Origin of deep saline groundwaters in the Vienne granitic rocks (France): constraints inferred from boron and strontium isotopes

As part of a preliminary geological characterization programme to assess the feasibility of an underground laboratory in granitic rock, a series of 17 deep boreholes (maximum depth, 900 m) was drilled by ANDRA in the Vienne district, France. A salinity gradient was demonstrated in the granitic waters with concentrations varying from approximately 1gL(-1) at 150 m depth at the top of the basement (beneath the sedimentary cover) to 10 g L-1 in the deeper part (from 400 to 600 m depth). Sr and B isotope ratios were measured in order to better understand the origin of the salinity and to evaluate the degree of water-rock interaction in the system. The results obtained were compared to those of mineral spring waters emerging from the granitic basement in the Massif Central. Evidence in support of a significant marine contribution include: (i) the Cl-Br investigations agree with a marine origin for the saline groundwaters without evolution from seawater; (ii) the Sr-87/Sr-86 ratio of the Vienne deep groundwaters (0.7078-0.7084) is in agreement with a palaeo-seawater isotopic signature; (iii) measured delta(11) B values for the deepest brine samples are enriched in B-11 (up to 36.1%) relative to the granitic springs. The combined use of delta B-11, Cl, B, Br, Sr contents and Sr-87/Sr-86 ratios makes it possible to define and quantify a mixing model between marine and crustal end-members in order to explain the origin of the deep saline groundwaters in the Vienne granitic rocks.