- © The Mineralogical Society Of America
One of the reasons noble gases make such good tracers of processes occurring in rocks is their scarcity. Thus, a process that converts a small fraction of a relatively rare element into a noble gas can have a large effect on the noble gas. Radioactive decay can be such a process. In a meteorite which has formed very early in the solar system’s history and been little altered since, the effect can be even more dramatic. Furthermore, since radioactive decay proceeds at a known rate, radiogenic noble gases, those produced by decay, have been crucial in deciphering the chronology of the solar system.
Several “radionuclides,” radioactive isotopes that decay to noble gases, are listed in Table 1⇓. In a radioactive decay, the radioactive isotope is referred to as the “parent” isotope, while the decay product, the noble gas isotope, is referred to as the “daughter” isotope. In some cases listed, there is more than one mode of decay possible. In other cases, a single decay starts a chain that will ultimately produce several noble gas atoms. To take both into account, the yield (the number of noble gas atoms produced for each parent atom) is also given. Finally, radionuclides that fission may produce any of the several different isotopes of Xe in a characteristic spectrum (Table 2⇓). All of the systems listed, with the exception of the decay of U and Th to Xe, have been exploited in extraterrestrial samples at one time or another.
A typical analysis of radiogenic noble gases involves trying to find the ratio of the daughter isotope to some other, stable, isotope of the parent element, for example, the ratio 129Xe*/127I or 40Ar*/40K, where …