Variability of 87Sr/86Sr in water draining granite revealed after a double correction for atmospheric and anthropogenic inputs

In a geochemical characterization of surface waters draining granites of the Vienne district, France, isotopic signatures strictly linked to water rock interactions were determined by applying a double correction for atmospheric and anthropogenic inputs; the former based on the conservative behaviour of Cl and the latter based on the Sr concentrations and isotopic signatures, which are the only variables with clearly identifiable characteristics. The double-correction concept was validated by comparing the results with the theoretical Sr isotopic signature of water in equilibrium with the parent rock. The weathering model is based on the preferential dissolution of the three main Sr-bearing mineral phases: plagioclase, potassium feldspar and biotite. The Sr isotopic signatures of the waters are in good agreement with the calculated ones. The relative scattering observed for waters draining the same plutonic intrusion can be explained by water-rock interactions within different layers of the weathering profile.