Isotopic characterization of the thermomineral waters of the Franco-Italian Northern Alps

Most of the (low enthalpy) thermomineral waters from the Franco-Italian Alps arise in fissured zones which promote mixing between deep waters, which are predominantly of the sodium chloride type, and intermediate and surface waters, which are mainly of the sulphate and/or calcium bicarbonate type. The characteristics and the origins of the waters from the deep circulations have been precisely determined by isotope analysis of the water molecules themselves (3H, 2H and 18O) and of certain dissolved compounds, such as the sulphates (34S and 18O), the sulphides (34S) and of carbon dioxide or the bicarbonates (13C, 18O). The meteoric origin of the deep waters was proved in this way. These waters are old and devoid of tritium; the age of one of the sources (Saint-Vincent) was also confirmed using 14C. Where gas is given off, the CO2 was shown to be endogenous, with isotope concentrations similar to those found in the perialpine carbon dioxide gas belt. Isotope measurements on 34S proved concentrations to be relatively uniform, which would indicate that the dissolved sulphates may have a common origin, having been flushed out of Permotriassic evaporites. Also, the 18O-34S geothermometer confirms that these waters are not magmatic in origin, having temperatures of between 150 to 200 deg. C which are in good agreement with the indications of other geothermometers. Nevertheless, the conventional geothermometers cannot be used in this context. With a geothermal gradient of 3.3 deg. C per 100 m, the results tend to indicate that the depth limit for these circulation pathways is at over 4000 m, which seems to strengthen the hypothesis that there are, at great depths, residual thin layers of Triassic evaporites, as deduced from the seismic profiles, or films of sodium chloride brines beneath the large, overlapping blocks of the platform. (author). 30 refs, 9 figs, 3 tabs