Extinct radionuclide
An extinct radionuclide is a short-lived radioactive isotope that has entirely decayed but left traces in meteorites.
An extinct radionuclide is a radioactive isotope that existed when the Solar System formed but has completely decayed since. The term emphasises that we cannot measure the parent directly; we can only see its fingerprints as excess daughter isotopes.
The first one recognised was iodine-129, which decays to xenon-129. In 1960 the physicist John Reynolds found excess xenon-129 in the Richardton chondrite, showing that iodine-129 was alive when the meteorite formed. That meant the time between the making of the elements and the formation of meteorites was short, a few tens of millions of years at most.
Since then, many more extinct radionuclides have been identified, including aluminium-26, manganese-53, iron-60, hafnium-182 and plutonium-244. Each gives a different window on early events. The method is to measure daughter excesses in several minerals with different parent concentrations: a straight-line relationship confirms the parent was present and gives a relative age.
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Level: Expert Advanced