CALCULATION OF THE VAPOR-SATURATED LIQUIDUS FOR THE NACL-CO2-H2O SYSTEM

The polybaric liquidus surface for the H2O-rich corner of the NaCl-CO2-H2O ternary is calculated, relying heavily on (1) a Henry's law equation for CO2 in brines (modified from DRUMMOND, 1981), (2) the assumption that the contributions of dissolved NaCl and CO2 in lowering the activity of H2O are additive, and (3) data on the CO2 clathrate solid solution (nominally CO2 . 7.3H2O, but ranging from 5.75 to 8 or 9 H2O) from Bozzo et al. (1975). The variation with composition of the activity of CO2.7.3H2O, or any other composition within the clathrate field, is small, thereby simplifying the calculations appreciably. Ternary invariant points are (1) ternary eutectic at - 2 1.5 0 C, with ice + clathrate + hydrohalite (NaCl - 2H2O) + brine (M(NaCl) = 5.15, M(CO2) = 0.22) + vapor (P(total) almost-equal-to P(CO2) = 5.7 atm); (2) peritectic at -9.6-degrees-C, with clathrate + hydrohalite + liquid CO2 + brine (M(NaCl) = 5.18, M(CO2) = 0.55) + vapor (P(total) almost-equal-to P(CO2) = 26.47 atm); and (3) peritectic slightly below +0.1-degrees-C, with halite + hydrohalite + liquid CO2 + brine (m(NaCl) almost-equal-to 5.5, M(CO2) almost-equal-to 0.64) + vapor (P(total) almost-equal-to P(CO2) almost-equal-to 34 atm). CO2 isobars have been contoured on the ternary liquidus and also on the 25-degrees-C isotherm. An important caveat regarding the application of this information to the interpretation of the freezing-thawing behavior of fluid inclusions is that metastable behavior is a common characteristic of the clathrate.