Implantação da metodologia de espectrometria 'alfa' para a determinação de isótopos de U e Th em rochas ígneas: aplicação ao estudo do desequilíbrio radioativo na Ilha da Trindade

Abstract

This work describes the implementation of experimental procedures for alpha spectrometry measurement of 238U, 234U and 230Th activities in silicates. The best experimental conditions were defined using 233U, 232U and 229Th radioactive tracers and simulating the usual conditions found in processing silicates. The chemical procedures consists of the following steps: radioactive tracer addition and sample dissolution by acid digestion, U and Th pre-concentration by co-precipitation, element separation and purification by ion exchange chromatography and electrodeposition in inox steel disks. In order to evaluate its effectiveness, the procedure was applied to the Brazilian geological standards BB-1 (basalt) and GB-1 (granite). The obtained chemical yields for The 238U and 232Th series disequilibrium conditions were investigated by alpha spectrometry, together with neutron activation analysis and natural gamma-ray spectrometry. 234U/238U, 238U/232Th and 230Th/232Th activity ratios and the 234Th, 214Pb, 214Bi, 235U, 228Ac, 212Pb, 212Bi and 208Tl specific activities were obtained. These results were interpreted with the help of additional constraints given by the larger and smaller elements concentrations, measured by X-ray fluorescence. The 232Th series is in secular radioactive equilibrium in all analysed samples. In the case of the 238U series, the equilibrium condition was verified, as expected, in the oldest rocks from the Trindade Island (Trindade Complex and Desejado Sequence). On the other hand, the results show that, in the samples from the last three volcanic episodes in the island (Morro Vermelho Formation, Valado Formation and Vulcão do Paredão), the 230Th and 238U are not in equilibrium. Furthermore the three Morro Vermelho Formation samples analysed by gamma spectrometry showed the disequilibrium between 226Ra and 238U. The obtained (238U/232Th) and (230Th/232Th) activity ratios allowed to estimate a 140,000 y age for the Morro Vermelho Formation uranium and thorium are of about 60% and 70%, respectively, for both matrices. The described methodology furnishes activity measurements with less than 4% relative precisions and accuracies of about 1%, that are essential for petrogenetic applications. The