Phase equilibria in the system CaO-MgO-SiO2-CO2-H2O-NaCl are calculated to illustrate phase relations in metacarbonates over a wide range of P-T-X[H2O-CO2-NaCl] conditions. Calculations are performed using the equation of state of Duan et al. (1995) for H2O-CO2-NaCl fluids and the internally consistent data set of Gottschalk (1997) for thermodynamic properties of solids. Results are presented in isothermal-isobarical plots showing stable mineral assemblages as a function of fluid composition. It is shown that in contact-metamorphic P-T regimes the presence of very small concentrations of NaCl in the fluid causes almost all decarbonation reactions to proceed within the two fluid solvus of the H2O-CO2-NaCl system.
Substantial flow of magma-derived fluids into marbles has been documented for many contact aureoles by shifts in stable isotope geochemistry of the host rocks and by the progress of volatile-producing mineral reactions controlled by fluid compositions. Time-integrated fluid fluxes have been estimated by combining fluid advection/dispersion models with the spatial arrangement of mineral reactions and isotopic resetting. All existing models assume that minerals react in presence of a single phase H2O-CO2 fluid and do not allow for the effect that fluid immiscibility has on the flow patterns.
It is shown that fluids emanating from calc-alkaline melts that crystallize at shallow depths are brines. Their salinity may vary depending mainly on pressure and fraction of crystallized melt. Infiltration-driven decarbonation reactions in the host rocks inevitably proceed at the boundaries of the two fluid solvus where the produced CO2 is immiscible and may separate from the brine as a low salinity, low density H2O-CO2 fluid. Most parameters of fluid-rock interaction in contact aureoles that are derived from progress of mineral reactions and stable isotope resetting are probably incorrect because fluid phase separation is disregarded.