Calculation of the vapor-saturated liquidus for the NaCl-CO2-H2O system
The polybaric liquidus surface for the H 2 O-rich corner of the NaCl-CO 2 -H 2 O ternary is calculated, relying heavily on 1. (1) a Henry's law equation for CO 2 in brines (modified from Drummond, 1981), 2. (2) the assumption that the contributions of dissolved NaCl and CO 2 in lowering the activity of H 2 O are additive, and 3. (3) data on the CO 2 clathrate solid solution (nominally CO 2 · 7.3H 2 O, but ranging from 5.75 to 8 or 9 H 2 O) from Bozzo et al. (1975). The variation with composition of the activity of CO 2 ·7.3H 2 O, or any other composition within the clathrate field, is small, thereby simplifying the calculations appreciably. Ternary invariant points are 1. (1) ternary eutectic at −21.5°C, with ice + clathrate + hydrohalite NaCl-·H 2 O + brine m NaCl = 5.15, m co 2 = 0.22 + vapor P total ≈ P co 2 = 5.7 atm; 2. (2) peritectic at −9.6°C, with clathrate + hydrohalite + liquid CO 2 + brine m NaCl = 5.18, m co 2 = 0.55 + vapor ( P total ≈ P co 2 = 26.47 atm ); and 3. (3) peritectic slightly below +0.1 °C, with halite + hydrohalite + liquid CO 2 + brine ( m NaCl ≈ 5.5, m co2 ≈ 0.64 ) + vapor ( P total ≈ P co 2 ≈ 34 atm). CO 2 isobars have been contoured on the ternary liquidus and also on the 25°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.