Abstract

Localized depletion of 18O and 13C in a thin subhorizontal marble layer in the Adamello contact aureole, Southern Alps, Italy, resulted from fluid infiltration focused along a crosscutting dike. Values of δ18O and δ3C in calcite from the 1 m long profile decrease systematically from sedimentary values of δ18O = 22‰ (SMOW) and δ13C = 0‰ (PDB) to δ18O = 12.5‰ and δ13C ≈ − 7‰ near the dike. The presence of clinozoisite and garnet in the 5–15 cm thick marble layers near the granodiorite dike indicates H2O-rich fluid conditions (XCO2 ≈ 0.01).

The O and C isotope profiles were compared with one- and two-dimensional models of advective-dispersive isotope transport. Individually the isotope profiles fit one-dimensional transport models well. However one-dimensional models, using equilibrium fluid-rock exchange or a kinetic formulation, do not explain the relative locations or shapes of the two isotope-exchange profiles given the petrologic constraint of XCO2 ≈ 0.01 for the infiltrating fluid. Excellent agreement with the δ18O and δ13C data is obtained using a two-dimensional model that specifies (1) a high-permeability zone in marble near the dike that focuses fluid flow parallel to the dike and (2) a lower permeability zone in marble away from the dike where isotope exchange is dominated by molecular diffusion. The combined constraints imposed by phase equilibria and two isotope tracers allow two-dimensional fluid flow to be inferred from one-dimensional data. The results emphasize that isotope distributions resulting from multidimensional flow may fortuitously fit one-dimensional transport models if isotope tracers are considered independently. The use of multiple tracers coupled to fluid-composition constraints is therefore essential to discriminate between various transport models.

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