MICROTHERMOMETRIC ANALYSIS OF SYNTHETIC FLUID INCLUSIONS IN THE HYDROTHERMAL DIAMOND-ANVIL CELL

The hydrothermal diamond-anvil cell (HDAC) was employed as a pressuriz ed fluid inclusion heating stage to determine temperatures of phase tr ansitions in synthetic fluid inclusions in quartz. Using this techniqu e, the common problem of decrepitation or stretching of inclusions hav ing high internal pressures was eliminated. Homogenization temperature s of pure H2O synthetic inclusions determined in the HDAC are inversel y related to the confining pressure exerted on the sample, suggesting a decrease in inclusion volume with increasing confining pressure. The very good reproducibility and reversibility of these experiments indi cate that volume changes during heating under confining pressure in th e HDAC are elastic in nature. However, results of microthermometric ex periments in the HDAC indicate that the change in homogenization tempe rature is significantly larger than would be predicted by the equation s of state for quartz and water. This difference reflects an additiona l component of volume change related to stress within the quartz host, causing displacement of the inclusion walls into the inclusion cavity , as predicted by theoretical models describing the behavior of inclus ions in minerals. Liquid-vapor homogenization temperatures [Th(L-V)] a nd halite dissolution temperatures (Tm halite) were determined for syn thetic fluid inclusions in the ternary H2O-NaCl-CO2. The measured homo genization temperatures were regressed as a function of confining pres sure at Th(L-V). The intersection of the Th(L-V)-confining pressure cu rve with an independently obtained P-T curve for the solvus of the sam e ternary composition provides the ''correct'' homogenization temperat ure, that is, the homogenization temperature corresponding to a confin ing pressure equal to the pressure on the solvus at that same temperat ure. This method for determining corrected Th(L-V) is based on the ass umption that the effect of elastic stress on Th(L-V) approaches zero a s the confining pressure approaches the internal pressure in the inclu sion at homogenization. The Th(L-V) values at these intersection point s were used to calculate lines of constant homogenization temperature (iso-Th lines). For a composition of H2O + 40 wt% NaCl + 10 mol% CO2, the iso-Th slopes decrease from about 53 bar/degrees C at Th(L-V) = 50 0 degrees C to 8.5 bar/degrees C for Th(L-V) = 650 degrees C, The hali te dissolution temperature (in the presence of liquid and vapor) avera ges 342 degrees C (range +/-6 degrees C) without recognizable pressure dependence. The slopes of iso-Th lines for a composition of H2O + 20 wt% NaCl + 20 mol% CO2 decrease from approximately 23 bar/degrees C at Th(L-V) = 475 degrees C to 6 bar/degrees C for Th(L-V) = 600 degrees C. Using the HDAC technique, the high-pressure portion of the halite l iquidus was redetermined for an H2O-NaCl solution containing 40 wt% Na Cl. These new measurements confirm the data of Bodnar (1994), which we re obtained using a gas-flow heating stage at 1 atm confining pressure . This indicates that Bodnar's (1994) observation of an essentially pr essure independent liquidus at pressures greater than or similar to 2 kbar is real and not a result of inclusion stretching.