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 methods1–4 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.