Uranium series dating of calcite coatings in groundwater flow systems of the Canadian Shield

Thin coatings of calcite on granitic fracture surfaces in contact with flowing groundwaters have been removed mechanically and dissolved. The resulting solutions were analyzed for their 238U, 234U, 228Th, 230Th and 232Th contents. Mean times of deposition for the U in these coatings were calculated from these data to establish lower limits for the time since the onset of flow through the fracture. In addition, the results indicate the limited variability of uranium water chemistry during this period. 11 refs.; 3 figs.; 3 tabs