Radiogenic nuclide

A radiogenic nuclide is a nuclide that is produced by a process of radioactive decay. It may itself be radioactive (a radionuclide) or stable (a stable nuclide).

Radiogenic nuclides (more commonly referred to as radiogenic isotopes) form some of the most important tools in geology. They are used in two principal ways:

Some naturally occurring isotopes are entirely radiogenic, but all those are radioactive isotopes, with half-lives too short to have occurred primordially and still exist today. Thus, they are only present as radiogenic daughters of either ongoing decay processes, or else cosmogenic (cosmic ray induced) processes that produce them in nature freshly. A few others are naturally produced by nucleogenic processes (natural nuclear reactions of other types, such as neutron absorption).

For radiogenic isotopes that decay slowly enough, or that are stable isotopes, a primordial fraction is always present, since all sufficiently long-lived and stable isotopes do in fact naturally occur primordially. An additional fraction of some of these isotopes may also occur radiogenically.

Lead is perhaps the best example of a partly radiogenic substance, as all four of its stable isotopes (²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb) are present primordially, in known and fixed ratios. However, ²⁰⁴Pb is only present primordially, while the other three isotopes may also occur as radiogenic decay products of uranium and thorium. Specifically, ²⁰⁶Pb is formed from ²³⁸U, ²⁰⁷Pb from ²³⁵U, and ²⁰⁸Pb from ²³²Th. In rocks that contain uranium and thorium, the excess amounts of the three heavier lead isotopes allows the rocks to be "dated", thus providing a time estimate for when the rock solidified and the mineral held the ratio of isotopes fixed and in place.

Another notable radiogenic nuclide is argon-40, formed from radioactive potassium. Almost all the argon in the Earth's atmosphere is radiogenic, whereas primordial argon is argon-36.

Some nitrogen-14 is radiogenic, coming from the decay of carbon-14 (half-life around 5700 years), but the carbon-14 was formed some time earlier from nitrogen-14 by the action of cosmic rays.

Other important examples of radiogenic elements are radon and helium, both of which form during the decay of heavier elements in bedrock. Radon is entirely radiogenic, since it has too short a half-life to have occurred primordially. Helium, however, occurs in the crust of the Earth primordially, since both helium-3 and helium-4 are stable, and small amounts were trapped in the crust of the Earth as it formed. Helium-3 is almost entirely primordial (a small amount is formed by natural nuclear reactions in the crust). Helium-3 can also be produced as the decay product of tritium (³H) which is a product of some nuclear reactions, including ternary fission. The global supply of helium (which occurs in gas wells as well as the atmosphere) is mainly (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. 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.