Phase relations in the CH 4-H 2O-NaCl system at 2 kbar, 300 to 600°C as determined using synthetic fluid inclusions

Abstract

Synthesis of fluid inclusions in the CH 4-H 2O-NaCl system was accomplished by subjecting fractured quartz or fluorite, along with known quantities of CH 4, H 2O, and NaCl, to a pressure of 2 kbar and temperatures of 300, 400, 500, or 600°C, in sealed Au capsules. Under the elevated P-T conditions, some of the fractures healed, trapping fluids as inclusions. Microthermometric measurements conducted on the fluid inclusions show that at 2 kbar and 400 to 600°C, there are very broad regions of fluid unmixing in the CH 4-H 2O-NaCl system. For those bulk fluid compositions that lie in the two-phase (i.e., immiscible fluids) field, the high-density phase is enriched in NaCl, whereas the low-density phase is enriched in CH 4. For any given bulk composition, the degree of NaCl enrichment in the high-density phase increases, whereas the degree of CH 4 enrichment in the low-density phase decreases, as temperature increases from 400 to 600°C. Our experimental constraints on the size of the two-phase field are generally consistent with results generated using the equation-of-state GEOFLUIDS (available at http://geotherm.ucsd.edu/geofluids/). However, when comparing the compositions of coexisting immiscible fluids, as determined experimentally vs. calculated using GEOFLUIDS, we find that some relatively small but probably significant differences exist between our experiments and this equation of state.