Boron behavior in the rivers of Réunion island, inferred from boron isotope ratios and concentrations of major and trace elements

International audienceA 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 (δ 11 B≈40‰), hydrothermalism (δ 11 B≈0‰), low temperature basalt weathering in steep sided basins (δ 11 B≈30‰), and cycling within soil and vegetation (δ 11 B>40‰). Réunion rivers have schematically two types of δ 11 B 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 11 B, with δ 11 B≥45‰

Geochemical Conditions Allowing the Formation of Modern Lacustrine Microbialites

International audienceInterpreting the environmental conditions of ancient microbialites rely on comparisons with modern analogues. Yet, we lack a detailed reference framework relating the chemical and mineralogical composition of modern lacustrine microbialites with the physical and chemical parameters prevailing in the lakes where they form. Here we performed geochemical analyses of water solutions and mineralogical analyses of microbialites in 12 Mexican crater lakes. We found a large diversity of microbialites in terms of mineralogical composition, with occurrence of diverse carbonate phases such as magnesian calcite, monohydrocalcite, aragonite, hydromagnesite, and dolomite as well as authigenic magnesium silicate phases. In parallel, the chemical compositions of the lakes differed particularly by their alkalinity, their concentration of ortho-silicic acid (H 4 SiO 4) and their Mg/Ca ratio. From this study, we infer a minimum alkalinity value for the formation of lacustrine microbialites, as well as several constraints given by the presence of mineral phases on the chemical composition of the lakes in which microbialites formed. Finally, we observe a general correlation between the alkalinity and the sodium content of the lakes. This may relate to variations in evaporation intensity and provide a historical model for lacustrine microbialite formation: microbialite start forming only when the lake is sufficiently old/evaporated