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

Abstract

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. Résumé Cette étude présente la caractérisation géochimique d'eaux de surface drainant des granitoïdes, dans la région de Vienne, en France. L'identification de la signature isotopique des eaux uniquement liée à l'interaction eau roche a été appréhendée par une double correction des apports atmosphériques et anthropiques. La correction atmosphérique est basée sur le comportement conservatif du Cl. La correction anthropique n'a été réalisée que sur les signatures isotopiques et les teneurs en Sr, qui sont les seules variables dont les caractéristiques sont clairement identifiables. Ce concept de double correction a été validé par comparaison avec la signature isotopique en Sr théorique d'une eau en équilibre avec une roche mère. Le modèle d'altération utilisé s'appuie sur la dissolution des trois principaux minéraux contenant du Sr (plagioclase, feldspath-K et biotite). Les signatures isotopiques en Sr des eaux sont similaires à celles issues du calcul. La relative dispersion des signatures au sein d'un même batholite peut s'expliquer par des interactions eau-roche dans différents niveaux du profil d'altération. Key words

Strontium isotope geochemistry of alluvial groundwater: a tracer for groundwater resources characterisation

“…7, it is noted that: (Négrel et al, 1997b;this study) and in river water (Négrel et al, 1997a;. Symbols are the same as in Figure 4 and Sr typical of water having interacted with granite (Négrel, 1999;Petelet-Giraud et al, 2003) (Négrel et al, 1997b, Briot et al, 2001. They confirm the range in waters draining these deposits since they are located in the typical lacustrine part of the Limagne.…”

Influences of natural and anthropogenic factors on surface and groundwater quality in rural and urban areas

“…Potassium ions in groundwater often come from orthoclase and muscovite minerals present in granite, and from pollution sources such as chemical fertilizer and domestic effluents. Chloride (Cl − ) may be derived from pollution sources such as industrial and domestic effluents, fertilizers and septic tanks (Bohlke and Horan 2000;Edmunds et al 2003;Negrel and Pauwels 2003;Petelet-Giraud et al 2003;Widory et al 2004; Valdes et al 2007), and from natural sources such as rainfall, the dissolution of fluid inclusions and chloridebearing minerals (Negrel and Roy 1998;Negrel 1999). Groundwater contamination due to nitrate has been a concern for some time.…”

Paleo water–rock interaction determined through isotopic tracing (Sr, O, C, U) in fracture-fill minerals: Evidence from the Vienne granitoids (France)

“…The theoretical Sr, Nd and Pb isotopic signature of water interacting with granite was constrained by a dissolution model (Négrel et al, , 2006Petelet-Giraud et al, 2003;Négrel, 2006) based on the hypothesis that the dissolution of main minerals (like plagioclase, K-feldspar and biotite in the case of Sr) contributes most of the elemental Sr, with the same minerals and the additional of apatite for Nd and allanite for Pb. This model assumes that the newly formed phases are in equilibrium with their parent solution, only the concentration of the fluid is modified, the isotopic ratio remaining constant.…”

“…This model assumes that the newly formed phases are in equilibrium with their parent solution, only the concentration of the fluid is modified, the isotopic ratio remaining constant. In the case of Sr, the theoretical isotopic composition (iSr th ) of water in equilibrium with granite requires the following parameters: (1) the Sr content of the three mineral phases (plagioclase, biotite and K feldspar); (2) the Sr isotopic composition of the three mineral phases; (3) the proportion of each mineral in the rock; and (4) the weatherability of each mineral phase The relative weatherability of the minerals was determined from the literature , the sensitivity analysis of this model was realized by testing various mineral weatherability ratios (Zuddas et al, 1995); the difference in the computed 87 Sr/ 86 Sr ratios is lower than 7 Â 10 À4 (Petelet-Giraud et al, 2003). The theoretical isotopic composition of water in equilibrium with granite was defined for Sr Petelet-Giraud et al, 2003) and a validation method was applied to seven granitoids and two basalts from the literature and two granites from France (Petelet-Giraud et al, 2003;Négrel, 2006).…”