The ability of fluids flowing through a given volume of crustal rock to affect mass and energy transfer is controlled by the physical and chemical nature of the fluid. Most numerical simulations of fluid-flow and fluid-rock interaction approximate the mass and energy transfer characteristics of crustal fluids using the physical, thermodynamic and transport properties of H2O. Results of fluid inclusion studies indicate that H2O is, in fact, the dominant component of fluids from most shallow to intermediate depth crustal environments, with other components usually present in varying amounts. In many crustal environments, properties of the fluids are adquately respresented by those of the system H2O-CO2-NaCl.

In the pure H2O system, mass and energy transport properties reach extrema and become very sensitive to small variations in temperature or pressure in a restricted region of P-T space near the critical point. This ''critical region'' migrates to higher and lower temperature as NaCl or CO2, respectively, are added to water, mass and energy transport properties become less sensitive to temperature or pressure variations when either NaCl or CO2 are added to water.

The effect of fluid composition on mass transfer is illustrated by the system SiO2-H2O-NaCl. The SiO2-H2O sub-system is characterized by a solubility maximumu, or region of retrograde solubility, at temperatures and pressures slightly above the critical point of water. Addition of NaCl increase the solubility of silica and shifts the quartz solubility maximum to higher temperatures and pressures relative to pure H2O. ¿American Geophysical Union 1991