Rhenium–osmium dating
Rhenium–osmium dating uses the decay of rhenium-187 to osmium-187 and is ideal for iron meteorites.
Rhenium–osmium dating uses rhenium-187, which decays to osmium-187 with a half-life of about 42 billion years. Both rhenium and osmium are siderophile, or iron-loving, so they are concentrated in metal. That makes the system one of the few that can directly date iron meteorites.
Isochrons from different iron meteorites within a group, or from metal phases within one meteorite, give the time when the metal crystallised and stopped exchanging these elements. Studies of IIIAB, IVA and IIAB irons have given ages close to 4.5 billion years, consistent with the very early formation of asteroid cores shown by hafnium–tungsten dating.
The method is also used to study the osmium isotopes of Earth's mantle and of impact deposits. Material from meteorites has a distinctive osmium signature, so rhenium–osmium analysis can detect even small amounts of meteoritic matter in impact melt rocks and in sediment layers, helping confirm that an ancient crater was made by an impact.
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