Oxygen-isotope zoning in garnet: A record of volatile transport

Abstract

This study examines oxygen-isotope zoning in garnets from a Barrovian metamorphic terrane in eastern Vermont using a CO 2 laser extraction system. Previous strontium isotopic and structural studies of these garnets have shown that they grew over an approximately 10 Ma interval during thrusting and nappe emplacement ( rosenfeld, 1970; Christensen et al., 1989). Our studies show that the garnets are strongly zoned in δ 18 O. This zoning is the result of equilibration of garnet with water derived from dehydration of subjacent pelites during nappe stage deformation. The magnitude and nature of δ 18 O zoning depends upon the garnet's location in the outcrop studied. The garnets examined in this study come from an isotopically low δ 18 O paragonitic schist ( δ 18 O whole rock ~ 9‰) that is adjacent to a relatively high δ 18 O schist ( δ 18 O whole rock ~12.5‰). Garnets from the paragonitic schists within 10 m of the contact with the isotopically heavier schists have relatively homogenous δ 18 O values varying from 9.5‰ in the core to 10.5‰ at the rim. Garnet in the paragonitic schists 85 m from the contact are more strongly zoned, with δ 18 O ranging from a low of ~6.0‰ in the cores to a high of ~9.0‰ at the rims of the garnet. These zoning patterns were produced by continuous infiltration of relatively high δ 18 O waters derived from the subjacent schists into the paragonitic schists during garnet-grade metamorphism. It is possible to determine the time-integrated fluid fluxes by comparison of observed δ 18 O zoning profiles in garnet with those calculated from the equation describing combined advection-diffusion of a tracer. Using this method, we calculate time-integrated fluid fluxes of ~ 1.5 × 10 4 cm 3/cm 2. Fluxes of this magnitude could have been produced by dewatering of ~ 1.5 km of schist during garnet-grade metamorphism.