Boron Behavior in the Rivers of Réunion Island, Inferred from Boron Isotope Ratios and Concentrations of Major and Trace Elements

A combined study of boron concentrations and isotopic ratios and of major and trace elements measured in Réunion waters, sampled up to four times between 1995 and 2012, illustrates boron behavior during water-rock interactions in tropical basaltic catchments. Boron isotope ratios measured in Réunion rivers and springs show a large range of variation between 1 and 48‰, that reflect mixed B sources and water-rock interaction processes: rain (δ11B≈40‰), hydrothermalism (δ11B≈0‰), low temperature basalt weathering in steep sided basins (δ11B≈30‰), and cycling within soil and vegetation (δ11B>40‰). Réunion rivers have schematically two types of δ11B signatures for low-temperature water-rock interactions: ≈30‰ for rivers with high weathering rates, and ≈45‰ for small forested catchments with relatively smoother slopes and lower weathering rates. High temperature water-rock interaction produces B enriched waters with a B isotopic signature close to that of the rocks. B behavior in the soil and vegetation cycle is more difficult to characterize but seems to result in soil solutions enriched in 11B, with δ11B≥45‰.