Origin
Rare earth elements are heavier than iron and thus are produced by supernova nucleosynthesis or the s-process in asymptotic giant branch stars. In nature, spontaneous fission of uranium-238 produces trace amounts of radioactive promethium, but most promethium is synthetically produced in nuclear reactors.
Rare earth elements change through time in small quantities (ppm, parts per million), so their proportion can be used for geochronology and dating fossils.
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