THE ORIGIN OF CORRELATED VARIATIONS IN INSITU O-18 O-16 AND ELEMENTALCONCENTRATIONS IN METAMORPHIC GARNET FROM SOUTHEASTERN VERMONT, USA/

CO2 laser-heating in a fluorinating atmosphere was used to obtain O-18 /O-16 analyses of coexisting garnet, staurolite, muscovite, chlorite, and quartz from a sample of the Gassetts schist, southeastern Vermont, USA. Garnet and quartz deltaO-18V-SMOW values vary on a millimeter sc ale while values for other minerals are uniform within analytical unce rtainties. Garnets exhibit O-18/O-16 zoning with core deltaO-18 values of 10.9 +/- 0.3 parts per thousand and rim values of 10.1 +/- 0.2. Th e depletion of O-18 in garnet rims correlates with reversals in cation zonation and intracrystalline textural unconformities. Quartz plucked from a garnet core yields a deltaO-18 of 14.8 parts per thousand whil e vein quartz deltaO-18 values vary from 14.3 +/- 0.2 in centers to 13 .8 +/- 0.1 at margins. Staurolite, muscovite, and chlorite have mean d eltaO-18 values of 10.8, 11.9, and 10.3, respectively. Two parageneses are distinguished on the basis of the mineral deltaO-18 and expected isotope partitioning and on the basis of textures. One consists of gar net cores, staurolite, muscovite, and quartz included in garnet. Garne t rims, chlorite, and vein quartz comprise the other. Quantitative mod els which explain the lower bulk deltaO-18 of the garnet-rim assemblag e show that garnet textural unconformities represent resorption during infiltration of externally derived aqueous fluid. Chlorite grew by th is hydration reaction and was the principal storage site for low O-18/ O-16 oxygen. Low-deltaO-18 garnet rims later grew at the expense of ch lorite with little change in modal abundances of staurolite and muscov ite. Retention of pre-infiltration deltaO-18 in muscovite precludes si gnificant diffusion of oxygen in this phase and indicates that the tot al duration of external fluid flow was less-than-or-equal-to 10(5) yea rs.