Extreme Boron Isotope Ratios in Groundwater

Wolfram Kloppmann (1) , Emmanuelle Petelet-Giraud (1) , Catherine Guerrot (1) , Lise Cary (1) , Hélène Pauwels (1)

1 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)

Examples of "extreme" boron isotope ratios in groundwaters are presented, both in the positive 11 B range (up to +75‰) and negative range (down to-30‰) relative to the "typical" 11 B groundwater values of-10‰ to +40‰. A conceptual model of 11 Benrichment in aquifers affected by salinization is provided. Rayleigh distillation by preferential sorption of 10 B-enriched borate on clays under open system conditions during progressing salinization explains the observed 11 B enrichment in modified salinized groundwater. The relative rarity and spatial limitation of extreme positive values >+50‰ is explained by a conjunction of factors necessary for such shifts from the seawater composition (11 B=39‰). In contrast, 11 B-depleted groundwater must have interacted with a solid phase itself depleted in 11 B (e.g. amphiboles, tourmalines, continental borates, coals…) as there is no known natural effect preferentially removing 11 B from the solution and the mobilization of boron from the solid phase is not associated with isotopic fractionation.

Mots clés: B isotopes; Rayleigh distillation; adsorption; groundwater salinization; water-rock interaction

Domaines: Sciences de la Terre

Wolfram Kloppmann, Emmanuelle Petelet-Giraud, Catherine Guerrot, Lise Cary, Hélène Pauwels. Extreme Boron Isotope Ratios in Groundwater. Procedia Earth and Planetary Science, 2015, 13, pp.296 - 300. ⟨10.1016/j.proeps.2015.07.069⟩. ⟨hal-03699932⟩