In metamorphic terranes, modification of the stable isotope compositions of marbles by fluid infiltration has been extensively modeled to try to constrain both the magnitudes and time scales of metamorphic fluid fluxes. We have made a study of key localities on the Aegean island of Naxos (Greece) in which we combined textural observations with micro-scale trace element and stable isotope analysis to test the assumptions upon which modeling of metamorphic fluid flow is based, and to identify fluid transport mechanisms. Integration of ion microprobe analyses of 18O/16O, 13C/12C, Mn, and Fe with detailed cathodoluminescence imaging provides evidence that water-rich fluids were channeled along cracks and grain boundaries within marble, with limited equilibration between the fluid and rock. Very low values of δ18OSMOW (down to 2‰) along grain boundaries indicate a meteoric source for fluids rather than a metamorphic source, in contrast to previous studies. In this example, the simplifying assumptions made previously in chromatographic theory, to constrain metamorphic fluid fluxes, time scales, and porosities from bulk isotope data, are incorrect. By making micro-scale measurements, we can determine infiltration mechanisms and distinguish separate fluid-flow events. These observations and measurements provide a basis for constructing more realistic models of metamorphic fluid flow.