REINTERPRETATION OF REACTION PROGRESS AS A RECORD OF THE GEOMETRY OF FLUID FLOW DURING METAMORPHISM: WAITS RIVER FORMATION, EAST-CENTRAL VERMONT

The spatial distribution of progress (ξ) of the infiltration-driven reaction Ms + 3Ank + 2Qtz=Bt + An + 2Cal + 4CO₂ records the geometry of reactive fluid flow through marls during regional metamorphism at P≈8 kbar and T≈550°C. Marl is the sole lithology at the study site. Variations in ξ were measured perpendicular to layering at mm to m scales. Along 9- and 16-cm traverses ξ varies between 0 and 1.0±0.05 (1σ) mol/L in layers 0.5-4 cm thick. Similar variations in ξ exist among samples 0 to 3.1 m apart along two other traverses 5 and 35 m long. The variations traditionally would be interpreted in terms of reactive fluid flow in cm-wide layer-parallel channels. If this explanation is correct, variations in fluid composition should occur on the same scale. Along both cm-scale traverses, however, X_CO2 and δ¹⁸O_Cal (proxy for δ¹⁸O_Fluid) are statistically consistent with single values whose best estimates are the weighted mean (±95% confidence interval): X_CO2=0.130±0.003 (MSWD=0.6); δ¹⁸O_Cal=18.9±0.1‰ VSMOW (MSWD=2.3) for the 9 cm traverse and X_CO2=0.180±0.008 (MSWD=0.5); δ¹⁸O_Cal=19.2±0.1‰ (MSWD=2.1) for the 16 cm traverse. Along the longer traverses some samples record statistically significant differences in X_CO2 and δ¹⁸O_Cal over distances as small as 0.4 m, while other samples record uniform fluid composition over distances up to 4.2 m. Simple models demonstrate that cm-scale variations in ξ instead result from (a) layer-by-layer variations in the composition and amount of mineral reactants prior to reaction (initial X_an=0.00-0.37 and Fe/(Fe+Mg)_Ank=0.1-0.4; initial Ank=1.5-3.0, Pl=0.2-2.0, Ms=0.4-1.2, and Cal=0-18 mol/L) and (b) cross-layer homogenization of fluid composition during subsequent infiltration and reaction. Significant m-scale cross-layer variations in X_CO2 and δ¹⁸O_Cal rule out fluid flow across layers and point to layer-parallel flow in channels in some cases ≤1.4 m wide. Because there are no local sources of H₂O, reactive fluid must be external. In general, time-integrated fluid flux and the geometry of fluid flow in rocks with mineral reactants and products that are solid solutions can never be determined from spatial variations in ξ at the scale over which fluid composition is homogenized during reaction. Data from these traverses place this distance between 15 cm and 4.2 m for Barrovian metamorphism.