Comparison of the 87 Rb → 87 Sr ages of the red granite of the Bushveld complex from measurements on the total rock and separated mineral fractions

Abstract

Age determinations on a portion of the total crushed rock, and on the felspar fraction of each of four widely separated samples of the red granite from the Bushveld complex are reported. A single determination from the separated biotite of one sample was made. These nine determinations lead to a mean age of 2.41 x 10 9 years [ t 1/2 = 6.3 x 10 10 years] or 1.92 x 10 9 years [ t 1/2 = 5.0 x 10 10 years]. There are no variations between individual determinations that are significant at the 99% confidence level. For the unweighted mean age the 99% confidence limits are ± 0.13 x 10 9 years. Despite the low enrichment of 87 Sr the ‘total rock ’ method shows 99% confidence limits of ± 0.22 x 10 9 years for the mean of four determinations.