Rubidium-Strontium age of amphoterite (LL) chondrites

Abstract

Eleven falls and three finds from the amphoterite (LL) group of chondrites have been analyzed for K, Rb, and Sr concentrations and Sr isotopic composition with negligible blank contributions. The samples measured show a sufficiently good spread in their Rb/Sr ratios to define a good isochron with a slope of 0.0654 ± 0.00037 (1s) and an intercept of 0.7005 ± 0.00015. Subject to the assumption that it is meaningful to consider these points as representing cogenetic systems with varying Rb/Sr ratios, this slope yields an age of 4.56 ± 0.150 × 10⁹ years (λ = 1.39 × 10¹¹ year^-1) for the amphoterites. Because of the brecciated appearance of these meteorites and because in some cases the Rb/Sr ratio varies within the meteorite, the extent to which this assumption is valid is unclear. In general, K and Rb tend to vary together in these meteorites. St. Severin is a notable exception with depleted Rb and normal K leading to a very high ratio of about 1500. Soko Banja samples show extreme inhomogeneity in Rb and K distributions.