Experimental study of the effect of temperature on water solubility in natural rhyolite melt to 100 MPa

The effect of temperature on water solubility in rhyolite melt was experimentally determined at 850-1200°C and 22-100 MPa. A natural high-silica rhyolite glass was equilibrated with pure water vapor, and the water content in the quenched glass was determined by IR spectroscopy. The results demonstrate that water solubility in rhyolite melt has a negative temperature dependence, which becomes weaker at high temperatures and low water contents. This temperature dependence can be modeled adequately on the basis of ideal mixing of water species and anhydrous rhyolite melt components. The model reproduces both the present and previously published solubility data for water in rhyolitic melts to 100 MPa and over a wide range of temperature from near the solidus to 1200°C, thereby permitting calculation of water saturation under varying temperature conditions. At 50-100 MPa, an increase in the fraction of excess water as a result of a rise in temperature can cause a four- to eight-fold increase in the fractional amount of volumetric expansion above that caused by pure thermal expansion, per unit temperature rise. Thus, the negative temperature dependence of water solubility could be of fundamental importance in the development of gravitational instability in shallow, water-saturated silicic magma chambers.