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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.
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