Thermodynamic model of the fluid system H2O-CO2-NaCl-CaCl2 at P-T parameters of the middle and lower crust

TL;DR

This paper develops a thermodynamic model for the H2O-CO2-NaCl-CaCl2 fluid system at crustal P-T conditions, enabling predictions of phase states, component activities, and densities relevant to deep petrogenesis.

Contribution

It introduces a novel equation of state for the four-component fluid system based on previous ternary models, expanding applicability to mixed salt compositions.

Findings

Model predicts phase states and component activities across P-T range.

Phase diagrams for various CO2 mole fractions are generated.

Pressure dependence of water activity in coexisting fluids is established.

Abstract

Based on the earlier obtained equations of state for the ternary systems H2O-CO2-CaCl2 and H2O-CO2-NaCl, an equation of state for the four-component fluid system H2O-CO2-NaCl-CaCl2 is derived in terms of the Gibbs excess free energy. A corresponding numerical thermodynamic model is build. The main part of the numerical parameters of the model coincides with the corresponding parameters of the ternary systems. The NaCl-CaCl2 interaction parameter was obtained from the experimental liquidus of the salt mixture. Similar to the thermodynamic models for H2O-CO2-CaCl2 and H2O-CO2-NaCl, the range of applicability of the model is pressure 1-20 kbar and temperature from 500C to 1400C. The model makes it possible to predict the physicochemical properties of the fluid involved in most processes of deep petrogenesis: the phase state of the system (homogeneous or multiphase fluid, presence or absence of solid salts), chemical activities of the components, densities of the fluid phases, and concentrations of the components in the coexisting phases. The model was used for a detailed study of the phase state and activity of water on the H2O-CO2-salt sections when changing the ratio xNaCl/(xNaCl+xCaCl2) from 1 to 0. Changes in the composition and density of coexisting fluid phases at a constant activity of water and changes in the total composition of the system are studied. A set of phase diagrams on sections H2O-NaCl-CaCl2 for different mole fractions of CO2 is obtained. Pressure dependencies of the maximal activity of water in the field of coexisting unmixable fluid phases are obtained for several salt compositions of the system. Due to removal of restrictions resulting from a smaller number of components in ternary systems, the thermodynamic behavior of systems with a mixed composition of the salt is significantly differ from the behavior of those with a single salt component.