radiogenic: Definition from Answers.com

A radiogenic nuclide is one 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.

Examples

Lead is perhaps the best example of a radiogenic substance, as it is produced from the radioactive decay of uranium and thorium. Specifically, Pb-206 is formed from U-238, Pb-207 from U-235, and Pb-208 from Th-232. Other substances considered radiogenic are argon-40, formed from radioactive potassium, and nitrogen-14, which is formed by the decay of carbon-14. U-238, U-235, and Th-232 themselves are likely to be radiogenic as well, being formed from the decay of those nuclei of the elements heavier than uranium which do not undergo spontaneous fission, just after they were formed in stellar supernovae. Other important examples of radiogenic elements are radon and helium, both of which form during the decay of heavier elements in bedrock. The global supply of helium is radiogenic.

Radiogenic isotopes 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.^[1] Extinct nuclides are not presently included. **indicates ultimate decay product of a series.

Parent nuclide	Product nuclide	Decay constant (yr^-1)	Half-life
¹⁹⁰Pt	¹⁸⁶Os	1.477 ×10^-12	469.3 Byr
¹⁴⁷Sm	¹⁴³Nd	6.54 ×10^-12	106 Byr
⁸⁷Rb	⁸⁷Sr	1.402 ×10^-11	49.44 Byr
¹⁸⁷Re	¹⁸⁷Os	1.666 ×10^-11	41.6 Byr
¹⁷⁶Lu	¹⁷⁶Hf	1.867 ×10^-11	37.1 Byr
²³²Th	²⁰⁸Pb**	4.9475 ×10^-11	14.01 Byr
⁴⁰K	⁴⁰Ar	5.81 ×10^-11	11.93 Byr
²³⁸U	²⁰⁶Pb**	1.55125 ×10^-10	4.468 Byr
⁴⁰K	⁴⁰Ca	4.962 ×10^-10	1.397 Byr
²³⁵U	²⁰⁷Pb**	9.8485 ×10^-10	0.7038 Byr
¹²⁹I	¹²⁹Xe	4.3 ×10^-8	16 Myr
¹⁰Be	¹⁰B	4.6 ×10^-7	1.5 Myr
²⁶Al	²⁶Mg	9.9 ×10^-7	0.7 Myr
³⁶Cl	³⁶Ar/S	2.24 ×10^-6	310 kyr
²³⁴U	²³⁰Th	2.826 ×10^-6	245.25 kyr
²³⁰Th	²²⁶Ra	9.1577 ×10^-6	75.69 kyr
²³¹Pa	²²⁷Ac	2.116 ×10^-5	32.76 kyr
¹⁴C	¹⁴N	1.2097 ×10^-4	5730 yr
²²⁶Ra	²²²Rn	4.33 ×10^-4	1600 yr

References

^ Dickin, A.P. (2005). Radiogenic Isotope Geology. Cambridge University Press. http://www.onafarawayday.com/Radiogenic/.

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