Extreme Boron Isotope Ratios in Groundwater
Abstract
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.
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