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Recently published thermodynamic and experimental data in a variety of chemical systems have been evaluated to derive Gibbs free energies for hedenbergite and pyrope. These were used to calculate the geobarometric equilibria

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Thermodynamic and phase equilibrium data for scapolite have been used to calculate CO2 activities (aco~) and to evaluate the presence or absence of a fluid phase in high-grade scapolite bearing meta-anorthosite, granulites, calc-silicates, and mafix xenoliths. The assemblage scapolite-plagioclase-garnet _+ quartz may be used to calculate or limit aco 2 by the reaction Meionite + Quartz = Grossular + Anorthite + CO 2. Granulites from four high-grade terranes (Grenville Province, Canada; Sargut Belt, India; Furua Complex, Tanzania; Bergen Arcs, Norway) yield aco~ =0.4-1, with most >0.7. For scapolite-bearing granulites from the Furua Complex, in which aco 2 > 0.9, calculated H20 activities (an2o) based on phlogopite dehydration equilibria are uniformly low (0.1-0.2). The aco2 calculated for meta-anorthosite from the Grenville Province, Ontario, ranges from 0.2 to 0.8. For Grenville meta-anorthosite also containing epidote, the aH2o calculated from clinozoisite dehydration ranges from 0.2 to 0.6. Calc-silicates from the Grenville, Sargur, and Furua tcrranes mostly yield aco~ < 0.5. The presence of calcite and/or wollastonite provides additional evidence for the low aco ~ in calc-silicates. Samples from six xenolith localities (Lashaine, Tanzania; Eifel, W. Germany; Lesotho; Delegate, Gloucester, and Hill 32, Australia) yield a wide range of aco ~ (0.1 to > 1). The calculated fluid activities are consistent with metamorphism (1) in the presence of a mixed CO2-H20 fluid phase in which CO2 is the dominant fluid species but other CO -H-S species are minor, (2) in the absence of a bulk fluid phase ("fluid-absent metamorphism"), or (3) in the presence of a fluid-bearing melt phase. The results for many granulites and Grenville meta-anorthosite are consistent with the presence of a CO2-rich, mixed CO2-H20 fluid phase. In contrast the relatively restricted and low values of aco2 for calc-sili-* Contribution No. 474 from the Mineralogical Laboratory, University of Michigan j **