CALCULATION OF THE ACTIVITY OF AL2SIO5 - APPLICATIONS TO THE GEOBAROMETRY AND GEOHYGROMETRY OF GARNET AND STAUROLITE ZONE METAPELITIC ROCKS

Geobarometry in garnet and staurolite zone metapelitic rocks is render ed difficult by the lack of an Al2SiO5 phase in the typically high var iance mineral assemblages, for example, garnet + biotite + muscovite plagioclase + quartz. The garnet-muscovite-plagioclase-biotite geobar ometer (GMPB) is most commonly used with the garnet-biotite Fe-Mg exch ange geothermometer (GB), but there is usually no independent check on the consistency of the calculated pressure and temperature using anot her geothermobarometer, Values of alpha Al2SiO5 (the activity of Al2Si O5 In the system) can be calculated from the equilibrium: 3 CaAl2Si2O8 = Ca3Al2Si3O12 + 2 Al2SiO5 + SiO2(GPAQ) at the P and T derived from G MPB and GB geothermobarometry, The standard state chosen is the alumin um silicate polymorph that would be stable at that P and T. The calcul ated activity of Al2SiO5 serves as a test of consistency of GMPB and G B geothermobarometry in aluminum silicate-free rocks (activity of Al2S iO5 should be <1.0). Data from the literature indicate that many rocks lacking an Al2SiO5 phase have alpha(Al2SiO5) < 1.0, and values of alp ha(Al2SiO5) generally increase from the garnet zone to the staurolite zone. The P,T, calculated alpha(Al2SiO5), and the activities of the Na AlSi3O8 phase component in plagioclase and the NaAl3Si3O10(OH)(2) + Si O2 = NaAlSi3O8 + Al2SiO5 + H2O to solve for the equilibrium values of alpha(H2O). Values for alpha(H2O) determined for rocks from southeaste rn British Columbia are generally low, varying between 0.2 and 0.4. Th ese values are lower than those calculated from interbedded, carbonate -bearing, calc-silicate rocks. Calculated values of alpha(Al2SiO5) and , consequently, values of alpha(H2O) are very sensitive to temperature estimates and to solution models for the albite and paragonite phase components.