Caracterização isotópica (Nd-Sr) e litogeoquímica de sedimentos atuais da Namíbia (África): implicações para proveniência sedimentar e potencial fonte de aerossol

Abstract

The purpose of this study is to verify wether the isotopic compositions of Nd and Sr of whole rock and the coarse silt fraction (36-64 μm) of present fluvial sediments in desert environment reflect the isotopic signatures of the nearby geological terrains. Another purpose is to define the field of isotopic compositions of Nd and Sr of the fine fraction (1.8-18 μm and <1.8 μm) of the same samples for provenance studies in the Namib Desert. The chemical and mineralogical characterization of sediments in whole rock and coarse silt (36-64 μm) was performed to corroborate the isotopic study and to verify if the sediment samples correspond to the weighted average of all the stratigraphic units of the sub-basin through the study of modeling and isotopic mixing. The mineralogical composition of nine samples in whole rock and eight of the coarse silt fraction was obtained by X-ray diffraction with semiquantitative estimative by Rietveld s refinement. The elemental composition of these samples in whole rock and five of the coarse silt fraction of the major, traces and rare earths elements was obtained by ICP-MS. The isotopic ratios 87Sr / 86Sr and 143Nd / 144Nd in the whole rock and eight fractions (36-64 μm, 1.8-18 μm and <1.8 μm) were analyzed by Thermal Ionization Mass Spectrometry (TIMS). The results show, in general, mineralogical and chemical fractionation, in the major, traces and rare earth elements between the whole rock and the coarse silt fraction. In the diagram εNd (0) X 87Sr / 86Sr (0), the isotopic results obtained in whole rock and coarse silt fraction (36-64 μm) were compared with the isotopic signatures of the main regional geological terrains using a database built from the literature. The analysis of the finer fractions were compared, and it was verified that there is, in general, a fractionation of Sr and Nd between them and the whole rock, but without linear tendency. The study concludes that the signature of each sample, as expected, reflects that of the nearby lithological associations, or with isotopic indications of isotopic mixing of two or more source areas. In the isotope modeling study, it was verified that the sediments did not correspond to the weighted average of all lithostratigraphic units exposured in each sub-basin, indicating that ephemeral fluvial systems are not efficient in promoting the mixing of all units. In four samples, the two-component isotopic mixing model (near units) shows that the difference of ε (Nd) measured and the weighted average of the literature diverge below 19%. On the other hand, four samples do not show a mixing curve, because the measured ε (Nd) value is close to the value of one unit adjacent to the collection. Comparing the isotopic compositions of the Saharan dust with the result of the Namib Desert, there is a great similarity between six samples (<18 μm) of this study with isotopic signatures of the Sahara, and two samples, an intermediate signature between the Kalahari and Sahara deserts.