Razão Isotópica 87Sr/86Sr em Sedimentos Recentes Implicações na Datação Radiométrica das Rochas Sedimentares

Resumo

Radiometric ages of geological significance are obtained when the Rb/Sr isocronic method is applied to argillaceous sedimentary rocks. In spite of these results many questions arise when the Rb/Sr methodology is applied to sedimentary rocks, notably the behavior of the rubidium (Rb) and strontium (Sr) elements during sedimentation and during the diagenetic process. The greatest uncertainty lies in the mechanism and the timing of the strontium isotopic homogenization, with constitutes the basic assumption behind radiometric dating by the Rb/Sr method. The purpose of this work is to contribute to the clarification of the conditions for the strontium isotopic homogenization, the relation between Rb and Sr and the dimension and nature of sedimentary particles, the early process associated with the clay minerals, with sedimentation process and with intersticial fluids. Recent sediments were sampled from different environments: the deltaic region of the Paraíba do SuI River (RJ), the deltaic region of the Açu River (RN), the continental shelf associated with the Amazon River mouth (AM), the continental slope and Jacarepaguá coastal plain (RJ). The samples were analysed by X-ray diffraction, scanning electron microscope and chemical and isotopic techniques. Results obtained show that some samples reached uniform strontium isotopic composition under certain conditions. These conditions include close proximity between samples, clay fraction enriched in expansive minerals such as smectite and degraded illite associated with deposition preferentially in marine waters.