Uranium-series disequilibrium studies of granitic rocks

Igneous plutons of the Canadian Shield which are to be considered as possible storage sites for radioactive waste, should be free from any evidence of past mobility of chemical species through the rock. However, cores taken from some plutons, to depths of up to 1 km, reveal the presence of hydrous low-temperature alteration along joints and fractures, and within the body of the rock. To test whether such rocks, or any others, have been subjected to chemical transport in geologically recent times, we may study the state of radioactive equilibrium between 238U, 235U, and 232Th and their short-lived daughterisotopes. Members of the decay series of these parent-isotopes have half-lives ranging from a few hours to 0.25 Ma. Also the chemical characteristics of the various daughters vary significantly, allowing us to test the mobility of isotopes of similar half-life but differing chemistry. If the rock has been chemically undisturbed for 10 Ma or more then the activity (in d.p.m./g) of each daughter in these decay schemes will be equal to that of its parent-isotope. Conversely, if the rock was chemically disturbed 10n a ago then some daughter-isotopes with half-lives t ⩾ 10n a will be out of equilibrium with their parents. Preliminary results are presented showing disequilibrium effects in samples from Atikokan, Ontario.