U-Th ISOTOPIC SYSTEMATICS OF SÃO PEDRO-SÃO PAULO ROCKS: INDICATION FOR ALTERATION PROCESSES BY SEAWATER

• Grant number: 09/01755-0
• Support Opportunities: Scholarships in Brazil - Scientific Initiation
• Start date: June 01, 2009
• End date: December 31, 2009
• Field of knowledge: Physical Sciences and Mathematics - Geosciences - Geophysics
• Principal Investigator: Rosana Nunes dos Santos
• Grantee: Karine Harumi Moromizato
• Host Institution: Instituto de Astronomia, Geofísica e Ciências Atmosféricas (IAG). Universidade de São Paulo (USP). São Paulo , SP, Brazil

Abstract

The 238U, 235U and 232Th natural radioactive series are composed of radioactive nuclides with different physical and chemical properties. When the series decay occurs in a closed system, it tends to reach, with time, the state of secular radioactive equilibrium. Disequilibrium, however, is evidenced when the state of equilibrium is disturbed by chemical, physical or biological processes, as parent or daughter nuclides are added or leached. After the disturbance has ended, the radioactive equilibrium is gradually attained again; the rate of such a re-equilibrium depends on the half-life of all intermediate daughter products. This time-dependent process allows dating the moment of disturbance. In the investigation of radioactive disequilibria in 238U series it should be considered magmatic processes do not provoke any fractionation between 234U and 238U isotopes. After the system is closed, 230Th and 238U only reach secular equilibrium in about 450,000 years. Thus, radioactive disequilibria in 238U series can be used for geochronological and petrogenetic studies in igneous rocks of the recent Quaternary.The use of the radioactive disequilibria of 232Th series to the study of the magmatic systems is much more limited, since all decay products reach the condition of secular equilibrium in about 40 years. Taking into account the time interval involved in the process of magma generation, and its transport to the surface, which is, in general, between 30 and 8,000 years, 228Ra and 232Th are very frequently in secular radioactive equilibrium.This study presents the results of a systematic investigation about the condition of radioactive disequilibrium in 238U and 232Th series in rocks from São Pedro-São Paulo Archipelago. These rocks are essentially constituted by mylonitized peridotites, which are localized in a geologically active region of the Equatorial Atlantic Ocean. In order to identifying and quantifying disequilibrium in those series, U and Th concentrations, 238U, 234U, and 230Th activity concentrations, which belong to the 238U series, as well as 232Th activity concentration, were determined by alpha spectrometry.