The determination of phase relations in the CH₄-H₂O-NaCl system at 2 and 5 kbars, 300 to 600° C using synthetic fluid inclusions

Abstract

Fluid inclusions were synthesized, using quartz and fluorite as host minerals, to determine the phase relations of the CH₄-H₂O-NaCl system at pressures of 2 and 5 kbars and temperatures of 300, 400, 500, and 600°C . Known quantities of CH₄, H₂O, and NaCl were sealed in Au capsules, along with inclusion-free quartz or fluorite, and elevated to a constant pressure (P) & temperature (T) in cold seal hydrothermal vessels. After the inclusions were formed, microthermometric measurements were made using a modified USGS heating/freezing stage to determine the composition of the trapped fluids. For each experimental P & T, bulk fluid inclusion compositions were determined to lie either in a single phase field or a two-phase field consisting of a high density phase and a low density phase. For those bulk fluid compositions that lie in the two-phase (i.e., immiscible fluids) field, the high density phase is enriched in NaCl and H₂O, whereas the low density phase is enriched in CH₄ with lesser amounts of H₂O. Bulk fluid compositions were plotted on ternary diagrams showing the ene- and two-phase fiends separated by a solvus. As pressure is held constant at 2 kbars, the two-phase field becomes smaller as temperature increases from 300 to 600°C. As temperature is held constant at 400°C, the size of the two-phase field changes relatively little with pressure. This information, in conjunction with pre-existing data of other fluid systems, will permit quantitative modeling of a variety of geologically important processes, including fluid/rock interaction in the diagenetic, metamorphic, igneous, and ore-forming environments.