| Nuclear physics |
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High-energy processes |
Aradiogenic nuclide is anuclide that is produced by a process ofradioactive decay. It may itself be radioactive (aradionuclide) or stable (astable nuclide).
Radiogenic nuclides (more commonly referred to asradiogenic isotopes) form some of the most important tools in geology. They are used in two principal ways:
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 bynucleogenic processes (natural nuclear reactions of other types, such as neutron absorption).
For radiogenic isotopes that decay slowly enough, or that arestable isotopes, a primordial fraction is always present, since all sufficiently long-lived and stable 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, asall four of its stable isotopes (204Pb,206Pb,207Pb, and208Pb) are present primordially, in known and fixed ratios. However,204Pb isonly present primordially, while the other three isotopes may also occur as radiogenic decay products ofuranium andthorium. Specifically,206Pb is formed from238U,207Pb from235U, and208Pb from232Th. 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 isargon-40, formed from radioactivepotassium. Almost all the argon in the Earth's atmosphere is radiogenic, whereas primordial argon is argon-36.
Somenitrogen-14 is radiogenic, coming from the decay ofcarbon-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 areradon andhelium, 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 have occurred primordially. Helium, however, occurs in the crust of the Earth primordially, since bothhelium-3 andhelium-4 are stable, and small amounts were trapped in the crust of the Earth as it formed. Helium-3 is almost entirely primordial (a small amount is formed by natural nuclear reactions in the crust). Helium-3 can also be produced as the decay product oftritium (3H) which is a product of some nuclear reactions, includingternary fission. The global supply of helium (which occurs in gas wells as well as the atmosphere) is mainly (about 90%–99%) radiogenic, as shown by its factor of 10 to 100 times enrichment in radiogenic helium-4 relative to the primordial ratio of helium-4 to helium-3. This latter ratio is known from extraterrestrial sources, such as someMoon rocks and meteorites, which are relatively free of parental sources for helium-3 and helium-4.
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 insupernovas, but are known asextinct radionuclides, since they are not seen directly on the Earth today.
An example of an extinct radionuclide isiodine-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 arealuminium-26 (also inferred from extra magnesium-26 found in meteorites), and iron-60.
The following table lists some of the most important radiogenic isotope systems used in geology, in order of decreasinghalf-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 occurs as a result of the release of heat energy fromradioactive decay[4] during the production of radiogenic nuclides. Along with heat from thePrimordial Heat (resulting from planetary accretion), radiogenic heating occurring in themantle andcrust make up thetwo main sources of heat in theEarth's interior.[5] Most of the radiogenic heating in the Earth results from the decay of the daughter nuclei in thedecay chains ofuranium-238 andthorium-232, andpotassium-40.[6]
