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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.

Radon is entirely radiogenic, since it has too short a half-life to occur 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).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 (Th.In rocks that contain uranium and thorium, the excess amounts of the three heavier lead isotopes allows the rocks to be "dated," or the time estimate from when the rock solidified and the mineral held the ratio of isotopes fixed and in place.Other notable nuclides that are partly radiogenic are argon-40, formed from radioactive potassium, and nitrogen-14, which is formed by the decay of carbon-14.Other important examples of radiogenic elements are radon and helium, both of which form during the decay of heavier elements in bedrock.