T)(p) ranges from -7.1 +/- 0.810(-3) to -5.6 +/- 1.3.10(-3) cm(3) /mol degrees C, becoming slightly less negative as the melts become more al uminous. The (V) over bar(H2O)(melt) values were combined with published partial mol ar volume information for anhydrous oxides in silicate melts to estimate de nsities of water-rich dacitic magmas in shallow magma chambers associated w ith explosive volcanism. For a chamber of constant bulk composition during a comparatively short explosive event, such as that of Mount Pinatubo in Ju ne 1991 or Mount St. Helens in May 1980, the average density of the magma a fter eruption is similar to 3% higher than before the eruption occurred. Fu rthermore, because of removal of overburden during an eruption, the H2O sat uration value of remaining magma is less than that prior to eruption. From density calculations of the residual hydrous magma after eruption, its dens ity decreases from top to bottom in the magma chamber. Consequently, this m agma is gravitationally unstable.