Oxygen isotope constraints on metamorphic fluid flow, Townshend Dam, Vermont, U.S.A.

Abstract

Fluid-rock interaction during amphibolite-facies metamorphism has been investigated for rocks exposed in a single 400 m long, lithologically heterogeneous outcrop near Townshend, Vermont, USA. Oxygen isotopic compositions have been measured in profiles across single garnet crystals from thirteen samples, and from hornblende and garnet separates from thirty-three fine-grained samples. This outcrop was previously studied by Chamberlain and Conrad (1991,1993), who measured large oxygen isotope gradients (3%.) in garnets from one sample, and inferred large amounts of pervasive fluid flow. All of the garnets that we have analyzed show less isotopic zonation, ≤1%., twelve of them have zonation <0.5%., and the mineral separate data imply both a strong correlation of isotopic composition with rock type and large gradients in peak metamorphic fluid isotopic compositions. Although devolatilization reactions in these rocks must have produced metamorphic fluids, the data preclude cross-foliation timeintegrated fluid fluxes greater than 300–600 cm3/cm2 during prograde amphibolite-facies metamorphism. The isotopic trends can all be interpreted in terms of closed-system behavior, channeled fluid flow, or diffusive exchange of oxygen in an interconnected grain-boundary fluid, and within uncertainty cross-strike advective fluxes could have been zero. Any significant flow at this locality was dominantly either layer parallel or channeled out of the system in veins. If all fluid flow occurred in veins, then their spacing must have been less than 300–600 m. The data are inconsistent with massive and pervasive metamorphic fluid flow across strike but do not address layer parallel flow.