MODELING SILICATE-RICH FLUIDS AT HIGH PRESSURES: AN EXAMPLE FOR THE CAO-AL2O3-SIO2-H2O (CASH) SYSTEM

Comprehension of rock-forming processes at depths corresponding to the lower crust and active subduction zones requires an understanding of silicate-rich fluids coexisting with rock-forming minerals. However, existing models for silicate solutions are difficult to extrapolate to pressures higher than 0.5 – 1.0 GPa. An alternative approach is to treat H₂O in terms of a statistical thermodynamic model involving a dynamic equilibrium between "gas-like" disordered and "liquid-like" clustered molecules (Gerya and Perchuk (1997), where X(liq) is the mole fraction of the latter. Gerya et al. (2004) successfully tested this approach for SiO₂-H₂O solutions to 1300°C and 2 GPa, and we use it here to combine both available and also new experimental high-pressure solubility data for quartz (SiO₂), corundum (Al₂O₃), wollastonite (CaSiO₃), grossular (Ca₃Al₂Si₃O₁₂), and zoisite (Ca₂Al₃Si₃O₁₂(OH)) to develop a simple thermodynamic description for aqueous fluids in the model system CASH. ΔGr of the dissolution reaction is thus described by the standard-state molar thermodynamic parameters ΔH°r, ΔS°r, ΔCpr, ΔVr, the molality of the species and X(liq). The model successfully returns the available experimental data. The formalism requires data on speciation to define the influence of X(liq) on the reaction and to determine the number of species involved. However, a simplified "component" approach for oxides can be developed to approximately model aqueous CASH fluids from 0.2 to 3 GPa. "Mixing terms” are needed for strongly interacting oxide pairs like SiO₂/Al₂O₃ or CaO/Al₂O₃, which can be directly derived from a comparison of fluids in the presence of grossular as opposed to zoisite+grossular+corundum. Analogous data for quartz or wollastonit and quartz+wollastonite suggest little interaction of CaO- and SiO₂-species in the solution. First calculations (Margules formulation of Berman and Brown, 1984) show nearly symmetric mixing for SiO₂/Al₂O₃ and strongly asymmetric behaviour of CaO/Al₂O₃.

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