The equilibrium conditions of the following reaction 2 zoisite --1 C02,~3 anorthite + 1 calcite+ 1 HsO 2 CasAls[O/OH/SiO~/SisO:J+ 1 COs~3 CaAlsSieOs-r I CaC03+ 1 I-IsO have been determined experimentally at total pressures of P/~ 2000 bars, P/= 5000 bars, and P/~ 7000 bars.
Owing to the vertical position of the equilibrium curves in isobaric T--Xco~ diagrams, the composition of the binary H20--CO 2 fluid phase coexisting with zoisite is independent of temperature in the temperature interval investigated. According to our experiments, orthorhombic zoisite is only stable in equilibrium with a fluid phase at a concentration of CO s which is less than, respectively, ca. 2 Mol% CO 2 at P/=2000 bars, ca. 6 Mol% at PI=5000 bars, and ca. 10 Mol% at P/--7000 bars. Thus, the fluid phase coexisting with zoisite is rich in HsO. While this is independent of temperature the experimental data demonstrate that the influence of pressure cannot be neglected: With increasing pressure the concentration of CO s of the fluid phase coexisting with zoisite can rise a little. The position of the reaction studied, which is independent of temperature and exhibits small values of Xco: ,leads to two important petrogenetic conclusions:
a) The occurrence of zoisite is an indicator for a COs-poor and H20-rich fluid composition during metamorphism of marly calcsilicates.
b) If the concentration of CO~ of the fluid phase coexisting with zoisite exceeds the equilibrium value of XCO~ calcite+ anorthitoq-~t20 is formed from zoisite+ CO 2 . Thus, a considerable increase in the anorthite-content of plagioclase is possible.