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 mineral fractions appear to show that whereas uranium tends to be removed from all mineral fractions, Ra is strongly readsorbed onto feldspars, presumably as these undergo alteration to clay minerals. This necessitates making a distinction between intermineral mobility of U or Ra, and the net loss of U or Ra from the whole rock.An attempt has been made to model, quantitatively, the transport rate of Ra, though with little success. Failure of the modelling appears to indicate that the mobility of 228Ra is higher than might be predicted by the mobility of 226Ra; that is, the two isotopes are not strict analogues of one another in their response to initial weathering.