Extinct nuclide dating
Helium-3 is almost entirely primordial (a small amount is formed by natural nuclear reactions in the crust).
The global supply of helium (which occurs in gas wells and well as the atmosphere) is almost entirely (about 90-99%) radiogenic, as shown by its factor of 10 to 100 times enrichment in radiogenic helium-4 relative to the primordial ratio of helium-4 to helium-3.
While the moment in time at which a particular nucleus decays is unpredictable, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-life, usually given in units of years when discussing dating techniques.Such nuclides are formed in supernovas, but are known as extinct radionuclides, since they are not seen directly on the Earth today.An example of an extinct radionuclide is xenon-129, a stable isotope of xenon which appears as a relative excess against other xenon isotopes.This latter ratio is known from extraterrestrial sources, such as some moon rocks and meteorites, which are relatively free of parental sources for helium-3 and helium-4.As noted in the case of lead-204, a radiogenic nuclide is often not radioactive.
Additionally, elements may exist in different isotopes, with each isotope of an element differing in the number of neutrons in the nucleus.