Thermodynamic Model of the Fluid System H2O–CO2–NaCl at P-T Parameters of the Middle and Lower Crust


A numerical thermodynamic model is proposed for one of the most important geological fluid systems, the ternary H2O–CO2–NaCl system, at P-T conditions of the middle and lower crust and crust-mantle boundary (up to P = 20 kbar and up to T = 1400°C). The form of the model is analoguous to the model developed earlier for the system H2O–CO2–CaCl2. The model is based on an equation for the Gibbs excess free energy for the fluid H2O–CO2–NaCl, also including terms responsible for the interaction of the fluid with the solid phase of NaCl. The model allows predicting physical-chemical properties of fluids, which participate in the majority of deep petrogenic processes: the phase state of the system (homogeneous or multi-phase fluid, presence or absence of solid NaCl), chemical activities of the components, densities of the fluid phases, and concentrations of the components in the co-existing phases. The P-T dependencies of coefficients of the equation for the Gibbs excess free energy are expressed via molar volumes of water and CO2 at corresponding pressure and temperature. The numerical parameters of the model are obtained by fitting experimental data on the phase state of the fluid system in the range of pressures from 1 to 9 kbar and temperatures from 500 to 930°C. Our parametrization of the P-T dependencies provides applicability of the model for pressures and temperatures beyond the experimental region, namely up to P = 20 kbar and up to T = 1400°C. The validity of the model above these P-T parameters is limited due the properties of the thermodynamic description of the CO2 employed.

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The authors are grateful to L.Ya. Aranovich for a productive discussion and helpful comments.


The work was carried out within the framework of the research themes of the laboratory of fluid processes, IPGG RAS. Computer equipment on which the computing part of the work was performed (graphic computer station), and also the equipment used for the study of fluid micro-inclusions in minerals when determining fluid parameters in a real geological surroundings (Raman spectral analyzer with a RamMix M532 microscope), was acquired by IPGG RAS under the program “Updating the instrument base of leading research and development organizations, academic sector science” within the framework of the national project “Science”.

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Ivanov, M.V., Bushmin, S.A. Thermodynamic Model of the Fluid System H2O–CO2–NaCl at P-T Parameters of the Middle and Lower Crust. Petrology 29, 77–88 (2021).

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  • middle and lower crust
  • fluid system H2O–CO2–NaCl
  • equation of state
  • thermodynamic model
  • phase state
  • density and compressibility of fluid
  • composition and activity of components