The relative mobility of U, Th and Ra isotopes in the weathered zones of the Eye-Dashwa Lakes granite pluton, northwestern Ontario, Canada

Radioisotope activity ratios of weathered surface rocks from the Eye-Dashwa Lakes granite pluton show that, in the 238U decay series, 226Ra has been leached more rapidly than the grandparent 234U and 238U isotopes. The observation of 238Th232Th disequilibria in the 232Th decay series shows that either 228Th (half-life of 1.9 yrs) or 228Ra (half-life of 5.75 yrs) have been removed from these rocks very rapidly indeed. Isotope ratios of separated …