Geological applications of synthetic fluid inclusions

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Virginia Polytechnic Institute and State University


Vapor-saturated solubility relationships in the system NaCl-KCl-H₂0 have been determined by experimentally synthesizing fluid inclusions in quartz in the presence of known brine compositions and then measuring the dissolution temperatures of halite and/or sylvite daughter crystals within the inclusions using a microscope equipped with a heating stage. These data, along with other literature values have been used in a regression routine to generate a series of equations describing vapor-saturated solubility relations within the halite, sylvite and hydrohalite stability fields. These equations, together with a recently published equation for the ice stability field (Hall et al., 1987), have been used to construct the complete vapor-saturated solubility surface in the NaCl-KCl-H₂O ternary system. The diagram may be used in the interpretation of microthermometric data to determine the compositions of fluid inclusions approximated by the NaCl-KCl-H₂O system.

P-T conditions inferred from fluid inclusions in metamorphic rocks often disagree with values predicted from mineral equilibria calculations suggesting that inclusions formed during early stages of regional metamorphism continue to re-equilibrate during burial and subsequent uplift in response to differential pressure. P-T conditions accompanying burial and uplift were experimentally simulated by forming pure-H₂0 inclusions in quartz, and then re-equilibrating the inclusions such that final confining pressures ranged from 5 kbar above to 4 kbar below the original formation pressure.

Homogenization temperatures of re-equilibrated inclusions indicated densities intermediate between the initial and final P-T conditions. In short-term experiments (7 days) where the initial internal overpressure exceeded 1 kbar most H₂O inclusions re-equilibrated until their internal pressures were between ~75O and 1500 bars above the confining pressure, regardless of the initial pressure differential. In a long-term experiment (52 days) some inclusions were found to have completely re-equilibrated.

In experiments where the confining pressure during re-equilibration exceeded the original formation pressure, the densities of pure-water inclusions increased to values intermediate between the initial and final P-T conditions. Additionally, these inclusions were generally surrounded by a halo of smaller inclusions also of intermediate density and resulting in a texture similar to that previously ascribed to decrepitation resulting from internal overpressure. Pressure-volume-temperature-composition (PVTX) relations in the CO₂-H₂O system have been experimentally determined from 2 to 6 kbar and 400° to 700°C for fluid compositions between 12.5 and 87.5 mole % CO₂ using the synthetic fluid inclusion technique. The method involves trapping CO₂-H₂O fluids of known composition as inclusions in quartz at elevated pressures and temperatures (PF and TF) and then calculating the desired fluid properties using microthermometric data combined with available PVTX data for this system at low pressures and temperatures.

PTX properties of CO₂-H₂O mixtures were determined from the total homogenization temperatures (Th (total)) of fluid inclusions trapped in the one-fluid phase field. Internal pressures on the solvus (at Th (total)) were calculated using the equation of state of Connolly and Bodnar (1983) and the inclusion densities as determined above. The pressure and temperature of total homogenization of each inclusion defines a point on the solvus unique to that particular bulk fluid composition and density. Thus, the array of many such points determined in this manner delineates the boundary between the one-phase and two-phase fields over a portion of PTX-space.