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Development of spatial variations in reaction progress during regional metamorphism of micaceous carbonate rocks, Northern new England

Sarah C. Penniston-Dorland^* and John M. Ferry

^* Present address: Department of Geology, University of Maryland, College Park, Maryland 20742, USA

Corresponding author: jferry{at}jhu.edu

Progress () of the infiltration-driven reaction, muscovite + ankerite + quartz + rutile + H₂O = biotite + calcite + plagioclase + CO₂, which occurred during Barrovian and Buchan regional metamorphism in east-central Vermont and south-central Maine, can vary by a factor of ten or more at all spatial scales down to that of adjacent lithologic layers <1 cm thick. Values of proxies for the activity of CO₂ and ¹⁸O_fluid, K_s(6) [(a_phl)(a_an)(a_cal)²]/[(a_ms)(a_dol)³(a_qtz)²] and ¹⁸O_Cal, are uniform within error of measurement over distances up to 1 m across layering. The conventional explanation of cm- to dm-scale variations in in terms of layer-parallel channeled fluid flow cannot explain the uniformity in the proxies. Observed cm- to dm-scale variations in are better explained by (a) mineral reactants and products that are solid solutions, (b) layer-by-layer variations in the amounts and compositions of minerals prior to reaction, and (c) uniformity of K_s(6) on a spatial scale larger than the scale of variations in during subsequent infiltration and reaction. The m-scale uniformity in K_s(6) and ¹⁸O_Cal is interpreted as homogenization of a_CO2 and ¹⁸O_fluid caused by the combined effects of intergranular diffusion and hydrodynamic dispersion. Reaction progress therefore was driven by the interplay of layer-parallel advection of chemically-reactive H₂O-rich fluid at decameter and larger scales and cross-layer transport of CO₂ and H₂O by diffusion/dispersion at scales of 1 m and less. Regardless of whether mineral reactants and products are solid solutions, the geochemical tracer considered, or the mechanism of fluid-rock reaction, the geometry of fluid flow can never be determined at a scale smaller than the one over which the concentration of the tracer is homogenized in the fluid within error of measurement by diffusion and dispersion.

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