A thermodynamic basis for predicting water solubilities in silicate melts and implications for the Low Velocity Zone

Abstract

A binary asymmetric solution model involving Margules type expansions is used in conjunction with the thermodynamic criterion of equilibrium and published isothermal-polybaric solubility data for basalt, andesite and granitic pegmatite to generate activity coefficients for coexisting water saturated liquid and vapor (containing dissolved solids) phases. Values of Margules parameters so obtained are extrapolated to mantle P-T conditions and show miscibility gaps to persist in all systems at high temperatures. Harding pegmatite alone exhibits complete miscibility at 700°C with the critical parameters P c=26.1 kb and x c=0.78 H2O. Margules parameters when combined with enthalpy, heat capacity and P-V-T data for H2O in liquid and vapor phases yield values for the solubility of water in magmas at mantle P- T conditions. A consequence of these thermodynamic relations is the fact that solubility isotherms are not monotonically increasing functions of pressure and, in fact, go through maxima. Application of these results to the LVZ (Low Velocity Zone) provides a mechanism whereby the LVZ is stabilized with respect to bulk vertical flow despite the obvious gravitational instability intrinsic to stratified solid-liquid systems.