Thermodynamic model of the h2o-licl-nacl system for fluid inclusions study: calculation using the pitzer equations

A thermodynamic model of the ternary fluid system H₂O-LiCl-NaCl is proposed for the temperature range from –77 to +300°C. This model incorporates low-temperature phase transitions of freezing products of water-salt inclusions. The model is based on the Pitzer equations using new interaction parameters of Na, Cl and the corresponding equilibrium constants of reactions involving the solid and liquid phases. Based on microthermometry data of fluid inclusions (T of phase transitions during heating after freezing), the model allows determine the salt concentrations. Characteristics (T, wt% LiCl and NaCl) of triple points with solid phases including the eutectic E''(ice + LiCl·5H₂O + NaCl·2H₂O), peritectic ${\text{P}}_1^{\text{''}}$ (LiCl·5H₂O + + NaCl·2H₂O + NaCl) and ${\text{P}}_2^{\text{''}}$ (LiCl·5H₂O + LiCl·3H₂O + NaCl) and cotectic, peritectic curves separating the phase fields (ice + L, NaCl·2H₂O + L, NaCl + L), as well as solubility isotherms of ice, hydrohalite and halite calculated by the model showed good agreement with experimental data. As an example of the application of the model to the natural object, we determined the salt contents in lithium-bearing brine inclusions in late quartz veins of the Bolshie Keivy area (Fennoscandian Shield).