Phase State and Thermodynamic Parameters of the Fluid System H2O–CO2–CH4 at P-T Conditions of the Crust and Lithosphere Mantle

Calculations of the thermodynamic properties of the ternary fluid system H 2 O–CO 2 –CH 4 in a broad range of P-T parameters above the critical point of water are presented. The calculations are based on the known equations of state for binary systems H 2 O–CO 2 and H 2 O–CH 4 , which were extended up to equation of state of the ternary system by means of the Lorentz–Berthelot mixing rules. The obtained equation for the Gibbs excess free energy allows determining the phase state of the system (homogeneous of two-phase fluid) and obtaining chemical activities of components, densities and compressibility of fluid phases, as well as other thermodynamic properties of the fluid. The equations and methods of calculations can be applied in the range of P-T parameters of P = 0.1–100 kbar and T = 400–2300°C, i.e. in the range from metamorphic processes of the greenschist facie up to deep layers of the lithosphere mantle. We obtained phase diagrams of the system H 2 O–CO 2 –CH 4 and investigated dependencies of the chemical activity of water on concentrations of the components and temperature and pressure. The conditions for splitting of the fluid into co-existing immiscible phases are investigated in detail. The splitting in the system H 2 O–CO 2 –CH 4 appears to be possible in the temperature range from 400 up to 735°C. In the range of temperatures 400–480°C and pressures 1.1–10 kbar broad fields of co-existing phases are present on phase diagrams. For the three binary boundary subsystems, the splitting at these P-T conditions is possible only in the system H 2 O–CH 4 . Nevertheless, the presence of methane in the ternary system enables the emergence of immiscible fluid phases with high concentration of CO 2 , which could exceed the concentration of the methane. At temperatures above 500°C the phase behavior of the system H 2 O–CO 2 –CH 4 is similar to the behavior of the majority of salt-free gas-water fluids, i.e. the splitting into two immiscible phases occurs at high pressures, corresponding to P-T conditions of the cold subduction.