CO2 laser-heating in a fluorinating atmosphere was used to obtain analyses of coexisting garnet, staurolite, muscovite, chlorite, and quartz from a sample of the Gassetts schist, southeastern Vermont, USA. Garnet and quartz δ18Ov-smowvalues vary on a millimeter scale while values for other minerals are uniform within analytical uncertainties. Garnets exhibit zoning with core δ18O values of 10.9 ± 0.3‰ and rim values of 10.1 ± 0.2. The depletion of 18O in garnet rims correlates with reversals in cation zonation and intracrystalline textural unconformities. Quartz plucked from a garnet core yields a δ18O of 14.8‰ while vein quartz δ18O values vary from 14.3 ± 0.2 in centers to 13.8 ± 0.1 at margins. Staurolite, muscovite, and chlorite have mean δ18O values of 10.8, 11.9, and 10.3, respectively.Two parageneses are distinguished on the basis of the mineral δ18O and expected isotope partitioning and on the basis of textures. One consists of garnet cores, staurolite, muscovite, and quartz included in garnet. Garnet rims, chlorite, and vein quartz comprise the other. Quantitative models which explain the lower bulk δ18O of the garnet-rim assemblage show that garnet textural unconformities represent resorption during infiltration of externally derived aqueous fluid. Chlorite grew by this hydration reaction and was the principal storage site for low oxygen. Low-δ18O garnet rims later grew at the expense of chlorite with little change in modal abundances of staurolite and muscovite. Retention of pre-infiltration δ18O in muscovite precludes significant diffusion of oxygen in this phase and indicates that the total duration of external fluid flow was ≤ 105 years.