Extreme Boron Isotope Ratios in Groundwater

AbstractExamples of “extreme” boron isotope ratios in groundwaters are presented, both in the positive δ11B range (up to +75‰) and negative range (down to -30‰) relative to the “typical” δ11B groundwater values of -10‰ to +40‰. A conceptual model of 11B-enrichment in aquifers affected by salinization is provided. Rayleigh distillation by preferential sorption of 10B-enriched borate on clays under open system conditions during progressing salinization explains the observed 11B 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 (δ11B=39‰). In contrast, 11B-depleted groundwater must have interacted with a solid phase itself depleted in 11B (e.g. amphiboles, tourmalines, continental borates, coals…) as there is no known natural effect preferentially removing 11B from the solution and the mobilization of boron from the solid phase is not associated with isotopic fractionation