GEOBAROMETRY OF LOW-TEMPERATURE ECLOGITES - APPLICATIONS OF ISOTHERMAL PRESSURE-ACTIVITY CALCULATIONS

Estimation of metamorphic pressures in low temperature eclogite (Type C) is difficult because of the high variance mineral assemblages and p roblems in geothermometry, solution properties of low-temperature omph acite, and the thermodynamic properties of clinozoisite. We have consi dered equilibria in the CaO-FeO-MgO-TiO2-Al2O3-SiO2-H2O (CFMTASH) syst em involving the phase components, quartz, rutile, kyanite, ilmenite, almandine, pyrope, grossular, clinozoisite, sphene, diopside, and H2O- fluid There are four linearly independent equilibria involving the pha se components in this system. Because kyanite can crystallize as a nea rly pure phase, the lack of kyanite in a rock indicates that a(Al2SiO5 ) is <1.0. If we can estimate temperature independently, we can solve for a(Al2SiO5) and pressure by using two of the equilibria in isotherm al pressure-activity diagrams. We have applied this approach to eclogi tes from New Caledonia and from southwestern Oregon. For the New Caled onia eclogites, calculated pressures range from 11.2 to 13.6 kbar at 5 00-degrees-C, and are consistent with the minimum pressures based upon the presence of jadeitic pyroxene + quartz and the lack of stable alb ite. Oregon eclogites come from different tectonic blocks and calculat ed minimum pressures of 11-12 kbar are based upon the presence of jade itic pyroxene + rutile + garnet and lack of stable albite and ilmenite at reduced values of a(SiO2) (0.7-0.9).