Note on the UPb perovskite method for dating kimberlites: Examples from the Wesselton and De Beers mines, South Africa, and Somerset Island, Canada

The previously reported UPb perovskite method of dating kimberlite has been substantially improved by obtaining high-purity perovskite concentrates using a combination of chemical, magnetic and hand-picking techniques. Reliable results are obtained from subhedral to euhedral perovskite, but not from anhedral perovskite commonly displaying atoll-like grain shapes. The latter may be weathered or may have been partially digested in the chemical treatment used in the initial concentration step. An age of 89 Ma was obtained from the Wesselton kimberlite and one of two De Beers Mine samples, while a second De Beers sample gave a younger age of ∼ 83 Ma. These ages are in agreement with previous age determinations by various other methods. The ThPb age for perovskite from De Beers Mines is 81 Ma. An age of ∼ 105 Ma for a Somerset Island kimberlite is in agreement with a RbSr errorchron age of ∼ 100 Ma for micas from a second Somerset Island occurrence, but a perovskite age of 40 Ma for a third locality is not realistic. Fresh kimberlitic perovskite has U and Pb concentrations of ∼ 150 and 15 ppm, respectively. Th concentrations fresh perovskite are of the order of 1000 ppm, though this is based on only one sample from the De Beers Mine. ⁸⁷Sr/⁸⁶Sr ratios and presumably ¹⁴³Nd/¹⁴⁴Nd ratios are easily obtained from groundmass perovskites, and reflect the initial Sr and Nd isotopic compositions of kimberlite at the time of emplacement.