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$\cdot $41 $\times $ 10$^{9}$ years [t$_{\frac{1}{2}}$ = 6$\cdot $3 $\times $ 10$^{10}$ years] or 1$\cdot $92 $\times $ 10$^{9}$ years [t$_{\frac{1}{2}}$ = 5$\cdot $0 $\times $ 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 $\pm $ 0$\cdot $13 $\times $ 10$^{9}$ years. Despite the low enrichment of $^{87}$Sr the 'total rock' method shows 99% confidence limits of $\pm $ 0$\cdot $22 $\times $ 10$^{9}$ years for the mean of four determinations.