The polybaric liquidus surface for the H₂O-rich corner of the NaCl-CO₂-H₂O ternary is calculated, relying heavily on

• 1.

  (1) a Henry's law equation for CO₂ in brines (modified from Drummond, 1981),

• 2.

  (2) the assumption that the contributions of dissolved NaCl and CO₂ in lowering the activity of H₂O are additive, and

• 3.

  (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.

  (1) ternary eutectic at −21.5°C, with ice + clathrate + hydrohalite NaCl-·H₂O + brine m_NaCl = 5.15, m_{co2 = 0.22 + vapor}P_total ≈ P_co2 = 5.7 atm;

• 2.

  (2) peritectic at −9.6°C, with clathrate + hydrohalite + liquid CO₂ + brine m_NaCl = 5.18, m_{co2 = 0.55} + vapor (P_total ≈ P_{co2 = 26.47 atm}); and

• 3.

  (3) peritectic slightly below +0.1 °C, with halite + hydrohalite + liquid CO₂ + brine (m_NaCl ≈ 5.5, m_{co2 ≈ 0.64}) + vapor (P_total ≈ P_co2 ≈ 34 atm).

CO₂ 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.