Infiltration of Aqueous Fluid and High Fluid: Rock Ratios During Greenschist Facies Metamorphism: A Reply

Wood & Graham (1986) suggest that fluid: rock ratios ofFerry (1984) were over-estimated because the temperature ofthe biotite isograd, inferred from biotite-garnet and calcite-dolomitegeothermometry, was underestimated by {small tilde}40?C. Wood& Graham's critique of calcite-dolomite geothermometry isincomplete because it fails to account for the effect of MnCO₃substitutions in calcite on estimated temperatures. New datathat consider the effects of both FeCO₃ and MnCO₃ substitutionssuggest that published calcite-dolomite temperatures need nosignificant correction. Their proposed correction to biotite-garnettemperatures, based on the garnet solid solution model of Ganguly& Saxena (1984), has an uncertainty of at least 100 percent and is not significantly different from zero. An empiricalscheme that corrects for the effects of grossular and spessartinesubstitution in garnet on estimated temperature, along withthe large uncertainty in Wood and Graham's proposed correction,suggest that published biotite-garnet temperatures also needno significant revision. The preferred temperature for the biotiteisograd remains {small tilde}400?C. In turn, the preferred estimatesof fluid: rock ratios remain unchanged; pelitic phyllites interactedwith at least 1-2 rock volumes fluid during the biotite-formingreaction at 400?C. Varied geologic, petrologic, and isotopic data are reviewedthat support fluid:rock ratios > 1 during low-grade metamorphismboth in south-central Maine and worldwide. Chemical interactionbetween rocks and fluids that are initially out of equilibriummay be an essential driving force behind many instances of low-graderegional metamorphism. Fluid:rock ratios > 1 and recent stableisotope studies should encourage serious consideration of amulti-pass model for metamorphic fluid flow.