Relation between U-series nuclide migration and microstructural properties of sedimentary rocks

Abstract

Natural radionuclide migration in Miocene sedimentary rocks has been studied at the Tono U deposit in Japan, which is a potentially useful analogue of radioactive waste isolation in geological environments. Uranium series disequilibrium studies have revealed that natural radionuclides have been redistributed during the past 3.5 × 10 5 a in the U-mineralized zone. Permeability tests using core samples of the U-mineralized zone have shown that the microfabrics of sedimentary rocks, such as the connectivity of pores which control the groundwater movement, directly influence the redistribution of nuclides. Detailed observations using cathodoluminescence (CL) and electron probe microanalysis (EPMA) were carried out after a dye-impregnation test in order to improve the understanding of the importance of microfabrics in relation to nuclide migration. Results of the observations show that U has migrated within detrital grains, such as biotite and quartz, as well as between these grains. Uranium-series disequilibrium studies of each detrital quartz and biotite grain have been shown that these detrital minerals behave differently in respect of the migration of U, owing to their different textural properties. The detrital biotite flakes, especially along the (001)-cleavage planes, appear to have fixed U for a long period of time, whereas the U has remained mobile within the microfractures in detrital quartz grains.