T-XCO2 stability relations and phase equilibria of a calcic carbonate scapolite
At a total pressure of 5 kb, calcic, Cl-free scapolite (Me 83 ) is stable relative to plagioclase-bearing assemblages at T ≧ 625°C "> T ≧ 625°C , X CO 2 ≧ 0.12 "> X CO2 ≧ 0.12 . With decreasing temperature, scapolite breaks down to plagioclase + calcite. Scapolite is replaced by plagioclase + grossular + cancrinite + CO 2 in the presence of H 2 O-rich fluids. The stable coexistence of scapolite and calcite, an assemblage typical of most natural occurrences of calcic scapolite, is limited by the reaction: scapolite + calcite → grossular + cancrinite + CO 2 , which occurs at 750°C, X CO 2 = 0.46; 700°C, X CO 2 = 0.33; 650°C, X CO 2 = 0.18, for the chosen bulk composition. Generalization of the experimental results to encompass the complete range of fully carbonated scapolite compositions indicates that mizzonite (Me 75 ) has the largest T - X CO 2 stability field. For scapolite more calcic than mizzonite, stable growth is restricted to conditions of increasingly higher temperature and X CO 2 . The experimental results are consistent with various petrologic features of scapolite-bearing rocks, particularly scapolite-clinopyroxene granulites, and indicate that such rocks were formed in the presence of CO 2 -rich fluids.