Osmium (Os) has seven naturally occurring isotopes, 6 of which are stable: 184Os, 187Os, 188Os, 189Os, 190Os, and (most abundant) 192Os. The other natural isotope, 186Os, has an extremely long half-life (2×1015 years) and for practical purposes can be considered to be stable as well. 187Os is the daughter of 187Re (half-life 4.56×1010 years) and is most often measured in an 187Os/188Os ratio. This ratio, as well as the 187Re/188Os ratio, have been used extensively in dating terrestrial as well as meteoric rocks. It has also been used to measure the intensity of continental weathering over geologic time and to fix minimum ages for stabilization of the mantle roots of continental cratons. However, the most notable application of Os in dating has been in conjunction with iridium, to analyze the layer of shocked quartz along the Cretaceous–Paleogene boundary that marks the extinction of the dinosaurs 66 million years ago.
There are also 30 artificial radioisotopes, the longest-lived of which is 194Os with a half-life of 6 years, all others have half-lives under 94 days. There are also 9 known nuclear isomers, the longest-lived of which is 191mOs with a half-life of 13.10 hours.
Relative atomic mass: 190.23(3).