Many studies have demonstrated the importance of the fluid phase during regional metamorphism of impure marble, yet uncertainty still exists as to the scale of fluid exchange and the composition of metamorphic fluids. This is due largely to difficulties in evaluation of the effects of solid solution, the importance of fluid components other than H2O and CO2 and the possibility that no fluid phase existed during metamorphism with fluid activities buffered by solid phases. It would clearly be desirable to be able to generalize about fluid compositions as has been done in the past. Turner (1967) and Winkler (1976) stated that metamorphism of marbles occurs with PF ≈ PCO2. Such CO2-rich fluids might be expected if marbles were closed systems relative to externally derived fluids, controlling their own fluid compositions, because many devolatilization reactions involve the breakdown of large amounts of calcite or dolomite and generate XCO2 > 0.75. It is well known from deformation studies (Griggs and others, 1960) and textures in thin section that calcite readily recrystallizes during high-grade metamorphism, and thus it is often proposed that calcite marbles are impervious to fluid flow, form a closed system, and buffer their own fluid composition (Turner, 1967). Turner also predicts that a dolomite marble is less likely to form a closed system than a calcite marble because it is less ductile. If these generalizations are universally true, then the many isobaric, univariant assemblages that are fourid in regional metamorphic terranes are controlled only by P + T and should be mappable as isograds.
Unfortunately, the results of field studies do not support uniformly high XCO2 or constant fluid compositions in marble, casting doubt upon such simplifying generalizations. This was dramatically demonstrated in the Whetstone Lake area, southern Ontario, by Carmichael (1970), who showed that the isograd biotite + calcite + quartz = Ca-amphibole + K-teldspar + CO2 + H2O in marbles crossed other dehydration and solid-solid isograds in metapelites because of variations in SH2O in marbles compared to pelites. He suggested that this was due to dilution by H2O derived from nearby pelites or granite. Ferry (1976a, 1976b) has also reported regional and local variations in XH2O in calc-pelites approaching quartz-monzonite and granodiorite plutons in the Waterville-Vassalboro area of Maine. Sobol and Essene (1973) investigated reactions in Grenville marble of the upper amphibolite facies from the Hastings-Haliburton region northeast of Whetstone Lake, and also inferred high XH2O. They mapped seven isograds which were compared with isotherms contoured from calcite-dolomite thermometry and found to be subparallel. They concluded that fluids do not greatly change in composition across the 3,000 km2 area, and that water migrated into the marble from underlying rock units such as a dehydrating pelite. Sobol intentionally avoided contacts with meta-igneous rocks in his area which had been previously mapped as contact aureoles. Allen (1976) showed that for one of these, the Tudor Gabbro, 13 silicate-carbonate isograds can be mapped in calcareous rocks without detectable change in temperature due to increasing XH2O toward the gabbro contact. Thus Sobol's success in locating approximately isothermal isograds was at least partly due to avoiding localized H2O-rich fluid anomalies in an amphibolite facies terrane.