RbSr age determinations on South African kimberlite pipes

Abstract

A study has been made to establish to what extent reliable age-measurements can be obtained by the RbSr isochron method on phlogopite micas from kimberlite and its associated inclusions. Essentially linear isochrons were usually obtained for the micas when grouped according to their source. The data for the Wesselton and Roberts Victor pipes are examined in detail to assess the validity of the method. The available evidence suggests that mica which occurs in kimberlite groundmass ahd associated peridotite and eclogite inclusions yields valid ages for pipe emplacement by the isochron method. Phlogopite nodule samples appear to yield an age that is anomalously high due to radiogenic Sr retention, and an association with carbonatite is implied.

Results for the following pipes are reported: Wesselton, DuToitspan, De Beers, Bultfontein and Monastery (mean age 86±3 m.y.); Roberts Victor (127±3 m.y., possibly less reliable); Swartruggens (147±4 m.y.); Gross Brukkaros (∼84 m.y., not considered reliable).