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An extinct radionuclide is a radionuclide that was formed by nucleosynthesis before the formation of the Solar System, about 4.6 billion years ago, and incorporated into it, but has since decayed to virtually zero abundance, due to having a half-life shorter than about 100 million years.Short-lived radioisotopes that are found in nature are continuously generated or replenished by natural processes, such as cosmic rays (cosmogenic nuclides), background radiation, or the decay chain or spontaneous fission of very long-lived isotopes such as uranium or thorium.Thus, although "extinct", these nuclides are present in meteorites, but produced by a more recent process."The idea that Rb-Sr is the most used chronometer for meteorites is largely based on work done 10-30 years ago.Short-lived isotopes that are not generated or replenished by natural processes are not found in nature, so they are known as extinct radionuclides.Their former existence is inferred from a superabundance of their stable decay products.
Additional evidence for xenon isotopic evolution of mantle reservoirs has been obtained from MORBs (Staudacher and Allegre, 1982) and diamonds (Ozima and Zashu, 1991). The I-Xe method of dating gives the time elapsed between nucleosynthesis and the condensation of a solid object from the solar nebula.Xenon isotopes are also a powerful tool for understanding terrestrial differentiation.The use of 14C in meteorite dating is solely based on its production by cosmic rays (and for terrestrial samples, with its production in the atmosphere).26Al and some other nuclides not mentioned are also used in this way.