A New Interpretation of Centimetre-scale Variations in the Progress of Infiltration- driven Metamorphic Reactions: Case Study of

Progress ( j) of the infiltration-driven reaction, 4olivineþ 5CO2þ H2O talc þ 5magnesite, that occurred during Barrovian regional metamorphism, varies at the cm-scale by a factor of 35 within an 3 m3 volume of rock. Mineral and stable isotope compositions record that XCO2, d 18Ofluid, and d 13Cfluid were uniform within error of measurement in the same rock volume. The conventional interpretation of small-scale variations in j in terms of channelized fluid flow cannot explain the uniformity in fluid composition. Small-scale variations in j resulted instead because (a) reactant olivine was a solid solution, (b) initially there were small-scale variations in the amount and composition of olivine, and (c) fluid composition was completely homogenized over the same scale by diffusion–dispersion during infiltration and subsequent reaction. Assuming isochemical reac-tion, spatial variations in j image variations in the (Mg þ Fe)/ Si of the parent rock rather than the geometry of metamorphic fluid flow. If infiltration-driven reactions involve minerals fixed in composition, on the other hand, spatial variations in j do directly image fluid flow paths. The geometry of fluid flow can never be determined from geochemical tracers over a distance smaller than the one over which fluid composition is completely homogenized by diffusion–dispersion. KEY WORDS: Alpine Barrovian metamorphism; diffusion; metamorphic fluid composition; metamorphic fluid flow; reaction progres