Primordial element | Wikipedia audio article
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Primordial element
00:05:54 1 Naturally occurring nuclides that are not primordial
00:07:19 2 Primordial elements
00:08:10 3 Naturally occurring stable nuclides
00:08:51 4 List of 33 radioactive primordial nuclides and measured half-lives
00:10:13 4.1 List legends
00:11:27 5 See also
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SUMMARY
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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 (113Cd, 116Cd; 128Te, 130Te; 144Nd, 150Nd; 147Sm, 148Sm; and 235U, 238U).
Because the age of the Earth is 4.58×109 years (4.6 billion years), this means that the half-life of the given nuclides must be greater than about 1×108 years (100 million years) for practical considerations. For example, for a nuclide with half-life 6×107 years (60 million years), this means 77 half-lives have elapsed, meaning that for each mole (6.02×1023 atoms) 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 232Th, 238U, 40K, and 235U.
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 244Pu, with a half-life of 8.08×107 years. 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 146Sm has a half-life of 6.8×107 years, about double that of the third-longest-lived non-primordial 92Nb (3.5×107 years). Taking into account that all these nuclides must exist since at least 4.6×109 years, 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 di ...
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