Assinaturas isotópicas de Sr em águas minerais de Portugal e sua relação com o enquadramento geológico

摘要

This work presents the 87Sr/86Sr isotope compositions of nine samples of bottled waters from several regions of Portuguese mainland. The Sr isotopic variability displayed by the analysed waters is strongly correlated with the age and mineralogical composition of the aquifer source rocks, suggesting that the 87Sr/86Sr isotope signatures of these mineral waters are dominantly controlled by fluid-rock and/or fluid-mineral interaction processes. The lowest 87Sr/86Sr ratios are found in water samples from the Monchique aquifer (87Sr/86Sr = 0.70447), located in Late Cretaceous alkaline magmatic rocks of mantle origin. The Vimeiro waters hosted in carbonate and evaporite formations of Jurassic age have 87Sr/86Sr values of 0.70808 and appear to have reached bulk isotopic equilibrium with whole-rock, whereas the waters sourced in Cenozoic siliciclastic sediments tend to exhibit 87Sr/86Sr ratios close to the rainwater value (São Silvestre; 87Sr/86Sr = 0.71078). Finally, the waters coming from granitic and/or metamorphic terrains of the Iberian Variscan basement (Vitalis, Luso, Carvalhelhos, Fastio and Serra da Estrela) display the most radiogenic Sr isotopic signatures (87Sr/86Sr > 0.7136). In the modern commercial context, the differences encountered provide a powerful tool for fingerprinting the mineral water origin and may be used for purposes of mineral water authentication. The results obtained also show the relevance of Sr isotopes as geochemical tracers in hydrogeology.