The effect of CO2 and N2 on phase relations, fluid composition, and quartz solubility in amphibolite facies metamorphic rocks

Abstract

Phase equilibria in the system SiO2–TiO2–Al2O3–Fe2O3–MnO–MgO–CaO–Na2O–K2O–P2O5–H2O–CO2–N2 are calculated to illustrate phase relations in amphibolite facies metasediments over a wide range of X[H2O–CO2–N2] conditions at 600 °C and 4.4 kb. Calculations are performed using the Gibbs free energy minimization technique. Results are presented in plots showing stable mineral assemblages as a function of total carbon in the system at varying water (\(a_{{{\text{H}}_{2} {\text{O}}}}\) = 1) content in the presence/absence of N2 in the fluid. The calculations indicate that the typical assemblage plagioclase—quartz—biotite—ilmenite—garnet—apatite is restricted to the rocks with CO2 saturation and \(X_{{{\text{H}}_{ 2} {\text{O}}}}\) higher than 60% in the fluid. Significant decrease in \(X_{{{\text{CO}}_{2} }}\) favors the stability of muscovite rather than garnet, whereas the decrease in \(X_{{{\text{H}}_{ 2} {\text{O}}}}\) leads to the stability of microcline over all range of \(X_{{{\text{CO}}_{2} }}\). This paper also presents the composition and parameters (pH, Eh) of the fluid equilibrated with mineral assemblage. It is shown that the presence of low concentrations of N2 causes the fluid to consist of two phases when an aqueous supercritical solution (AS) coexists with a supercritical fluid with gas-like properties (SF). At high concentration of N2, the fluid consists of SF alone; in the absence of nitrogen, the fluid consists of AS alone. The solubility of monomer SiO20 and dimer Si2O40 decreases with increasing CO2 and after CO2 saturation point is held constant. The magnitude of the silica solubility at CO2 saturation depends upon the water content in AS. The effect of nitrogen on quartz solubility has been demonstrated to be negligible.