A
radiogenic nuclideA nuclide is an atomic nucleus characterized by its specific constitution, i.e., by its number of protons, its number of neutrons, and its energy content...
is one that is produced by a process of
radioactive decayRadioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, named the daughter...
.
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 datingRadiometric dating is a technique used to date materials, 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.
A
radiogenic nuclideA nuclide is an atomic nucleus characterized by its specific constitution, i.e., by its number of protons, its number of neutrons, and its energy content...
is one that is produced by a process of
radioactive decayRadioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, named the daughter...
.
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 datingRadiometric dating is a technique used to date materials, 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-leadUranium-lead is one of the oldest and most refined 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 geochemistryIsotope 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
LeadLead is a main-group element with symbol Pb and atomic number 82. Lead is a soft, malleable poor metal, also considered to be one of the heavy metals. Lead has a bluish-white color when freshly cut, but tarnishes to a dull grayish color when exposed to air...
is perhaps the best example of a radiogenic substance, as it is produced from the radioactive decay of
uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. Besides its 92 protons, a uranium nucleus can have between 141 and 146 neutrons. The most common uranium isotopes are U-238 and U-235 . A uranium atom has...
and
thoriumThorium is a chemical element with the symbol Th and atomic number 90. It is a naturally occurring, slightly radioactive metal. Thorium is estimated to be about three to four times more abundant than uranium in the earth's crust...
. Specifically, Pb-206 is formed from U-238, Pb-207 from U-235, and Pb-208 from Th-232. Other substances considered radiogenic are
argonArgon is a chemical element designated by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is present in the Earth's atmosphere at 0.94%. Terrestrially, it is the most abundant and most frequently used of the noble gases...
-40, formed from radioactive
potassiumPotassium is the chemical element with the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash...
, and
nitrogenNitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.Many industrially important...
-14, which is formed by the decay of
carbon-14Carbon-14,
14C, or radiocarbon, is a radioactive isotope of carbon discovered on February 27, 1940, by Martin Kamen and Sam Ruben at the University of California Radiation Laboratory in Berkeley, though its existence had been suggested already in 1934 by Franz Kurie. Its nucleus contains...
. 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 fissionSpontaneous fission is a form of radioactive decay characteristic of very heavy isotopes. It is theoretically possible for any atomic nucleus whose mass is greater than or equal to 100 atomic mass units , i.e. elements near ruthenium. In practice, however, spontaneous fission is only energetically...
, just after they were formed in stellar supernovae.
Other important examples of radiogenic elements are
radonRadon 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 radium. It is one of the heaviest substances that remains a gas under normal conditions and is considered to be a health...
and
heliumHelium is the chemical element with atomic number 2, and 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. 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-lifeHalf-life is the period of time, for a substance undergoing decay, to decrease by half. The name originally was used to describe a characteristic of unstable atoms , but may apply to any quantity which follows a set-rate decay....
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>
|
|
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>