CL-ISOTOPIC SIGNATURES OF GROUNDWATERS, PORE WATERS AND CALCITE FLUID INCLUSIONS: CONSTRAINTS ON FLUID ORIGIN AND TRANSPORT IN A MARL SEQUENCE

A sequence of low permeability marls containing porewaters under subhydrostatic pressures is encountered at Wellenberg, Switzerland. Na-Cl waters located in the center of the sequence are bound by dilute, Na-HCO₃-type waters of Holocene age above, and moderately mineralized Na-HCO₃ Pleistocene glacial waters below. Previous investigations of the stable isotope signatures and noble gas analyses of porewaters extracted from core material indicate that diffusion is the dominant transport process in the marls. In addition, the salinities, dissolved gas composition, and deuterium signatures of the Na-Cl groundwaters were found to be very similar to those of the metamorphic aqueous fluid inclusions in vein calcites, suggesting a common origin. In the current study, the chloride concentrations and delta ³⁷Cl signatures of groundwaters and rock matrix leachates are compared to those of fluid inclusions in vein calcites to provide further constraints on the origin of these fluids, and on the initial conditions for diffusion.

The Na-Cl groundwaters have chlorine isotopic signatures in the range of +0.2 to +0.9 per mil, relative to Standard Mean Oceanic Chloride. The delta ³⁷Cl signatures of pore waters determined from rock matrix leaching fall within the same range (+0.2 to +0.8 per mil). A mixed 1^st and 2^nd generation, and a 3^rd generation vein calcite have delta ³⁷Cl signatures of +0.4 per mil, which is within the range of values measured for both the Na-Cl waters and the rock matrix leachates. The similar, positive signatures are consistent with a common origin for the fluids. A Na-HCO₃-Cl groundwater, which is thought to be transitional between the Na-Cl waters and the overlying Na-HCO₃-type waters, has a delta ³⁷Cl signature of -0.4 per mil, suggesting that upward diffusion of chloride from the Na-Cl waters has occurred. Given sufficient time, diffusion will shift the chlorine isotopic signature in the center of the sequence towards more positive values compared to the fluid inclusions, as observed for the Na-Cl waters and rock leachates.