Rb-Sr age of stony meteorites

Abstract

A suite of samples of stony meteorites with a spread in Rb 87 Sr 86 ratios from about 0.02 to 3.0 has been analysed for Rb and Sr contents and Sr isotopic compositions. These whole-meteorite analyses all coincide within experimental error with a single isochron. The slope of this isochron yields an age of (4.52 ± 0.12) × 10 9 yr using a decay constant of 1.39 × 10 −11 yr −1 for Rb 87. The meteorites that form the isochron are all observed falls, and consequently are comparatively fresh (unweathered) samples. Each point on the isochron represents several analyses. On the same basis, the intercept of the isochron yields an initial Sr 87 Sr 86 ratio of 0.698 ± 0.001. This value is based on a normalization to 0.1194 of the meteoritic Sr 86 Sr 88 ratio, and is relative to a value of 0.7085 for the Sr 87 Sr 86 ratio in a standard sample of SrCO 3 (Eimer and Amend, lot No. 492327) which was analysed alternately and repeatedly with the meteorite Sr samples during this investigation. It is inferred from this investigation that the source materials of the stony meteorites contained Sr whose Sr 87 Sr 86 ratio was 0.698, and that this source material was chemically differentiated into a variety of Rb-Sr systems. The close fit of the samples to a single isochron indicates that this chemical differentiation must have occurred within a relatively short interval of time, and that the individual meteorites are samples of systems that have remained closed ever since. This investigation favors a decay constant of about 1.38 × 10 −11 yr −1 for Rb 87, in so far as this value is required to make the Rb-Sr age of the meteorites coincide with Patterson's (1956) Pb-isotope age for the meteorites and Earth of 4.55 × 10 9 years.