Iodine–xenon dating
Iodine–xenon dating uses the extinct isotope iodine-129, which decayed to xenon-129, to date early events.
Iodine–xenon dating is based on iodine-129, a short-lived isotope with a half-life of about 16 million years, which decays to xenon-129. All the original iodine-129 has long since decayed, but its xenon remains trapped in minerals that originally contained iodine.
To measure the age, scientists irradiate a sample with neutrons, converting some stable iodine-127 into xenon-128. They then heat the sample in steps and measure the released xenon. The ratio of xenon-129 to xenon-128 is proportional to the iodine-129 to iodine-127 ratio when the mineral formed or last closed. Comparing this with a standard gives a relative age.
The method has dated chondrules, aqueous alteration products and early metamorphism in chondrites. It played a historic role: the 1960 discovery of excess xenon-129 in a meteorite was the first proof that short-lived radionuclides existed in the early Solar System and that meteorites formed soon after the elements were made.
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