Grain-scale stable isotope disequilibrium during fluid-rock interaction; 2, An example from the Penninic-Austroalpine tectonic contact in eastern Switzerland

Abstract

At the tectonic contact between the Penninic Platta Nappe and the Lower-Austroalpine Err Unit in eastern Switzerland carbon and oxygen isotope fronts are preserved at the interface of serpentinites and low-grade regional metamorphic metacarbonates. The carbon and oxygen isotope compositions of calcite are delta 13 C = 2.3 per mil (PDB) and delta 18 O = 21.0 per mil (SMOW) in the unaltered metacarbonates at a distance of about 20 m from the lithologic contact, and they are delta 13 C = 0.0 per mil and delta 18 O = 14.0 per mil at the metacarbonate-serpentinite interface. The concomitant shift in the oxygen isotope composition of quartz is from 24.0 to 17.0 per mil. A peculiarity of the isotope pattern is the systematic variation of the quartz-calcite oxygen isotope fractionations across the isotopic front. Quartz-calcite fractionations are relatively small ( nearly equal 3.0 per mil) in the unaltered metacarbonate and at the metacarbonate-serpentinite interface, and they increase in the transitional zone in between with a maximum of 7.1 per mil at a distance of about 2 m from the contact. This pattern is interpreted as a result of advective-dispersive material transport across the lithologic contact and coupled first-order kinetic mineral-fluid exchange. The oxygen isotope systematics indicate that calcite-fluid isotope exchange was fast, and local calcite-fluid equilibrium prevailed during fluid-rock interaction. In contrast, quartz-fluid exchange was relatively sluggish, and the quartz-calcite oxygen isotope fractionations deviated significantly from their equilibrium values. Front geometries suggest that dispersive processes contributed substantially to material transport across the metacarbonate-serpentinite contact. Cross-layer advective flow was minor with a maximum time-integrated volumetric flux of 21 m 3 /m 2 . From the influence of quartz-fluid exchange kinetics on the intermineral fractionations quantitative relations among transport velocities, the rates of mineral-fluid exchange and the duration of fluid-rock interaction are derived. These relations are then combined with geologic and experimental data to identify feasible scenarios of fluid-rock interaction.