Metodologia de datação em carbonatos pela série de desequilíbrio do urânio por espectrometria de massa

AUTOR(ES)

Barbara Alcantara Ferreira Lima

DATA DE PUBLICAÇÃO

2006

RESUMO

There are many dating methods largely used according to the kind of material to be dated. Methods that rely on the time-dependency of geochemical disequilibrium 238 235 232 U, U and Th natural decay system, between daughter and parent isotope, from the whose final members are stable Pb isotopes, are the most useful in a broader variety of geological, hydrological and archeological problems. The use of this method in cave deposits like speleothems and clastic sediments proved to be a powerful tool to study past climatic and environmental changes. Speleothems like stalagmites are frequently used in paleoenvironmental interpretations. This method may be applied to rocks and underground water dating, erosion studies, transport and sedimentation processes, interactions between rock-groundwater, among others. The present work focuses on the chemical preparation of the U-Th method including the chemical procedures to avoid laboratory contamination and the mass spectrometer precise calibration in order to get high reproducibility. The process has many steps: samples preparation; digestion and chromatographic separation using standards and samples already dated in other laboratories; determination of the isotopic ratios by mass spectrometers, and a case study based on a stalagmite from João Arruda cave, Bonito, Mato Grosso State, Brazil. The obtained data shows the reliability of the method works, mostly in relation to co precipitation of U and Th and to the chromatographic separation of these metals. It is also observed that the obtained dates are within the errors for each sample and are very similar to the dates obtained in other laboratories (validated to this methodology). Further improvements are necessary to achieve better results, mostly related to the reading method in the MC-ICP-MS. The case study in the JA-5 stalagmite shows that it stopped growing around 2,0 ky. This is a different result from that obtained on JA-3 stalagmite, from the same cave, but placed in a spot with different hydrodynamics.

ASSUNTO(S)

cromatografia geologia espeleotemas espectometria de análise ambiental espectrometria de massas urânio

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