Numerical models for hydrothermal fluid flow commonly include the premise that the migrating fluid is pure water. This simplifying assumption implies that small amounts of dissolved gases (e.g., CO₂) and solids (e.g., NaCl and SiO₂) have negligible effects on the thermophysical properties and phase relations of H₂O-rich hydrothermal solutions. Thus, it is significant that recently acquired density and phase equilibrium data for CO₂-H₂O mixtures (Seitz and Blencoe, 1997; Seitz et al., 1999; Blencoe et al., 2001; Seitz et al., in prep.) can be used to calculate highly accurate relative bouyancies for the fluids at 300-400°C, P £ 100 MPa, using pure H₂O as the reference fluid. The relative bouyancy (RB) of a CO₂-H₂O mixture at a given P and T is defined by the relation RB = r_H2O/r, where r_H2O is the density of pure water, and r is the density of the CO₂-H₂O mixture. Peak values for RB at 300, 350 and 400°C, calculated from the density data for CO₂-H₂O mixtures—with densities for pure H₂O given by a highly accurate equation of state (Wagner and Pruß, 1995)—are, respectively: 11.8 (P = 9.94 MPa, X_CO2  = 0.10); 5.4 (P = 17.44 MPa, X_CO2 =  0.20); and 2.2 (P = 34.94 MPa, X_CO2 =  0.30). Therefore, at 300-400°C, P £ 100 MPa: (1) the most bouyant CO₂-H₂O mixtures are H₂O-rich, and (2) the bouyancy contrast between the most bouyant CO₂-H₂O fluid and pure H₂O increases sharply with decreasing temperature and pressure. These results strongly suggest that small amounts of CO₂ can have profound effects on the rates and patterns of hydrothermal fluid flow in sedimentary basins, geothermal fields and shallow contact metamorphic aureoles.

*Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy under contract DE-AC05-00OR22725, Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.