Thermoelastic equilibrium of fluid inclusion–host mineral systems under homogenization measurement

Abstract

Microthermometric data of fluid inclusions can elucidate the physicochemical properties of lithospheric fluids, but the inclusions must satisfy several criteria to yield proper fluid information. One is the ‘constant volume criterion’: the inclusion volume must remain constant after being trapped. However, volume changes of fluid inclusions occur in nature during emplacement underground or uplifting to the surface. They can also occur during sample preparation or data collection in the laboratory. Therefore, thermoelastic deformation of the crystal lattice surrounding fluid inclusions during experiments might increase uncertainty about microthermometric data. Herein, we introduce and assess a method using the equation of state of pure CO₂ and thermoelastic equilibrium between a fluid inclusion and a host mineral to estimate fluid inclusion volume changes accurately during measurements of homogenization temperature. The estimation is valid if the inclusion and host are isotropically elastic, concentric spheres. Subsequently, we extended the equation of state to a more comprehensive Redlich and Kwong equation of state, which is applicable to more complicated fluid systems. Furthermore, for accurate treatment of the elastic effects involving an elastically anisotropic host mineral, we propose the method’s application to an anisotropic host mineral. If physical properties of the host mineral and the molar volume of the fluid inclusion at an arbitrary temperature are known, then one can use this method for accurate estimation of the molar volume at a given temperature. This microthermometric data based method can accurately elucidate characteristics of the crust and mantle–fluid activity.