Fluid inclusion geobarometry: Pressure corrections for immiscible H2O–CH4 and H2O–CO2 fluids

Temperatures and pressures of heterogeneous carbo-aqueous fluids are usually derived from the isochore of the gaseous phase calculated using an equation of state for the anhydrous gas. Theoretical calculations with binary CH4–H2O and CO2–H2O systems have shown that this approach is justified only for trapping pT conditions far away from the critical curve, and for the partial homogenization of the gaseous phase to vapour. In contrast, partial homogenization to the liquid phase is significantly affected by volume effects associated with the reaction of the gas to hydrates, and/or with the phase transformation of the supercooled water to hexagonal ice. The curvature and slope of the two-phase isochoric trajectory of the water-containing gaseous inclusion deviate slightly from that of a homogeneous supercritical anhydrous gas with an equivalent density, thus contributing to the inaccuracy of the calculated trapping pressure. The calculated geologic pressure can be significantly overestimated in CO2–H2O inclusions with the partial homogenization of the carbonic phase in the presence of water, and in CH4–H2O inclusions trapped at temperatures lower than 270 °C. All heterogeneous gas–aqueous fluids trapped close to their critical conditions yield erroneous results when interpreted in terms of an anhydrous supercritical gas. Special care must be exercised when calculating the “minimum trapping pressure” from the partial homogenization temperature of the gaseous phase in aqueous liquid-dominated fluid inclusions. In this special case, a triple partial homogenization at extremely varying temperatures can be observed in H2O–CO2 inclusions with 30–40 vol.% of the gaseous phase at room temperature: a metastable partial homogenization in the presence of ice and clathrate below Te, stable partial homogenization in the presence of aqueous liquid and clathrate, and a second metastable partial homogenization in the presence of aqueous liquid alone.