SYNTHETIC FLUID INCLUSIONS .10. EXPERIMENTAL-DETERMINATION OF P-V-T-X PROPERTIES IN THE CO2-H2O SYSTEM TO 6-KB AND 700-DEGREES-C

Pressure-volume-temperature-composition (P-V-T-X) relations in the CO2-H2O system have been experimentally determined from 2 to 6 kb and 400-degrees to 700-degrees-C for fluid compositions between 12.5 and 87.5 mole percent CO2 using the synthetic fluid inclusion technique. Molar volumes (MVs) of CO2-H2O mixtures (calculated from densities of fluid inclusions) are used in conjunction with the equations of state of Haar, Gallagher, and Kell (1984) for H2O and Shmonov and Shmulovich (1974) for CO2 to generate an empirical expression that predicts molar volumes of all CO2-H2O compositions over this P-T range. From 2 to 6 kb and 400-degrees to 700-degrees-C, CO2-H2O fluids display very limited departures from ideal mixing with maximum excess volume of +1.31 cm3/mole (4.3 percent) occurring at 6.0 kb, 500-degrees-C, and X(CO)2 = 0.5. For all temperatures between 400-degrees and 700-degrees-C, at any given composition, the smallest excess volumes are found along the 3.0 kb isobar and increase toward both higher and lower pressures. Molar volumes of CO2-H2O mixtures determined using synthetic inclusions are in good general agreement with those at 400-degrees and 500-degrees-C and P less-than-or-equal-to 4 kb reported by Shmulovich and others (1980). Compared with molar volumes at 2 kb reported by Franck and Todheide (1959), our equation predicts somewhat larger values-particularly at 700-degrees-C. At low pressure good agreement is found between the isochores predicted by various published equations of state and those derived from synthetic fluid inclusions. Below approx 3 kb maximum discrepancies are typically less than 300 bar. At higher pressures, isochores calculated by using the various formulations begin to diverge; our results generally predict the highest pressures. Maximum differences between isochores predicted by the various equations occur for equi-molar or moderately CO2-rich compositions at pressures above 3 kb where discrepancies as large as 600 to 700 bars are noted. We have determined the CO2-H2O solvus between approx 0.5 and 3.0 kb for 0.125 less-than-or-equal-to X(CO)2 less-than-or-equal-to 0.75 using total homogenization temperatures of synthetic fluid inclusions. The results are in close agreement with the experimental determination of Todheide and Franck (1963). Somewhat higher temperatures (approx 15-degrees - 20-degrees-C) are recorded for CO2-poor (X(CO)2 = 0.125) and CO2-rich (X(CO)2 = 0.75) compositions compared with their determination while considerably smaller (2-degrees to 10-degrees-C) discrepancies are found for intermediate compositions. Compared with the experimental determination of Takenouchi and Kennedy (1964) our study indicates considerably higher solvus temperatures for CO2-rich compositions with maximum disagreement at about 1.5 kb (approx 40-degrees-C for X(CO)2 = 0.625; approx 90-degrees-C for X(CO)2 = 0.75). According to our data, the saddle point on the CO2-H2O solvus occurs at approx 2 kb and X(CO)2 = 0.37 and at a temperature of 269-degrees-C. This temperature is 3-degrees to 4-degrees-C higher than that predicted by the above studies.