Inclusions in minerals were studied from the point of view of thermobarogeochemistry. On the basis of the Peng-Robinson two-parameter equation of state and conditions of diffusional and mechanical stability of fluid mixtures, programs were developed to calculate isochores of entrapping of gas-liquid inclusions and to estimate critical parameters of gas-liquid transition in the quaternary water–carbon dioxide–methane–nitrogen mixture. The difficulties related to the sensitivity to selection of initial conditions in Newton’s method on solving systems of nonlinear equations have been overcome by using experimental and calculated data for ternary mixtures. The obtained P-T characteristics of critical mixtures are given in the form of sections of a multidimensional domain at a constant content of nitrogen and are in agreement with the known characteristics for the ternary H2O–CO2–CH4 mixtures. The critical data for quaternary mixtures were used as initial values for finding parameters of homogenization, from which isochores of entrapment of gas-liquid inclusions in minerals have been calculated. The calculations show that ignoring even a small portion (relative to CO2) of water in highly carbon dioxide inclusions can lead to a serious error in estimation of P and T of mineral formation.

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