STUDY OF THE RADIOACTIVE DISEQUILIBRIUM OF THE VOLCANIC ROCKS FROM THE TRINDADE ISLAND (BRAZIL)
ROSANA N. SANTOS1, 2 AND LEILA S. MARQUES1
1Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, 01065-970, São Paulo, SP.
2Departamento de Física, Pontifícia Universidade Católica de São Paulo, 01303-050, São Paulo, SP.
Presented by ANTONIO C. ROCHA-CAMPOS
The radiochemical procedures employed for the determination of 238U, 234U and 230Th activities in silicates by alpha spectrometry are presented. The best experimental conditions were defined using 233U, 232U and 229Th radioactive tracers and simulating the usual conditions found in processing silicate samples. The adapted procedure 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 stainless 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 chemical yields obtained for uranium and thorium are of about 60% and 70%, respectively, for both matrices. The described methodology provides activity measurements with less than 4% relative precision and accuracy of about 1%, that are essential for petrogenetic applications.
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 were obtained, as well as the 234Th, 214Pb, 214Bi, 235U, 228Ac, 212Pb, 212Bi and 208Tl specific activities. These results were interpreted along with additional constraints given by major and minor element concentrations, determined by X-ray fluorescence.
The 232Th series is in secular radioactive equilibrium in all analyzed 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 of the island (Morro Vermelho Formation, Valado Formation and Vulcão do Paredão), the 230Th and 238U are not in secular equilibrium. Furthermore, three Morro Vermelho Formation samples analyzed by gamma spectrometry showed 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. (December 14, 2001).
Publication Dates
-
Publication in this collection
09 Oct 2002 -
Date of issue
Sept 2002
