Rb-Sr dating of two kimberlites from the newly discovered kimberlite field in the Lac de Gras area within the Slave Craton in the Northwest Territories, Canada, shows that these diamondiferous occurrences are Eocene in age. Leached phlogopite and whole-rock isochrons yield Rb-Sr ages of 47.5+ or -1.6 Ma and 52.1+ or -1.0 Ma for the two kimberlites, confirming the palaeontological evidence based on fossils in mudstones preserved as xenoliths within the kimberlites.