Radiogenic nuclide
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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:
- 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).
- In comparison with the quantity of a non-radiogenic isotope of the same element, the quantity of the radiogenic isotope is used to define its isotopic signature (e.g. 206Pb/204Pb). This technique is discussed in more detail under the heading isotope geochemistry.
Examples
Some naturally occurring isotopes are entirely radiogenic, but all those are radioactive isotopes, with half-lives too short to have occurred primordially and still exist today. 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
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", thus providing a time estimate for when the rock solidified and the mineral held the ratio of isotopes fixed and in place.
Another notable radiogenic nuclide is argon-40, formed from radioactive potassium. Almost all the argon in the Earth's atmosphere is radiogenic, whereas primordial argon is argon-36.
Some nitrogen-14 is radiogenic, coming from the decay of carbon-14 (half-life around 5700 years), but the carbon-14 was formed some time earlier from nitrogen-14 by the action of cosmic rays.
Other important examples of radiogenic elements are
As noted in the case of lead-204, a radiogenic nuclide is often not radioactive. In this case, if its precursor nuclide has a half-life too short to have survived from primordial times, then the parent nuclide will be gone, and known now entirely by a relative excess of its stable daughter. In practice, this occurs for all radionuclides with half lives less than about 50 to 100 million years. Such nuclides are formed in supernovas, but are known as extinct radionuclides, since they are not seen directly on the Earth today.
An example of an extinct radionuclide is iodine-129; it decays to xenon-129, a stable isotope of xenon which appears in excess relative to other xenon isotopes. It is found in meteorites that condensed from the primordial Solar System dust cloud and trapped primordial iodine-129 (half life 15.7 million years) sometime in a relative short period (probably less than 20 million years) between the iodine-129's creation in a supernova, and the formation of the Solar System by condensation of this dust. The trapped iodine-129 now appears as a relative excess of xenon-129. Iodine-129 was the first extinct radionuclide to be inferred, in 1960. Others are aluminium-26 (also inferred from extra magnesium-26 found in meteorites), and iron-60.
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.[1]
** indicates ultimate decay product of a series.
Units used in this table
Gyr = gigayear = 109 years
Myr = megayear = 106 years
kyr = kiloyear = 103 years
| Parent nuclide | Daughter nuclide | Decay constant (yr−1) | Half-life |
|---|---|---|---|
| 190Pt | 186Os | 1.477 ×10−12 | 483 Gyr[2] |
| 147Sm | 143Nd | 6.54 ×10−12 | 106 Gyr |
| 87Rb | 87Sr | 1.402 ×10−11 | 49.44 Gyr |
| 187Re | 187Os | 1.666 ×10−11 | 41.6 Gyr |
| 176Lu | 176Hf | 1.867 ×10−11 | 37.1 Gyr |
| 232Th | 208Pb** | 4.9475 ×10−11 | 14.01 Gyr |
| 40K | 40Ar | 5.81 ×10−11 | 11.93 Gyr[3] |
| 238U | 206Pb** | 1.55125 ×10−10 | 4.468 Gyr |
| 40K | 40Ca | 4.962 ×10−10 | 1.397 Gyr |
| 235U | 207Pb** | 9.8485 ×10−10 | 0.7038 Gyr |
| 129I | 129Xe | 4.3 ×10−8 | 16 Myr |
| 10Be | 10B | 4.6 ×10−7 | 1.5 Myr |
| 26Al | 26Mg | 9.9 ×10−7 | 0.70 Myr |
| 36Cl | 36Ar (98%) 36S (2%) |
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 |
Radiogenic heating
Radiogenic heating occurs as a result of the release of heat energy from
See also
References
- ^ Dickin, A.P. (2018). Radiogenic Isotope Geology. Cambridge University Press.
- .
- ^ Note: this not the half-life of 40K, but rather the half-life that would correspond to the decay constant for decay to 40Ar. About 89% of the 40K decays to 40Ca.
- ^ Allaby, Alisa; Michael Allaby (1999). "radiogenic heating". A Dictionary of Earth Sciences. Retrieved 24 November 2013.
- ^ Mutter, John C. "The Earth as a Heat Engine". Introduction to Earth Sciences I. Columbia University. p. 3.2 Mantle convection. Retrieved 23 November 2013.
- ^ Dumé, Belle (27 July 2005). "Geoneutrinos make their debut; Radiogenic heat in the Earth". Physics World. Institute of Physics. Retrieved 23 November 2013.
External links
- National Isotope Development Center Government supply of radionuclides; information on isotopes; coordination and management of isotope production, availability, and distribution
- Isotope Development & Production for Research and Applications (IDPRA) U.S. Department of Energy program for isotope production and production research and development