GRAIN-SCALE STABLE-ISOTOPE DISEQUILIBRIUM DURING FLUID-ROCK INTERACTION .2. AN EXAMPLE FROM THE PENNINIC-AUSTROALPINE TECTONIC CONTACT IN EASTERN SWITZERLAND
H. Eppel et R. Abart, GRAIN-SCALE STABLE-ISOTOPE DISEQUILIBRIUM DURING FLUID-ROCK INTERACTION .2. AN EXAMPLE FROM THE PENNINIC-AUSTROALPINE TECTONIC CONTACT IN EASTERN SWITZERLAND, American journal of science, 297(7), 1997, pp. 707-728
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.