VII. The natural radioactivity of rubidium

The ages of some African lepidolites determined from the 87 Rb- 87 Sr decay

Using isotope dilution the ages of fourteen lepidolite hand samples representing eight different areas have been determined. Lepidolites from Pope’s Claim, Southern Rhodesia, Bikita, Southern Rhodesia and Alto Ligonha, Portuguese East Africa, were investigated in a series of statistical experiments and the ages and 95% confidence limits for the mean of the determinations are: Pope’s Claim ' A ' 2931 ± 63 million years, Pope’s Claim ' B ' 2955 ± 75 million years; Bikita ' A ' 2978 ± 43 million years, Bikita ' B ' 2961 ± 25 million years; Alto Ligonha ' A ' 533 ± 8·3 million years, Alto Ligonha ' B ' 528 ± 8·5 million years. Ḟurther samples from Bikita show ages ranging from 2830 to 3040 million years and all lying within the 95% confidence limit for a single determination. The Karibib area, South-West Africa, is dated from three separate samples as 600 ± 70 million years. The other ages determined are Hombola, Tanganyika (2990 ± 200 million years), Antelope Mine, Southern Rhodesia (2675 ± 150 million years), Tsarafara, Madagascar (652 ± 45 million years) and Mica Siding North-East Transvaal (2040 ± 150 million years). In these cases the uncertainty quoted covers the range of the determinations made. The relationship of these ages to ages determined using 87 Rb— 87 Sr decay elsewhere and to U—Pb ages for the same areas is discussed, and it is shown that for the Rhodesian shield a value of λ = 1·26 ± 0·04 x 10 -11 for 87 Rb decay would lead to agreement with the U-Pb ages.