A New Interpretation of Centimetre-scale Variations in the Progress of Infiltration-driven Metamorphic Reactions: Case Study of Carbonated Metaperidotite, Val d'Efra, Central Alps, Switzerland

Abstract

Progress ( j) of the infiltration-driven reaction, 4olivineþ 5CO2 þ H2O 1⁄4 talc þ 5magnesite, that occurred during Barrovian regional metamorphism, varies at the cm-scale by a factor of 3 5 within an 3 m volume of rock. Mineral and stable isotope compositions record that XCO2, d Ofluid, and d Cfluid 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 reaction, 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.