STABLE ISOTOPIC EVIDENCE FOR FLUID ROCK INTERACTIONS IN THE IVREA ZONE, ITALY

Stable isotope compositions of Ivrea Zone marbles and associated lithologies are in general heterogeneous. The oxygen isotope composition of quartz in pelites ranges from δ18O ∼ +9‰ to + 17‰ (SMOW) and does not vary systematically with metamorphic grade. Peridotites retain oxygen isotope signatures close to mantle values. Marble calcites vary in isotopic composition from δ13C ∼ + 2‰(PDB),δ180 ∼+24‰(SMOW)to δ13C ∼ -6‰(PDB), ∼18O ∼ + 13‰ (SMOW). Depletions in 18O and 13C may be explained dominantly by interaction with fluids derived from within the observed metasedimentary sequence during prograde metamorphism.δ18O and δ13C show gradients of greater than 5‰/m across marble margins and within marbles. The preservation of such isotopic gradients is not consistent with the long-term presence of grain-boundary-scale interconnected fluid films in and around marbles. There is a general lowering of δ18O within individual marble bodies although large carbon and oxygen isotopic gradients are present. Calcite in marbles may attain oxygen isotope equilibrium, but rarely carbon isotope equilibrium, with surrounding metapelites. Infiltration of marbles must involve a component of channelized fluid flow.The general lack of isotopic equilibration within the sequence requires channelized fluid flow and limited fluid-rock ratios. Large pervasive mantle to crust fluid fluxes are not consistent with the observations. © 1990 Oxford University Press.