GRAIN-SCALE STABLE-ISOTOPE DISEQUILIBRIUM DURING FLUID-ROCK INTERACTION .2. AN EXAMPLE FROM THE PENNINIC-AUSTROALPINE TECTONIC CONTACT IN EASTERN SWITZERLAND

At the tectonic contact between the Penninic Platta Nappe and the Lowe r-Austroalpine Err Unit in eastern Switzerland carbon and oxygen isoto pe fronts are preserved at the interface of serpentinites and low-grad e regional metamorphic metacarbonates. The carbon and oxygen isotope c ompositions of calcite are delta(13)C = 2.3 permil (PDB) and delta(18) O = 21.0 permil (SMOW) in the unaltered metacarbonates at a distance o f about 20 m from the lithologic contact, and they are delta(13)C = 0. 0 permil and delta(18)O = 14.0 permil at the metacarbonate-serpentinit e interface. The concomitant shift in the oxygen isotope composition o f quartz is from 24.0 to 17.0 permil. A peculiarity of the isotope pat tern is the systematic variation of the quartz-calcite oxygen isotope fractionations across the isotopic front. Quartz-calcite fractionation s are relatively small (approximate to 3.0 permil) in the unaltered me tacarbonate 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 inter preted as a result of advective-dispersive material transport across t he lithologic contact and coupled first-order kinetic mineral-fluid ex change. The oxygen isotope systematics indicate that calcite-fluid iso tope 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 fractionati ons deviated significantly from their equilibrium values. Front geomet ries suggest that dispersive processes contributed substantially to ma terial transport across the metacarbonate-serpentinite contact. Cross- layer advective now was minor with a maximum time-integrated volumetri c flux of 21 m(3)/m(2). From the influence of quartz-fluid exchange ki netics on the intermineral fractionations quantitative relations among transport velocities, the rates of mineral-fluid exchange and the dur ation of fluid-rock interaction are derived. These relations are then combined with geologic and experimental data to identify ''feasible'' scenarios of fluid-rock interaction.