Chemical interaction of aqueous solutions with epidote-feldspar mineral assemblages in geologic systems; II, Equilibrium constraints in metamorphic/geothermal processes

Abstract

Thermodynamic analysis of the system Na/sub 2/O-K/sub 2/O-CaO-FeO-Fe/sub 2/O/sub 3/-Al/sub 2/O/sub 3/-SiO/sub 2/-H/sub 2/O-HCl-CO/sub 2/ at pressures and temperatures to 5 kb and 600/sup 0/C affords quantitative description and interpretation of phase relations among epidote, garnet, plagioclase, and alkali feldspar solid solutions in hydrothermal systems. Comparison of computed and observed compositions of these minerals and coexisting fluids suggests that the calculations afford close approximation of equilibrium and mass transfer in metamorphic/geothermal processes. The standard molal enthalpies of decomposition reactions for plagioclase are of the order of -4 to -50 kcal mole/sup -1/, which requires the activity of the anorthite component of plagioclase coexisting with epidote solid solutions to increase with increasing temperature at constant pressure. In contrast, because the standard molal volumes of these reactions are also negative, a/sub CaAl/sub 2/Si/sub 2/O/sub 8// decreases with increasing pressure at constant temperature. Either stoichiometric epidote or clinozoisite is compatible with plagioclase, quartz, calcite, and an aqueous phase, but only at X/sub CO/sub 2// less than or equal to 0.2. The complex zoning commonly exhibited by both plagioclase and epidote solid solutions in geologic systems can be attributed to minor isothermal/isobaric changes in the composition of coexisting aqueous solutions. Logarithmic activity and fugacitymore » diagrams constructed with provision for solid solution permit documentation of such changes and facilitate thermodynamic interpretation of mineral and fluid compositions in metamorphic/geothermal systems.« less