Exotic stable isotope compositions of saline waters and brines from the crystalline basement

The overwhelming majority of groundwaters reported in literature show isotopic compositions that place them on or right of the meteoric water line (MWL) in a delta(2)H vs. delta(18)O diagram. A closer look to the steadily increasing data from the deeper crystalline basement reveals nevertheless that points left of the MWL are not so unusual as has been suggested during the last decades. Their supposed rareness seems to be due to a bias of sampling: The concerned fluids develop only at greater depths and in low permeability environments, which makes access to them so difficult. Paradoxically, exotic stable isotope compositions may be the rule in a restricted depth zone in all crystalline basements with slow groundwater movements but not too high temperatures. The present study takes stable isotope data on saline fluids from the deep French granitic basement as a starting point. In order to situate the French data from the Vienne region in a more general context, a literature review has been undertaken assembling more than 1300 stable isotope analyses of waters and brines in crystalline rocks from stable cratons and mountain belts world-wide. The French fluids show some particular features with respect to the majority of other studies as their major ion composition close to seawater or a significant isotopic shift at relatively low salinities. Detailed knowledge on the hydrological and chemical history of the site allowed to develop a conceptual model of isotopic evolution by interaction of fracture fluids with fracture minerals. This model considers interactions of the last fluids flushing the system (marine water, unaltered, evaporated or diluted) with previously precipitated mineral phases and the neoformation of the last generation of fracture minerals. The model has been tested quantitatively for several combinations of dissolution-precipitation reactions involving formation of carbonates, clay minerals and Fe-hydroxides. Most of the modelled scenarios would leave the residual liquid phase depleted in O-18 and either enriched or depleted in H-2 and produce waters with isotopic signatures corresponding to the observed delta(2)H vs. delta(18)O correlation. It is therefore suggested that the general phenomenon of an isotopic shift in deep fissured silicate rocks, which seems to be rather independent of lithology, may result from a combination of different reaction mechanisms implying not solely the rock matrix but also the different generations of fracture minerals. (C) 2002 Elsevier Science B.V. All rights reserved.