Radiogenic heat
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 these are isotopes that are radioactive, with half-lives too short to occur primordially. 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," 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.
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