Evolutionary paths for the formation of different types of fluid inclusions in the H2O-NaCl system

Abstract

Previous studies revealed very complicated phase ratios and microthermometric behavior of fluid inclusions, which can be attributed to the various phase relations in the H2O-NaCl system and different evolutionary paths after their entrapment at different conditions. This numerical calculation work aims at presenting a comprehensive illustration of fluid inclusion formation in the H2O-NaCl system and facilitating the interpretation of microthermometric data. We firstly illustrate a series of contours figures in the pressure–temperature projection for fluid salinities at 0.1, 1, 10, 30, 50, and 70 wt% NaCl, for the bubble curve and the dew curve, and for the halite liquidus of the H2O-NaCl system to demonstrate the changes of salinities and densities that lead to the variations of room-temperature phase ratios of fluid inclusions. Then we comprehensively construct the evolutionary paths of fluids trapped at variable conditions. Isochores of the entrapped halite-undersaturated fluids may intersect the vapor + liquid surface of the H2O-NaCl system on the dew curve, the bubble curve, or the locus on the critical curve, or intersect the bubble curve of the vapor + halite region. Isochores of the entrapped halite-saturated fluids may intersect the bubble curve of the vapor + liquid surface at elevated temperature, the dew curve of the vapor + liquid surface, the halite liquidus, or their junction. After the intersections, the fluid inclusions evolve in the two-phase regions (vapor + liquid, liquid + halite, or vapor + halite), and the detailed evolution of fluid pressure, temperature, density, salinity, and phase ratios of the coexisting two phases are systematically discussed. Evolutionary paths of the halite-saturated fluids and the extremely low salinity fluids will eventually intersect the vapor + liquid + halite surface and then evolve to room temperature.The evolutionary paths above define nine types of fluid inclusions with characteristic phase ratios and microthermometric behavior, making a necessary complement to previous studies for the understanding of fluid inclusion formation and interpretation of microthermometric data. For the first time, we present the evolutionary path for a halite-bearing fluid inclusion that experiences partial homogenization by halite dissolution and total homogenization by liquid disappearance. Underestimation of the homogenization temperature is inevitable for vapor-rich fluid inclusions formed in the vapor + halite region, therefore, such fluid inclusions, typically formed in very shallow porphyry systems, provide invalid temperature information for fluid evolution. The residual vapor phase at halite dissolution for halite-bearing fluid inclusions may lead to an over-estimation of the bulk fluid salinity by over 3 % at elevated temperature conditions. The contour figures can facilitate the understanding of post-entrapment modifications of fluid inclusions which alters fluid inclusion phase ratios and homogenization behavior.