An experimental study of phase equilibria in the systems H_2O-CO_2-CaCl_2 and H_2O-CO_2-NaCl at high pressures and temperatures (500-800 ℃, 0.5-0.9 GPa): geological and geophysical applications

摘要

Phase equilibria in the ternary systems H_2O-CO_2-NaCl and H_2O-CO_2-CaCl_2 have been determined from the study of synthetic fluid inclusions in quartz at 500 and 800 ℃, 0.5 and 0.9 GPa. The crystallographic control on rates of quartz overgrowth on synthetic quartz crystals was exploited to prevent trapping of fluid inclusions prior to attainment of run conditions. Two types of fluid inclusion were found with different density or CO_2 homogenisation temperature (Th(CO_2)): a CO_2-rich phase with low Th(CO_2), and a brine with relatively high Th(CO_2). The density of CO_2 was calibrated using inclusions in the binary system H_2O-CO_2. Mass balance calculations constrain tie lines and the miscibility gap between brines and CO_2-rich fluids in the H_2O-CO_2-NaCl and H_2O-CO_2-CaCl_2 systems at 500 and 800 ℃, and 0.5 and 0.9 GPa. The miscibility gap in the CaCl_2 system is larger than in the NaCl system, and solubilities of CO_2 are smaller. CaCl_2 demonstrates a larger salting-out effect than NaCl at the same P-T conditions. In ternary systems, homogeneous fluids are H_2O-rich and of extremely low salinity, but at medium to high concentrations of salts and non-polar gases fluids are unlikely to be homogeneous. The two-phase state of crustal fluids should be common. For low fluid-rock ratios the cation compositions of crustal fluids are buffered by major crustal minerals: feldspars and micas in pelites and granitic rocks, calcite (dolomite) in carbonates, and pyroxenes and amphiboles in metabasites. Fluids in pelitic and granitic rocks are Na-K rich, while for carbonate and metabasic rocks fluids are Ca-Mg-Fe rich. On lithological boundaries between silicate and carbonate rocks, or between pelites and metabasites, diffusive cation exchange of the salt components of the fluid will cause the surfaces of immiscibility to intersect, leading to unmixing in the fluid phase. Dispersion of acoustic energy at critical conditions of the fluid may amplify seismic reflections that result from different fluid densities on lithological boundaries.