Plagioclase-garnet-Al 2 SiO 5 -quartz; a potential geobarometer-geothermometer

Abstract

For the reaction 3CaAl/sub 2/Si/sub 2/O/sub 8/ = Ca/sub 3/Al/sub 2/Si/sub 3/O/sub 12/ + 2Al/sub 2/SiO/sub 5/ + SiO/sub 2/, the logarithm of the apparent distribution coefficient (K/sub D/) varies from -0.5 to -1.4 for kyanite-bearing assemblages and from -2.1 to -3.4 for sillimanite and/or andalusite-bearing assemblages. Using equilibrium constant equations derived from experimental data on pure phases and an ideal solution model for garnet and plagioclase solid solutions, individual P-T curves can be calculated for different values of log K/sub D/. If P or T can be independently estimated, these curves can be used as either geothermometers or geobarometers. The degree of non-ideality of grossular solid solution in garnet can be estimated from calculations using the activity coefficients of anorthrite in plagioclase and the kyanite--sillimanite P-T equilibrium curves as a limiting case for kyanite and sillimanite-bearing assemblages.