  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.
