Fluid-mineral equilibria and thermodynamic mixing properties of fluid systems

Abstract

The paper presents a review of an experimental method to quantitatively constrain thermodynamic mixing properties of fluid systems at high temperature T and pressure P. The method is based on bracketing equilibrium parameters of simple fluid-mineral reactions. Experimental data obtained with this technique for the H2O-CO2, H2O-N2, and H2O-H2 binary systems were utilized to calculate mixing parameters corresponding to the simplified van Laar model W12VL, according to which the equation for the integral excess Gibbs free energy of a binary mixture Gex is Gex =X1X2W12VL/(X1V10 + X2V20), where Xi is the mole fractions of the components, and Vi0 are pure species molar volumes at given P and T (in cm3). The W12VL for the three mixtures correspond to 202, 219, and 331 kJ cm3/mol. The empirical correlation \(W_{H_2 O - X}^{VL}\) (kJ cm3/mol) = 887.012 QX − 16.674, where Q = Pc (critical pressure, bar)/Tc (critical temperature, K) for gas X (where X = CH4, CO, H2S, O2, Ar, and NH3) is used to evaluate the van Laar parameters for a number of petrologically important water-gas mixtures. The H2O-H2 system is characterized by the greatest positive deviation from the ideal mixing and can thus decompose into two immiscible fluid phases under the P-T parameters typical of deep lithospheric zones. The exsolution of the H2O-CO2 and H2O-N2 systems is expected to occur only under high pressure and low temperature. This combination of parameters may be expected only in the environments of cold subduction. Salts (highly soluble simple salts and/or silicates) should significantly expand the exsolution regions in petrologically important fluids.