Geothermometry, geobarometry and the Al2SiO5 triple point at Mt. Moosilauke, New Hampshire

The distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the Acadian Orogeny to have been approximately those of the Al 2 SiO 5 invariant point, thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the Al 2 SiO 5 invariant point of Holdaway (1971). Four fluid-independent equilibria yield P-T estimates consistent with Holdaway (1971). Two fluid-dependent equilibria yield internally consistent but unrealistically high temperatures. Our data are consistent with the conclusions that: (1) only pyrope-grossular mixing in the quaternary garnet system is significantly non-ideal; and (2) the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point P-T conditions.--Modified journal abstract.