Isotopic composition of osmium in terrestrial samples determined by accelerator mass spectrometry

Abstract

Interest has revived in the rhenium/osmium system due to significant advances in detection methods^1–4 and because of its use for the identification of extraterrestrial material at the Cretaceous/Tertiary boundary⁵. Another, perhaps more important application is its use as a tracer for the origin of crustal material. This application is, however, severely restricted by the very low concentrations typical of crustal rocks. We present here the first results for the isotopes ¹⁸⁷Os, ¹⁸⁸Os, and ¹⁸⁹Os measured in sub-p.p.b. (parts per 10⁹) level samples by accelerator mass spectrometry (AMS). Our first exploratory measurements on extraterrestrial and crustal material with Os concentrations between 2 p.p.m. (parts per 10⁶) and 0.006 p.p.b. show a detection limit of ∼0.01 p.p.b. and a precision of better than 10%. The ¹⁸⁷Os/¹⁸⁶Os ratios measured for samples for the Canyon Diablo meteorite and from the East Clearwater crater in Quebec, Canada, were close to 1, typical of extraterrestrial material. In contrast, ratios in samples from the Ries crater in Germany were around 10, which is evidence of predominantly crustal osmium in these rocks.