Editing Rare-earth element (section)

==Origin==
Rare-earth elements, except [[scandium]], are heavier than [[iron]] and thus are produced by [[supernova nucleosynthesis]] or by 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.

Due to their chemical similarity, the concentrations of rare earths in rocks are only slowly changed by geochemical processes, making their proportions useful for [[geochronology]] and dating fossils.