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In geochemistry, geophysics and geonuclear physics, primordial nuclides, also known as primordial isotopes, are nuclides found on Earth that have existed in their current form since before Earth was formed. Primordial nuclides were present in the interstellar medium from which the solar system was formed, and were formed in the Big Bang, by nucleosynthesis in stars and supernovae followed by mass ejection, by cosmic ray spallation, and potentially from other processes. They are the stable nuclides plus the long-lived fraction of radionuclides surviving in the primordial solar nebula through planet accretion until the present. Only 286 such nuclides are known.
All of the known 253 stable nuclides occur as primordial nuclides, plus another 33 nuclides that have half-lives long enough to have survived from the formation of the Earth. These 33 primordial radionuclides represent isotopes of 28 separate elements.
Cadmium, tellurium, neodymium, samarium and uranium each have two primordial radioisotopes (113
; and 235
Because the age of the Earth is (4.6 billion years), this means that the half-life of the given nuclides must be greater than about (100 million years) for practical considerations. For example, for a nuclide with half-life (60 million years), this means 77 half-lives have elapsed, meaning that for each mole of that nuclide being present at the formation of Earth, only 4 atoms remain today.
The four shortest-lived primordial nuclides (i.e. nuclides with shortest half-lives) are 232
, and 235
. These are the 4 nuclides with half-lives comparable to, or less than, the estimated age of the universe. (In the case of 232Th, it has a half life of more than 14 billion years, slightly longer than the age of the universe.) For a complete list of the 33 known primordial radionuclides, including the next 29 with half-lives much longer than the age of the universe, see the complete list in the section below. For practical purposes, nuclides with half-lives much longer than the age of the universe may be treated as if they really were stable. 232Th and 238U have half-lives long enough that their decay is limited over geological time scales; 40K and 235U have shorter half-lives and are hence severely depleted, but are still long-lived enough to persist significantly in nature.
The next longest-living nuclide after the end of the list given in the table is 244
, with a half-life of . It has been reported to exist in nature as a primordial nuclide, although later studies could not detect it. Likewise, the second-longest-lived non-primordial 146
has a half-life of , about double that of the third-longest-lived non-primordial 92
. Taking into account that all these nuclides must exist since at least , 244Pu must survive 57 half-lives (and hence be reduced by a factor of 257 ? 1.4 × 1017), 146Sm must survive 67 (and be reduced by 267 ? 1.5 × 1020), and 92Nb must survive 130 (and be reduced by 2130 ? 1.4 × 1039). Considering the likely initial abundances of these nuclides, possibly measurable quantities of 244Pu and 146Sm should persist today, while they should not for 92Nb and all shorter-lived nuclides. Nuclides such as 92Nb that were present in the primordial solar nebula but have long since decayed away completely are termed extinct radionuclides if they have no other means of being regenerated.
Although it is estimated that about 33 primordial nuclides are radioactive (list below), it becomes very difficult to determine the exact total number of radioactive primordials, because the total number of stable nuclides is uncertain. There exist many extremely long-lived nuclides whose half-lives are still unknown. For example, it is predicted theoretically that all isotopes of tungsten, including those indicated by even the most modern empirical methods to be stable, must be radioactive and can decay by alpha emission, but as of 2013 this could only be measured experimentally for 180
. Similarly, all four primordial isotopes of lead are expected to decay to mercury, but the predicted half-lives are so long (some exceeding 10100 years) that this can hardly be observed in the near future. Nevertheless, the number of nuclides with half-lives so long that they cannot be measured with present instruments--and are considered from this viewpoint to be stable nuclides--is limited. Even when a "stable" nuclide is found to be radioactive, the fact merely moves it from the stable to the unstable list of primordial nuclides, and the total number of primordial nuclides remains unchanged.
Because primordial chemical elements often consist of more than one primordial isotope, there are only 83 distinct primordial chemical elements. Of these, 80 have at least one observationally stable isotope and three additional primordial elements have only radioactive isotopes (bismuth, thorium, and uranium).
Some unstable isotopes which occur naturally (such as 14
, and 239
) are not primordial, as they must be constantly regenerated. This occurs by cosmic radiation (in the case of cosmogenic nuclides such as 14
), or (rarely) by such processes as geonuclear transmutation (neutron capture of uranium in the case of 237
). Other examples of common naturally occurring but non-primordial nuclides are isotopes of radon, polonium, and radium, which are all radiogenic nuclide daughters of uranium decay and are found in uranium ores. A similar radiogenic series is derived from the long-lived radioactive primordial nuclide 232Th. All of such nuclides have shorter half-lives than their parent radioactive primordial nuclides. Some other geogenic nuclides do not occur in the decay chains of 232Th, 235U, or 238U but can still fleetingly occur naturally as products of the spontaneous fission of one of these three long-lived nuclides, such as 126Sn, which makes up about 10-14 of all natural tin.
There are 253 stable primordial nuclides and 33 radioactive primordial nuclides, but only 80 primordial stable elements (1 through 82, i.e. hydrogen through lead, exclusive of 43 and 61, technetium and promethium respectively) and three radioactive primordial elements (bismuth, thorium, and uranium). Bismuth's half-life is so long that it is often classed with the 80 primordial stable elements instead, since its radioactivity is not a cause for serious concern. The numbers of elements are smaller, because many primordial elements are represented by more than one primordial nuclide. See chemical element for more information.
As noted, these number about 253. For a list, see the article list of elements by stability of isotopes. For a complete list noting which of the "stable" 253 nuclides may be in some respect unstable, see list of nuclides and stable nuclide. These questions do not impact the question of whether a nuclide is primordial, since all "nearly stable" nuclides, with half-lives longer than the age of the universe, are primordial also.
These 33 primordial nuclides represent radioisotopes of 28 distinct chemical elements (cadmium, neodymium, samarium, tellurium, and uranium each have two primordial radioisotopes). The radionuclides are listed in order of stability, with the longest half-life beginning the list. These radionuclides in many cases are so nearly stable that they compete for abundance with stable isotopes of their respective elements. For three chemical elements, a very long lived radioactive primordial nuclide is found to be the most abundant nuclide for an element that also has a stable nuclide. These unusual elements are tellurium, indium, and rhenium.
The longest has a half-life of , which is 160 trillion times the age of the Universe. Only four of these 33 nuclides have half-lives shorter than, or equal to, the age of the universe. Most of the remaining 29 have half-lives much longer. The shortest-lived primordial isotope, 235U, has a half-life of 704 million years, about one sixth of the age of the Earth and Solar System.
age of universe
|254||128Te||8.743261||2 ?-||2.530||160 trillion|
|256||136Xe||8.706805||2 ?-||2.462||150 billion|
|257||76Ge||9.034656||2 ?-||2.039||130 billion|
|259||82Se||9.017596||2 ?-||2.995||8 billion|
|260||116Cd||8.836146||2 ?-||2.809||2 billion|
|261||48Ca||8.992452||2 ?-||4.274, .0058||2 billion|
|262||96Zr||8.961359||2 ?-||3.4||1 billion|
|264||130Te||8.766578||2 ?-||.868||600 million|
|265||150Nd||8.562594||2 ?-||3.367||600 million|
|266||100Mo||8.933167||2 ?-||3.035||600 million|
|269||50V||9.055759||?+ or ?-||2.205, 1.038||10 million|
|279||138La||8.698320||K or ?-||1.737, 1.044||7|
|283||232Th||7.918533||? or SF||4.083||1|
|284||238U||7.872551||? or SF or 2 ?-||4.270||0.3|
|285||40K||8.909707||?- or K or ?+||1.311, 1.505, 1.505||0.09|
|286||235U||7.897198||? or SF||4.679||0.05|