A NEW INTERPRETATION OF CM-SCALE VARIATIONS IN REACTION PROGRESS PRODUCED BY INFILTRATION-DRIVEN METAMORPHISM

Variation in the progress (x) of devolatilization reactions at the cm-scale traditionally is interpreted as focused flow of reactive fluids in cm-sized channels. The interpretation assumes no chemical communication between high-x/high-flow and low-x/low-flow regions by diffusion/dispersion (D/D). Recent studies suggest fluid composition may be homogenized by D/D over a distance >1 m during regional metamorphism. If so, cm-scale variations in x only reflect cm-scale differences in the amount and composition of mineral solid solutions prior to reaction. Carbonated metaperidotite in Val d’Efra, southern Swiss Alps, offers an ideal test whether the old or new interpretation is correct. Olivine (Ol)-talc (Tlc)-magnesite (Mgs)-chlorite schist experienced the prograde reaction 4Ol + 5CO₂ + H₂O=Tlc + 5Mgs, driven by infiltration of reactive CO₂-rich fluid during Alpine Barrovian metamorphism near 650°C and 7 kbar. Measured x in 17 samples varies by a factor of 3-4 over 1 m both across and along foliation. The conventional interpretation is that high-x areas image an anastomosing network of tube-shaped flow channels. If fluid composition was homogenized by D/D over a distance >1 m, observed variations in x alternatively can be explained simply by variations in the amount (11.2-19.9 mol/l) and composition (0.902-0.920 X_Fo) of Ol prior to reaction. If the conventional interpretation is correct, there should be evidence for differences in fluid composition between samples. Because all minerals are essentially Fe-Mg solutions, Ol composition is a proxy for XCO₂. Measured values of X_Fo in the 17 samples are consistent with the same value [0.888±0.001 (weighted mean±2 standard error), MSWD=1.42]. Except for 4 samples <6 cm from a late vein, d¹⁸O and d¹³C of Mgs are also consistent with single values (d¹⁸O=9.37±0.06‰ VSMOW, MSWD=1.90; d¹³C=-6.80±0.06‰ VPDB, MSWD=1.71). The high-precision evidence for uniform fluid composition indicates that variations in x result from initial variations in the amount and composition of Ol that can be traced to differences in enstatite/olivine in the harzburgite protolith. As a generalization, mineralogical and isotopic information about the distribution and geometry of fluid flow is never preserved at a scale smaller than the distance over which fluid composition is homogenized by D/D.