Estimating the P-T-XH2O Conditions during Crystallization of Low Calcium Granites

The pressure and temperature at which the alkali feldspar-plagioclase boundary curve intersects the $$H_{2}O$$-saturated liquidus of a granitic composition defines the minimum pressure at which alkali feldspar can be the liquidus phase and the minimum temperature at which plagioclase can be stable at the liquidus. The intersection may also define the maximum water content for a magma that had plagioclase at the liquidus. The key pieces of information required to estimate a P-T projection from P-T-$$X_{H_{2}O}$$ space for a granite liquidus are: (1) the solubility of water in the melt composition of interest, (2) the location and shape of the liquidus surface in P-T-$$X_{H_{2}O}$$ space, and (3) the pressure and temperature at which the two-feldspar boundary curve intersects the saturated liquidus. Our review of recent data suggests that Burnham's (1979) model predicts water solubility in granitic melts within about 5-10 mole % near the saturated liquidus but that there are unresolved problems in the undersaturated region. The isobaric temperature depression of the liquidus curves of granites by $$H_{2}O$$ is similar to that known for albite so that if the temperature of the saturated liquidus can be estimated at various pressures, a first order estimate of the granite liquidus in P-T-$$X_{H_{2}O}$$ space may be constructed. Petrographic data for the Scituate granite, central Rhode Island, suggest that plagioclase was the liquidus phase. The experiments of James and Hamilton (1969) suggest that crystallization of the Scituate began at T > 750°C and that the two-feldspar boundary curve intersects the saturated liquidus at T > 750°C, P < 1 kb. Our estimated liquidus surface and the solubility of water calculated from Burnham (1979) suggest that the water content of the Scituate magma was probably less than 4.7 wt % $$H_{2}O$$. The presence of abundant alkali feldspar mesoperthite ($$Or_{50}$$) suggests that solidification was essentially completed at P < 3 kb and T > 665°C.