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A radiogenic nuclide is a
nuclide A nuclide (or nucleide, from nucleus, also known as nuclear species) is a class of atoms characterized by their number of protons, ''Z'', their number of neutrons, ''N'', and their nuclear energy state. The word ''nuclide'' was coined by Truman ...
that is produced by a process of
radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...
. It may itself be radioactive (a
radionuclide A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferr ...
) or stable (a
stable nuclide Stable nuclides are nuclides that are not radioactive and so (unlike radionuclides) do not spontaneously undergo radioactive decay. When such nuclides are referred to in relation to specific elements, they are usually termed stable isotopes. The ...
). 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 Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares ...
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 An isotopic signature (also isotopic fingerprint) is a ratio of non-radiogenic 'stable isotopes', stable radiogenic isotopes, or unstable radioactive isotopes of particular elements in an investigated material. The ratios of isotopes in a sample m ...
(e.g. 206Pb/204Pb). This technique is discussed in more detail under the heading
isotope geochemistry Isotope geochemistry is an aspect of geology based upon the study of natural variations in the relative abundances of isotopes of various elements. Variations in isotopic abundance are measured by isotope ratio mass spectrometry, and can reveal i ...
.


Examples

Some naturally occurring isotopes are entirely radiogenic, but all these are isotopes that are radioactive, with half-lives too short to occur primordially. 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 A nucleogenic isotope, or nuclide, is one that is produced by a natural terrestrial nuclear reaction, other than a reaction beginning with cosmic rays (the latter nuclides by convention are called by the different term cosmogenic). The nuclear reac ...
processes (natural nuclear reactions of other types, such as neutron absorption). For radiogenic isotopes that decay slowly enough, or that are
stable isotope The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundanc ...
s, 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 Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, l ...
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 Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weakly ...
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," or the time estimate from when the rock solidified and the mineral held the ratio of isotopes fixed and in place. Another notable radiogenic nuclide is
argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
-40, formed from radioactive
potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosp ...
. Almost all the argon in the earth's atmosphere is radiogenic, whereas primordial argon is argon-36. Some
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at sevent ...
-14 is radiogenic, coming from the decay of
carbon-14 Carbon-14, C-14, or radiocarbon, is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and col ...
(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
radon Radon is a chemical element with the symbol Rn and atomic number 86. It is a radioactive, colourless, odourless, tasteless noble gas. It occurs naturally in minute quantities as an intermediate step in the normal radioactive decay chains through ...
and
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. It ...
, 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 occur primordially. Helium, however, occurs in the crust of the Earth primordially, since both
helium-3 Helium-3 (3He see also helion) is a light, stable isotope of helium with two protons and one neutron (the most common isotope, helium-4, having two protons and two neutrons in contrast). Other than protium (ordinary hydrogen), helium-3 is the ...
and
helium-4 Helium-4 () is a stable isotope of the element helium. It is by far the more abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on Earth. Its nucleus is identical to an alpha particle, and consis ...
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 of
Tritium Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of ...
which is a product of some nuclear reactions, including
ternary fission Ternary fission is a comparatively rare (0.2 to 0.4% of events) type of nuclear fission in which three charged products are produced rather than two. As in other nuclear fission processes, other uncharged particles such as multiple neutrons and g ...
. The global supply of helium (which occurs in gas wells as well as the atmosphere) is almost entirely (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 some moon 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 exhibits a half life too short to survive 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
supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
s, but are known as
extinct radionuclide An extinct radionuclide is a radionuclide that was formed by nucleosynthesis before the formation of the Solar System, about 4.6 billion years ago, but has since decayed to virtually zero abundance and is no longer detectable as a primordial nuc ...
s, 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 Iodine-129 (129I) is a long-lived radioisotope of iodine which occurs naturally, but also is of special interest in the monitoring and effects of man-made nuclear fission products, where it serves as both tracer and potential radiological contami ...
(half life 15.7 million years) some time 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 Aluminium-26 (26Al, Al-26) is a radioactive isotope of the chemical element aluminium, decaying by either positron emission or electron capture to stable magnesium-26. The half-life of 26Al is 7.17 (717,000) years. This is far too short for the i ...
(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 Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...
of the radioactive parent isotope. The values given for half-life and decay constant are the current consensus values in the Isotope Geology community. ** indicates ultimate decay product of a series. * In this table Gyr = gigayear = 109 year, Myr = megayear = 106 year, kyr = kiloyear = 103 year


Radiogenic heating

Radiogenic heating occurs as a result of the release of heat energy from
radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...
during the production of radiogenic nuclides. Along with heat from the Primordial Heat (resulting from planetary accretion), radiogenic heating occurring in the mantle and crust make up the two main sources of heat in the Earth's interior. Most of the radiogenic heating in the Earth results from the decay of the daughter nuclei in the
decay chain In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". Most radioisotopes do not decay directl ...
s of
uranium-238 Uranium-238 (238U or U-238) is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However, it ...
and
thorium-232 Thorium-232 () is the main naturally occurring isotope of thorium, with a relative abundance of 99.98%. It has a half life of 14 billion years, which makes it the longest-lived isotope of thorium. It decays by alpha decay to radium-228; its decay ...
, and
potassium-40 Potassium-40 (40K) is a radioactive isotope of potassium which has a long half-life of 1.25 billion years. It makes up about 0.012% (120 ppm) of the total amount of potassium found in nature. Potassium-40 undergoes three types of radioactive de ...
.


See also

*
Geoneutrino A geoneutrino is a neutrino or antineutrino emitted in decay of radionuclide naturally occurring in the Earth. Neutrinos, the lightest of the known subatomic particles, lack measurable electromagnetic properties and interact only via the weak nucl ...
*
Radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares ...
*
Stable nuclide Stable nuclides are nuclides that are not radioactive and so (unlike radionuclides) do not spontaneously undergo radioactive decay. When such nuclides are referred to in relation to specific elements, they are usually termed stable isotopes. The ...


References

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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 Radioactivity