OXYGEN-ISOTOPE ZONING IN GARNET - A RECORD OF VOLATILE TRANSPORT

This study examines oxygen-isotope zoning in garnets from a Barrovian metamorphic terrane in eastern Vermont using a CO2 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 deltaO-18. 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 deltaO-18 zoning depends upon the garnet's location in the outcrop studied. The garnets examined in this study come from an isotopically low deltaO-18 paragonitic schist (deltaO-18 whole rock approximately 9 parts per thousand) that is adjacent to a relatively high deltaO-18 schist (deltaO-18 whole rock approximately 12.5 parts per thousand). Garnets from the paragonitic schists within 10 m of the contact with the isotopically heavier schists have relatively homogenous deltaO-18 values varying from 9.5 parts per thousand in the core to 10.5 parts per thousand at the rim. Garnet in the paragonitic schists 85 m from the contact are more strongly zoned, with deltaO-18 ranging from a low of approximately 6.0 parts per thousand in the cores to a high of approximately 9.0 parts per thousand at the rims of the garnet. These zoning patterns were produced by continuous infiltration of relatively high deltaO-18 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 deltaO-18 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 approximately 1.5 X 10(4) cm3/cm2. Fluxes of this magnitude could have been produced by dewatering of approximately 1.5 km of schist during garnet-grade metamorphism.