The isotopic effects of fluid-rock interaction are controlled by material transport and mineral-fluid exchange. Transport may either be focussed through fractures or, alternatively, the pathways may be distributed homogeneously on a grain-scale. In metamorphic rocks, textural evidence from fluid-rock interaction is usually obliterated by continuous deformation and recrystallization. The nature of transport and exchange can only be inferred from indirect, mineralogical or (isotope)geochemical evidence. In the following we discuss two examples from natural systems, where stable isotope signatures indicate contrasting regimes of material transport.
The first example is taken from the Penninic-Lower Austroalpine boundary in eastern Switzerland. There, carbon and oxygen isotope fronts developed at a metacarbonate-serpentinite contact during low-grade regional metamorphism. Within the metasediments, the isotopic compositions show a pronounced depletion towards the lithologic contact. Calcite is shifted from d18O(SMOW)=20.9” and d13C(PDB)=2.3” to 14.2 and 0.1”. A similar shift from 23.9 to 17.1” is observed in coexisting quartz. The isotopic depletion is due to the interaction of the metasediments with an isotopically light, water-rich fluid derived from the serpentinites. Whereas the oxygen isotope front is relatively smooth and located at about fifteen meters from the lithologic contact, the carbon front is relatively sharp and propagated only two meters into the metasediments. Retardation of the carbon- with respect to the oxygen-front is interpreted in terms of pervasive transport and coupled exchange. It indicates that the carbon to oxygen ratio in the fluid was significantly lower than in the rock. This agrees well with the composition of a serpentinite derived fluid. Quartz-calcite fractionations vary systematically across the oxygen-front. This indicates grain-scale oxygen isotope disequilibrium caused by differential rates of mineral-fluid exchange. The isotope signature of the metasediments is best explained by pervasive, advective-dispersive transport and coupled kinetic mineral-fluid exchange.
An alternative regime of material transport is inferred from the hydrogen and oxygen isotope systematics of the Ventina Ophicarbonate Zone (Bergell contact aureole, N Italy). The Ventina Ophicarbonate Zone is a 10- to 400-meter wide ophicarbonate layer within massive serpentinites. In profiles across the ophicarbonate zone, isotope compositions become successively depleted towards the lithologic contacts. Calcite is shifted from d18O(SMOW) = 14” to 8”, antigorite is shifted from d18O = 10 to 6” and from dD = -65, to -100”. The isotopic depletion is due to interaction of the ophicarbonate rocks with an isotopically light, water-rich fluid derived from serpentine dehydration. As in the previous example the antigorite-calcite fractionations vary systematically across the isotopic front, indicating grain-scale disequilibrium. However, in the Ventina Ophicarbonate Zone, the oxygen and hydrogen fronts coincide. The hydrogen to oxygen ratio in the fluid most likely was significantly larger than in the rock. If transport and exchange were pervasive, the hydrogen-front would be expected to propagate faster than the oxygen-front. The lack of retardation of the oxygen- with respect to the hydrogen-front suggests that transport was focussed through fractures. In this case, only a small fraction of the fluid could exchange isotopically with the rock during transport. Transport and exchange probably occurred sequentially, and they were not as intimately interrelated as in the case of pervasive transport and coupled exchange. The geometry of the isotopic fronts reflects a decrease in fracture density away from the lithologic contact rather than dispersion of the tracer isotopes.