ROLES OF DIFFUSION AND REACTIVE FLUID FLOW IN DRIVING DECARBONATION REACTIONS IN INTERBEDDED PELITE, PSAMMITE, AND MICACEOUS LIMESTONE DURING ACADIAN REGIONAL METAMORPHISM OF THE GILE MOUNTAIN FORMATION, EAST-CENTRAL VERMONT

Formation of prograde Bt, Chl, and Pl in pelite and psammite from the Bt zone at ≈8 kb and ≈475°C is described by 3 net-transfer reactions: (R1) Sd+Ank+Ms+Qtz+Rt=Bt+Pl+CO₂, (R2) Ank+Ms+Qtz+Rt=Bt+Pl+Cal+CO₂, and (R3) Ank+Pl+H₂O=Chl+Qtz+Cal+CO₂. Typically all 3 reactions occurred in pelite, only (R2) and (R3) in psammite, and no reaction in limestone. In one large exposure, the amount of Bt (n_Bt in mol/L) varies greatly in rocks that retain Ank+Ms+Qtz: 0.5-1.1 (7.3-16.4 modal %) in pelite, 0.2-0.5 (2.2-8.0%) in psammite, and 0 in limestone. Conventionally the variations are interpreted in terms of infiltration-driven decarbonation reaction, fluid flow in chemically isolated channels that correspond to individual metasedimentary layers, elevated flow in Bt-rich layers, reduced flow in Bt-poor layers, and no flow in Bt-free layers. Two new observations are not well explained by the conventional interpretation. (1) Fluid composition, independently recorded by two mineral equilibria in pelite and psammite, is uniform within error of measurement in all analyzed samples. For equilibrium (E1) dol+chl+ms+qtz=phl+an+CO₂+H₂O, lnK_s=-3.96±0.30 (95% confidence interval), MSWD=1.39 (n=7); for (E2) dol+ms+qtz=phl+an+cal+CO₂, lnK_s=-1.77±0.17, MSWD=0.83 (n=5). The corresponding X_CO2 recorded by (E2) is 0.023±0.001. The Bt- and Chl-free assemblage in limestone is consistent with the estimated X_CO2. (2) Reactants and products of (R2) and (R3) occur in psammite, reactant Sd is never observed in pelite, and product Bt and Chl are never observed in limestone. The observations can be explained by diffusion over distances of 1-10 m during fluid flow and mineral reaction. Specifically, CO₂-H₂O interdiffusion would homogenize fluid composition and account for uniform X_CO2. Simulations of Bt-forming reactions in the 3 rock types under the constraint of spatially uniform X_CO2 demonstrate that when reactants and products of (R2) and (R3) are stable in psammite, Sd is already consumed in pelite by (R1) (controlled by differences in carbonate modes of the protoliths) while no reaction occurs in limestone (controlled by its Mg-rich Ank). Variations in n_Bt therefore image the spatial distribution of mineral assemblages, modes, and mineral compositions prior to formation of Bt rather than channelways for reactive fluid flow.