Isotopes of osmium

Osmium has seven naturally occurring isotopes, five of which are stable: ¹⁸⁷Os, ¹⁸⁸Os, ¹⁸⁹Os, ¹⁹⁰Os, and ¹⁹²Os. The other natural isotopes, ¹⁸⁴Os and ¹⁸⁶Os, have extremely long half-lives and for practical purposes can be considered to be stable as well. ¹⁸⁷Os is the daughter of ¹⁸⁷Re and is most often measured by the 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.
There are also 31 artificial radioisotopes, the longest-lived of which are ¹⁹⁴Os with a half-life of 6.0 years, ¹⁸⁵Os with 92.95 days, and ¹⁹¹Os with 14.99 days; others are under 30 hours, with most under seven minutes. There are also 19 listed nuclear isomers, the longest-lived of which is ^191mOs with a half-life of 13.10 hours. All isotopes and nuclear isomers of osmium are either radioactive or observationally stable, meaning that they are predicted to be radioactive but no actual decay has been observed.

Osmium isotopes in radiometric dating

The isotopic ratio of osmium-187 and osmium-188 can be used as a window into geochemical changes throughout the ocean's history. The average marine ¹⁸⁷Os/¹⁸⁸Os ratio in oceans is 1.06. This value represents a balance of the continental riverine inputs of Os with a ¹⁸⁷Os/¹⁸⁸Os ratio of ~1.3, and the mantle/extraterrestrial inputs with a ¹⁸⁷Os/¹⁸⁸Os ratio of ~0.13. The lighter isotope, ¹⁸⁷Os, is produced by beta decay of ¹⁸⁷Re. This decay has actually increased the ¹⁸⁷Os/¹⁸⁸Os ratio of the bulk silicate earth by 33%. The difference between crust and mantle ratios is explained this way: crustal rocks have a much higher level of rhenium, which produces an excess of ¹⁸⁷Os. The combined input of the two sources to the marine environment results in the observed ratio in the oceans, and has fluctuated over the geologic history. These changes in the isotopic values of marine Os can be observed in the marine sediment that is deposited, and eventually lithified in that time period. This allows for researchers to estimate weathering fluxes, flood basalt volcanism, and impact events that may have caused some of our largest mass extinctions. The marine sediment Os isotope record has corroborated the K-T boundary impact, for example. The impact of this ~10 km asteroid massively altered the ¹⁸⁷Os/¹⁸⁸Os signature of marine sediments at that time - the average extraterrestrial ¹⁸⁷Os/¹⁸⁸Os of ~0.13 and the huge amount of Os this impact contributed lowered the global marine ¹⁸⁷Os/¹⁸⁸Os value of ~0.45 to a minimum of ~0.2.
Os isotope ratios may also be used as a signal of anthropogenic impact. The same ¹⁸⁷Os/¹⁸⁸Os ratios that are common in geological settings may be used to gauge the addition of anthropogenic Os through things like catalytic converters. While catalytic converters have been shown to drastically reduce the emission of NO_x and CO, they are introducing platinum group elements such as Os, to the environment. Other sources of anthropogenic Os include combustion of fossil fuels, smelting chromium ore, and smelting of some sulfide ores. In one study, the effect of automobile exhaust on the marine Os system was evaluated. Automobile exhaust ¹⁸⁷Os/¹⁸⁸Os has been recorded to be ~0.2. The effect of anthropogenic Os can be seen best by comparing aquatic Os ratios and local sediments or deeper waters. Surface waters thought to be affected have depleted values compared to deep ocean and sediments by a ratio larger than can be explained by cosmic inputs.
The alpha decay of ¹⁸⁴Os into ¹⁸⁰W has been proposed as a radiometric dating method for osmium-rich rocks or for differentiation of a planetary core.