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

Abstract

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 1 g L−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 87Sr/86Sr ratio of the Vienne deep groundwaters (0.7078–0.7084) is in agreement with a palaeo‐seawater isotopic signature; (iii) measured δ11B values for the deepest brine samples are enriched in 11B (up to 36.1‰) relative to the granitic springs. The combined use of δ11B, Cl, B, Br, Sr contents and 87Sr/86Sr 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.

Boron isotope signatures in the coastal groundwaters of French Guiana

“…Both the sum of the cations and the bicarbonate content increase during weathering, which implies an increase in the pH (HCO 3 versus pH in groundwater gives a correlation coefficient r close to 0.88). [Barth, 2000;Casanova et al, 2001]. Hydraulic tests around this drill hole suggest that the water reservoir is limited in space and has low hydraulic conductivity, implying a low water/rock ratio and probably a prolonged residence time for the water.…”

Br/Cl ratios and O, H, C, and B isotopic constraints on the origin of saline waters from eastern Canada

“…Regions of crystalline shield rocks in Canada and Europe commonly contain deep groundwaters with elevated salinity, up to 350 g/L in some cases (e.g., Fritz and Frape, 1982;Frape et al, 1984;Herut et al, 1990;McNutt et al, 1990;Casanova et al, 2001;Bottomley et al, 2002;Kloppmann et al, 2002;Starinsky and Katz, 2003;Gascoyne, 2004;Négrel and Casanova, 2005). Unlike saline groundwaters in sedimentary basins, those in shield regions commonly have oxygen and hydrogen isotopic compositions that are shifted to the left of the global meteoric water line.…”

Variability of87Sr/86Sr in water draining granite revealed after a double correction for atmospheric and anthropogenic inputs

“…Previous studies in the Vienne district (western France) focused mainly on deep-fluid geochemistry in order to identify water origin and understand groundwater circulations in the granite batholith (Michelot, 1997;Casanova et al, 2001;Négrel et al, 2001).…”