Rhenium–Osmium Geochronology: Sulfides, Shales, Oils, and Mantle Holly Stein a,b * and Judith Hannah a,b a AIRIE Program, Colorado State University, Fort Collins, CO, USA b CEED, University of Oslo, Oslo, Norway Synonyms Re–Os geochronology; 187 Re– 187 Os geochronology Definition Rhenium–osmium geochronology is based on radioactive decay of 187 Re to 187 Os with a half-life of 41.6 b.y. – about 10 times the age of the earth. Both Re and Os are siderophile-chalcophile elements, that is, they are both strongly partitioned into metals or sulfides rather than silicates. This distin- guishes them from other widely used geochronometers whose parent-daughter elements reside in silicates. Re–Os geochronology underpins dating of materials from meteorites, the mantle, and metallic ore deposits. Furthermore, both Re and Os are redox-sensitive metals, soluble when oxidized, and fixed by reduction. Thus, both are mobile in Earth’ s presently oxidized surface environments but are concentrated in sulfides and organic matter in anoxic-euxinic sediments. This is the basis for Re–Os dating of the depositional age of organic-rich sedimentary rocks and provides a temporally constrained record of changing redox conditions through earth history. Fundamentals of Re–Os Geochemistry There are two naturally occurring isotopes of Re: stable 185 Re and radioactive 187 Re. Mass fractionation of the two Re isotopes is readily observed during the extreme conditions imposed by mass spectrometry (e.g., Suzuki et al. 2004; Zimmerman et al. 2007). The few reported natural variations, however, are less than 0.3 ‰ (Miller et al. 2009) and thus introduce errors less than other sources of uncertainty for Re–Os geochronology. As with other high-mass elements used in geochronology, we therefore assume uniform present-day abundances of 185 Re (37.398 %) and 187 Re (62.602 %; Gramlich et al. 1973) as an underpinning assumption for radiometric dating. There are seven naturally occurring isotopes of Os: 184 Os, 186 Os, 187 Os, 188 Os, 189 Os, 190 Os, and 192 Os. Isotope 187 Os is the product of beta decay of 187 Re, with a decay constant of 1.666  10 11 year 1 (Smoliar et al. 1996). Isotope 186 Os is the product of alpha decay of 190 Pt with a decay constant of 1.542  10 12 years 1 (Walker et al. 1997). The abundances of the remaining isotopes may be considered constant for present-day Re–Os isotope geochemistry and geochronology. Prior to recognition of small, but measurable contributions of radiogenic 186 Os, Os isotopic compositions were reported as 187 Os/ 186 Os. Current convention is to normalize abundances of 187 Os to 188 Os, reporting isotopic variations in terms of the ratio 187 Os/ 188 Os. *Email: holly.stein@colostate.edu Encyclopedia of Scientific Dating Methods DOI 10.1007/978-94-007-6326-5_36-1 # Springer Science+Business Media Dordrecht 2014 Page 1 of 25