Géochimie isotopique du bore dans les cycles supergènes d'altération

Abstract : Large variations in boron (B) isotopic compositions were found in river waters, clay minerals from sediments and soils, rain waters, and snow. For this study, a new analytical method was developed to analyse boron isotopes in natural samples with low boron content (sometimes less than 1 ppb). Boron isotopie compositions of river waters, which are among the first to be measured, are highly variable ([environ]35%o). These variations suggest that boron is a potential tracer of weathering processes. Variations in boron isotopic compositions due to silicate alteration are sensitive to the extent of dissolution (total or partial) and to the pH of the soil where secondary minerais are formed. Large changes in boron isotopic compositions found in the Gange and the Brahmapoutre rivers between the monsoon and the dry seasons ([environ]15%o) reveal the extreme heterogeneity of inputs to the ocean, which has to be considered to understand the processes regulating the B isotopic composition of seawater. Boron isotopic compositions of clay minerals (<0.1[micro]m et <2[micro]m) from Himalayan sediments and soils display also large variations. However, it is possible to discriminate between kaolinite rich clays, which display typical low Il B enrichments, and smectiterich clays, which are usually enriched in 11B. These differences are enhanced by the monsoon. The newly measured boron isotopic compositions in rain waters and snows unravel information on the boron isotopic composition of the atmosphere. Experiments performed in the laboratory to mimic the evaporation of seawater showed that it is possible to reproduce concentrations and isotopic compositions of natural rain waters.
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Estelle Rose. Géochimie isotopique du bore dans les cycles supergènes d'altération. Sciences de la Terre. Institut National Polytechnique de Lorraine, 1999. Français. ⟨NNT : 1999INPL024N⟩. ⟨tel-01749930⟩

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