An Improved Set of a--X Parameters for Fe--Mg--Ca Garnets and Refinements of the Orthopyroxene-Garnet Thermometer and the Orthopyroxene--Garnet--Plagioclase--Quartz Barometer

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Abstract
An internally consistent set of activity-composition relations for Fe-Mg-Ca garnets constrained from calorimetric and phase equilibrium experiments provides the basis for refining the orthopyroxene-garnet thermometer and the orthopyroxene-garnet-plagioclase-quartz barometer based on the equilibria FeSiO3 + 1/3Mg3Al2Si3O12 = MgSiO3 + 1/3Fe3Al2Si3O12 (A) Mg2Si2O6 + CaAl2Si2O8 = 2/3 Mg3Al2Si3O12 + 1/3 Ca3Al2Si3O12 + SiO2 (B) Fe2Si2O6 + CaAl2Si2O8 = 2/3 Fe3Al2Si3O12 + 1/3 Ca3Al2Si3O12 + SiO2. (C) The major modification arises from better constrained values of mixing parameters for the pyrope-almandine binary. Thermodynamic analysis of reversed experimental data on Fe-Mg partioning between orthopyroxene and garnet (Kawasaki & Matsui, 1983; Lee & Ganguly, 1988), when combined with the enthalpy of solution data for the pyrope-almandine binary (Geiger et al.t 1987), yields and AGIT of the exchange reaction as cal in the temperature range 1248–1573 K. The retrieved data, combined with the known thermochemical parameters for the pyrope-grossular and almandine-grossular binaries and complementary phase equilibrium data, have been used to estimate temperatures from equilibrium (A) and pressures from equilibria (B) and (C). The geothermometer registers temperatures higher by 70–100C than those obtained from Harley's (1984) formulation and 20–40°C lower than those from Lee & Ganguly's (1988) formulation for granulite grade rocks; the scatter in estimated temperatures for all areas is lower when compared with the other two formulations. The estimated pressures from (B) and (C) are reasonable for granulite grade rocks in the pressure range 3.0–12.0 kb and agree, in most cases, within ± 1000'b. The smaller scatter in the computed temperatures and convergence of calculated pressure values support the claim that the formulations presented here represent improved versions of the orthopyroxene-garnet geothermometer and the orthopyroxene-garnet-plagioclase-quartz geobarometer.

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