Fluid inclusion phase ratios, compositions and densities from ambient temperature to homogenization, based on PVTX properties of H2O[sbnd]NaCl

Abstract

Fluid inclusions are the best tool to characterize the properties of fluids in paleo-geological environments, and the most widely used methods applied to fluid inclusions are petrographic analysis and microthermometry. Here, we present a comprehensive review of the phase characteristics of fluid inclusions, represented by the binary system H2ONaCl, from ambient temperature to homogenization. We quantify the changes in compositions, densities and volume fractions of liquid, vapor and halite as temperature is increased. Fluid inclusions of any bulk density, salinity, or homogenization behavior show a decrease in the volume fraction of vapor (i.e., shrinkage of the bubble) during heating to ~400 °C. For fluid inclusions that homogenize to vapor, this leads to a phenomenon referred to in previous studies as “homogenization by inversion,” wherein the volume fraction of vapor decreases during heating to ~380 °C, and increases with continued heating thereafter. Inclusions with salinity less than ~10 wt% NaCl show a rapid increase in volume fraction of vapor with heating above ~380 °C, and plateau at >90 vol% vapor, leading to apparent homogenization at temperatures well below the actual homogenization. Inclusions containing a fluid with the critical density may exhibit reversals in the volume fraction of vapor during heating. Inclusions having the critical density and low salinities will generally homogenize by fading of the meniscus while the volume fraction of vapor is quite low (<10 vol%) and may be misinterpreted as homogenizing to the liquid. These results aid in understanding and interpreting petrographic and microthermometric observations.