A radiogenic nuclide is a nuclide that is produced by a process of radioactive decay.
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:
1) In comparison with the quantity of the radioactive 'parent isotope' in a system, the quantity of the radiogenic 'daughter product' is used as a radiometric dating tool (e.g. uranium-lead geochronology).
2) In comparison with the quantity of a non-radiogenic isotope of the same element, the quantity of the radiogenic isotope is used as an isotopic tracer (e.g. 206Pb/204Pb). This technique is discussed in more detail under the heading isotope geochemistry.
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 cosmogenic (cosmic ray induced) processes that produced them in nature freshly.
For radiogenic isotopes that decay slowly enough, a primordial fraction is always present, since all sufficiently long-lived 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 (Pb-204, Pb-206, Pb-207, and Pb-208) are present primordially, in known and fixed ratios. However, Pb-204 is only present primordially, while the other three isotopes may also occur as radiogenic decay products of uranium and thorium. Specifically, Pb-206 is formed from U-238, Pb-207 from U-235, and Pb-208 from Th-232. In rocks that contain uranium and thorium, the excess amounts of the three heavier lead isotopes allows the rocks to be dated.
Other substances considered 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. 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 entirely primordial. However, the global supply of helium (which occurs in gas wells and well as the atmosphere) is almost entirely (about 99%) radiogenic, as shown by its factor of 100 enrichment in radiogenic helium-4 relative to the primordial ratio of helium-4 to helium-3. The latter ratio is known from extraterrestrial sources.
Radiogenic nuclides used in geology
The following table lists some of the most important radiogenic isotope systems used in Geology, in order of decreasing half-life of the radioactive parent isotope. The values given for half-life and decay constant are the current consensus values in the Isotope Geology community. Extinct nuclides are not presently included. **indicates ultimate decay product of a series.
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