Radiogenic

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. 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