Calculation of CO2 activities using scapolite equilibria: constraints on the presence and composition of a fluid phase during high grade metamorphism

Thermodynamic and phase equilibrium data for scapolite have been used to calculate CO₂ activities (aCO₂) 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 aCO₂ by the reaction Meionite+Quartz = Grossular+Anorthite+CO₂. Granulites from four high-grade terranes (Grenville Province, Canada; Sargut Belt, India; Furua Complex, Tanzania; Bergen Arcs, Norway) yield aCO₂=0.4-1, with most >0.7. For scapolite-bearing granulites from the Furua Complex, in which aCO₂≥0.9, calculated H₂O activities (aH₂O) 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 terranes mostly yield aCO₂< 0.5. The presence of calcite and/or wollastonite provides additional evidence for the low aCO₂ 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 aCO₂ (0.1 to >1). The calculated fluid activities are consistent with metamorphism (1) in the presence of a mixed CO₂−H₂O fluid phase in which CO₂ is the dominant fluid species but other C−O−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−H₂O fluid phase. In contrast the relatively restricted and low values of aCO₂ for calc-silicates require an H₂O-rich fluid or absence of a fluid phase during metamorphism. The range of values for xenoliths are most consistent with absence of a fluid phase. The primary implication of these results is that a CO₂-rich fluid accounts for the reduced aH2_O in scapolite-bearing granulites. However, scapolite may be stable with a wide range of fluid compositions or in the absence of a fluid phase, and the presence of scapolite is not a priori evidence of a CO₂-rich fluid phase. In addition, close association of scapolite-free mafic granulites with scapolite-bearing granulites having identical mineral compositions in the Furua Complex, and the absence of scapolite from most granulite terranes implies that a CO₂-rich fluid phase is not pervasive on an outcrop scale or common to all granulite terranes. Contribution No. 474 from the Mineralogical Laboratory, University of Michigan