Fluid Flow and Diffusion in the Waterville Limestone, South--Central Maine: Constraints from Strontium, Oxygen and Carbon Isotope Profiles

Abstract

Oxygen, carbon and strontium isotopic profiles across the margin of the Waterville limestone member are used to investigate advective and diffusive transport during metamorphism of the Waterville Formation in south–central Maine, USA. Rb–Sr isotopic systematics were homogenized on the ~10 cm hand-specimen scale at ages that are within error of the 366±6 Ma Rb–Sr whole-rock age of the syn-metamorphic Hallowell pluton. This is consistent with a plutonic heat source for this low-pressure andalusite- and sillimanite-grade Acadian metamorphic terrane. Advective displacements of all three isotope profiles at the garnet-grade Blue Rock Quarry indicate fluid flow to the east into the limestone, and the oxygen-isotope profile implies a time-integrated fluid flux of 3.2±1.5 m 3 /m 2 (2σ error). This cross layer flux is insufficient to cause the observed reaction progress of the muscovite+ankerite–Quartz to biotite+anorthite (in plagioclase)+calcite reaction in the ~100 m thick Waterville limestone member and much of the fluid flow responsible may have been layer parallel. The isotope profiles indicate advective–diffusive homogenization over distances of 1.5 m (δ 13 C) to 6 m (δ 18 O) and such homogenization distances are difficult to reconcile with observations of order of magnitude variations in reaction progression the centimetre scale or less. It is possible that infiltration occurred during events short lived compared with diffusion, that the reactions started at different temperatures dependent on bulk composition or that diffusion of water from layers with less reactants to layers with more reactants was important in driving the biotite-producing reaction. However, variations of fluid composition inferred from the mineral assemblages are apparently inconsistent with diffusion driving reactions progress, and models of precursor assemblages do not indicate significant compositional control of the temperature of the first appearance of biotite in the rocks. Irrespective of the details of flow and diffusive exchange on the centimetre scale, the average reaction progress in the Waterville limestone member requires significant layer-parallel fluid fluxes.