Localized depletion of O-18 and C-13 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
delta(18)O and delta(13)C in calcite from the 1 m long profile decrease
systematically from sedimentary values of delta(18)O = 22 parts per
thousand (SMOW) and delta(13)C = 0 parts per thousand (PDB) to
delta(18)O = 12.5 parts per thousand and delta(13)C approximate to -7
parts per thousand 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 (X(CO2) approximate to 0.001).
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
X(CO2) approximate to 0.01 for the infiltrating fluid. Excellent
agreement with the delta(18)O and delta(13)C 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 how 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.