A method of radiometric dating based on the decay of rhenium-187 (187Re) to osmium-187 (187Os), with a half-life of 4.16 × 1010 years. Since both rhenium and osmium are strongly siderophile and chalcophile, they are useful in dating sulphide ores.
http://www.geochronometria.pl/pdf/geo_27/Geo27_05.pdf A review of the physical foundations of the rhenium–osmium method.