Isotope exchange between fluid and rocks has been traditionally considered to be rate limited by two elementary processes: lattice diffusion in the matrix minerals and dissolution into the fluid followed by precipitation from it. In this study we show the results of high-pressure experiments on 18O-water infiltration into quartzite that point to a third, highly efficient process: grain growth accompanied by migration of the grain boundaries (GBs) that are isotopically enriched by GB diffusion and surface exchange. We predict on the basis of a mass-transfer mechanism discrimination diagram that this GB sweeping is the primary control on isotopic equilibration under hydrothermal conditions in various fine-grained rocks with low fluid fraction. Grain growth should be considered when interpreting and simulating isotope and chemical composition of rocks and fluids.

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