Exsolution enthalpy of water from silicate liquids

Because water dissolves in silicate melts as at least two species, H2O molecules and OH groups, the exsolution (evaporation) enthalpy of the H2O component from a silicate melt depends on species proportions, which in turn depends on H2O pressure. In this short communication, a formal analysis of the exsolution enthalpy of water from a silicate melt is presented, taking into account of the role of speciation. The exsolution enthalpy from a rhyolitic melt at 850°C is found to increase with pressure at ≤100 bar and this increase is mostly caused by the speciation reaction. Sahagian and Proussevitch [Sahagian, D.L., Proussevitch, A.A., 1996. Thermal effects of magma degassing. J. Volcanol. Geotherm. Res. 74, 19–38] also found an increase of the exsolution enthalpy with pressure up to 20 bar for an albitic melt. However, the rate of increase determined from this work is an order of magnitude less than that determined by Sahagian and Proussevitch. It is concluded that the strong pressure dependence of the exsolution enthalpy at low pressures they inferred is probably an artifact of their treatment. The method presented here is general and can be applied to other multi-species components.