Primordial nuclide

In geochemistry, geophysics and nuclear physics, primordial nuclides, or 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, or from other processes throughout the history of the universe. They are the stable nuclides plus the fraction of the long-lived radionuclides surviving from the primordial solar nebula through planet accretion until the present; 286 such nuclides are known.

All of the known 251 stable nuclides, plus another 35 nuclides that have half-lives long enough to have survived from the formation of the Earth, occur as primordial nuclides. These 35 primordial radionuclides represent isotopes of 28 separate elements. Cadmium, tellurium, xenon, neodymium, samarium, osmium, and uranium each have two primordial radioisotopes (¹¹³
Cd, ¹¹⁶
Cd; ¹²⁸
Te, ¹³⁰
Te; ¹²⁴
Xe, ¹³⁶
Xe; ¹⁴⁴
Nd, ¹⁵⁰
Nd; ¹⁴⁷
Sm, ¹⁴⁸
Sm; ¹⁸⁴
Os, ¹⁸⁶
Os; and ²³⁵
U, ²³⁸
U).

Because the age of the Earth is 4.58×10⁹ years (4.58 billion years), the half-life of the given nuclides must be greater than about 10⁸ years (100 million years) for practical detectability. 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 seven shortest-lived primordial nuclides (i.e., the nuclides with the shortest half-lives) to be detected as primordial are ⁸⁷
Rb (4.92×10¹⁰ years), ¹⁸⁷
Re (4.12×10¹⁰ years), ¹⁷⁶
Lu (3.70×10¹⁰ years), ²³²
Th (1.40×10¹⁰ years), ²³⁸
U (4.46×10⁹ years), ⁴⁰
K (1.25×10⁹ years), and ²³⁵
U (7.04×10⁸ years).

These are the seven nuclides with half-lives comparable to, or somewhat less than, the estimated age of the universe. (⁸⁷Rb, ¹⁸⁷Re, ¹⁷⁶Lu, and ²³²Th have half-lives somewhat longer than the age of the universe.) For a complete list of the 35 known primordial radionuclides, including the next 28 with half-lives much longer than the age of the universe, see the complete list below. For practical purposes, nuclides with half-lives much longer than the age of the universe may be treated as if they were stable. ⁸⁷Rb, ¹⁸⁷Re, ¹⁷⁶Lu, 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 remain present in significant amount on Earth.

The longest-lived isotope not proven to be primordial is ¹⁴⁶
Sm, which has a half-life of 9.20×10⁷ years, followed by ²⁴⁴
Pu (8.13×10⁷ years) and ⁹²
Nb (3.47×10⁷ years). ²⁴⁴Pu was reported to exist in nature as a primordial nuclide in 1971, but this detection could not be confirmed by further studies in 2012 and 2022.

Taking into account that all these nuclides must exist for at least 4.58×10⁹ years, ¹⁴⁶Sm must survive 50 half-lives (and hence be reduced by 2⁵⁰ ≈ 1×10¹⁵), ²⁴⁴Pu must survive 57 (and be reduced by a factor of 2⁵⁷ ≈ 1×10¹⁷), and ⁹²Nb must survive 130 (and be reduced by 2¹³⁰ ≈ 1×10³⁹). Mathematically, considering the likely initial abundances of these nuclides, primordial ¹⁴⁶Sm and ²⁴⁴Pu should persist somewhere within the Earth today, even if they are not identifiable in the relatively minor portion of the Earth's crust available to human assays, while ⁹²Nb and all shorter-lived nuclides should not. 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. As for ²⁴⁴Pu, calculations suggest that as of 2022, sensitivity limits were about one order of magnitude away from detecting it as a primordial nuclide.

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).