Radiogenic nuclide
Encyclopedia
A radiogenic nuclide is a nuclide
that is produced by a process of radioactive decay
. It may itself be radioactive, or stable.
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
.
processes (natural nuclear reactions of other types, such as neutron absorption).
For radiogenic isotopes that decay slowly enough, or that are stable istope
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
is perhaps the best example of a partly radiogenic substance, as all four of its stable isotopes (Pb-204, Pb-206, Pb-207, and Pb-208) are present primordially, in known and fixed ratios. However, Pb-204 is only present primordially, while the other three isotopes may also occur as radiogenic decay products of uranium
and thorium
. Specifically, Pb-206 is formed from U-238, Pb-207 from U-235, and Pb-208 from Th-232. 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.
Other notable nuclides that are partly radiogenic are argon
-40, formed from radioactive potassium
, and nitrogen
-14, which is formed by the decay of carbon-14
.
Other important examples of radiogenic elements are radon
and helium
, 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 and helium-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). The global supply of helium (which occurs in gas wells and 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 supernovas, but are known as extinct radionuclide
s, since they are not seen directly on the Earth today.
An example of an extinct radionuclide is xenon-129, a stable isotope of xenon which appears as a relative excess against other xenon isotopes, in meteorites that condensed from the primordial solar system dust cloud and trapped promordial iodine-129
(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 (also inferred from extra magnesium-26 found in meteorites), and iron-60.
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>
|186Os
|1.477 ×10−12
|469.3 Gyr *>
|143Nd
|6.54 ×10−12
|106 Gyr>
|87Sr
|1.402 ×10−11
|49.44 Gyr>
|187Os
|1.666 ×10−11
|41.6 Gyr>
|176Hf
|1.867 ×10−11
|37.1 Gyr>
|208Pb**
|4.9475 ×10−11
|14.01 Gyr>
|40Ar
|5.81 ×10−11
|11.93 Gyr>
|206Pb**
|1.55125 ×10−10
|4.468 Gyr>
|40Ca
|4.962 ×10−10
|1.397 Gyr>
|207Pb**
|9.8485 ×10−10
|0.7038 Gyr>
|129Xe
|4.3 ×10−8
|16 Myr>
|10B
|4.6 ×10−7
|1.5 Myr>
|26Mg
|9.9 ×10−7
|0.7 Myr>
|36Ar/S
|2.24 ×10−6
|310 kyr>
|230Th
|2.826 ×10−6
|245.25 kyr>
|226Ra
|9.1577 ×10−6
|75.69 kyr>
|227Ac
|2.116 ×10−5
|32.76 kyr>
|14N
|1.2097 ×10−4
|5730 yr>
|222Rn
|4.33 ×10−4
|1600 yr>
Nuclide
A nuclide is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its nuclear energy state....
that is produced by a process of radioactive decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
. It may itself be radioactive, or stable.
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
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
tool (e.g. uranium-lead
Uranium-lead dating
Uranium-lead is one of the oldest and most refined of the radiometric dating schemes, with a routine age range of about 1 million years to over 4.5 billion years, and with routine precisions in the 0.1-1 percent range...
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
Isotope geochemistry
Isotope geochemistry is an aspect of geology based upon study of the relative and absolute concentrations of the elements and their isotopes in the Earth. Variations in the abundance of these isotopes, typically measured with an isotope ratio mass spectrometer or an accelerator mass spectrometer,...
.
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 nucleogenicNucleogenic
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 nuclear reaction that produces nucleogenic nuclides is usually interaction with an alpha particle or the capture of fission or thermal neutron...
processes (natural nuclear reactions of other types, such as neutron absorption).
For radiogenic isotopes that decay slowly enough, or that are stable istope
Stable
A stable is a building in which livestock, especially horses, are kept. It most commonly means a building that is divided into separate stalls for individual animals...
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
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
is perhaps the best example of a partly radiogenic substance, as all four of its stable isotopes (Pb-204, Pb-206, Pb-207, and Pb-208) are present primordially, in known and fixed ratios. However, Pb-204 is only present primordially, while the other three isotopes may also occur as radiogenic decay products of uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
and thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
. Specifically, Pb-206 is formed from U-238, Pb-207 from U-235, and Pb-208 from Th-232. 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.
Other notable nuclides that are partly radiogenic are argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
-40, formed from radioactive potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
, and nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
-14, which is formed by the decay of carbon-14
Carbon-14
Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon with a 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 colleagues , to date archaeological, geological, and hydrogeological...
.
Other important examples of radiogenic elements are radon
Radon
Radon is a chemical element with symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tasteless noble gas, occurring naturally as the decay product of uranium or thorium. Its most stable isotope, 222Rn, has a half-life of 3.8 days...
and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
, 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 and helium-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). The global supply of helium (which occurs in gas wells and 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 supernovas, but are known as extinct radionuclide
Extinct radionuclide
An extinct radionuclide is one that scientists believe was formed by primordial processes, such as stellar nucleogenesis in the supernova that contributed radioisotopes to the early solar system, about 4.6 billion years ago...
s, since they are not seen directly on the Earth today.
An example of an extinct radionuclide is xenon-129, a stable isotope of xenon which appears as a relative excess against other xenon isotopes, in meteorites that condensed from the primordial solar system dust cloud and trapped promordial iodine-129
Iodine-129
Iodine-129 is long-lived radioisotope of iodine which occurs naturally, but also is of special interest in the monitoring and effects of man-made nuclear fission decay products, where it serves as both tracer and potential radiological contaminant....
(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 (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-lifeHalf-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
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.
!Product nuclide
!Decay constant (yr−1)
!Half-life>
|186Os
|1.477 ×10−12
|469.3 Gyr *>
|143Nd
|6.54 ×10−12
|106 Gyr>
|87Sr
|1.402 ×10−11
|49.44 Gyr>
|187Os
|1.666 ×10−11
|41.6 Gyr>
|176Hf
|1.867 ×10−11
|37.1 Gyr>
|208Pb**
|4.9475 ×10−11
|14.01 Gyr>
|40Ar
|5.81 ×10−11
|11.93 Gyr>
|206Pb**
|1.55125 ×10−10
|4.468 Gyr>
|40Ca
|4.962 ×10−10
|1.397 Gyr>
|207Pb**
|9.8485 ×10−10
|0.7038 Gyr>
|129Xe
|4.3 ×10−8
|16 Myr>
|10B
|4.6 ×10−7
|1.5 Myr>
|26Mg
|9.9 ×10−7
|0.7 Myr>
|36Ar/S
|2.24 ×10−6
|310 kyr>
|230Th
|2.826 ×10−6
|245.25 kyr>
|226Ra
|9.1577 ×10−6
|75.69 kyr>
|227Ac
|2.116 ×10−5
|32.76 kyr>
|14N
|1.2097 ×10−4
|5730 yr>
|222Rn
|4.33 ×10−4
|1600 yr>
- In this table Gyr = gigayear = 109 year, Myr = megayear = 106 year, kyr = kiloyear = 103 year
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