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In geochemistry, Primordial elements are chemical elements found on the earth that have existed in their current form since before the earth was formed, according to accepted stellar evolution theory. This means that the half-life on the given atoms must be greater than about 108 years.
All stable elements are primordial, as are some radioactive elements including thorium (due to isotope 232), and uranium. It has also been shown that plutonium is primordial, although just barely as its most stable isotope (plutonium-244) concentration in ores is nearly undetectable.
Some unstable isotopes which occur naturally (such as carbon-14, tritium, and plutonium-239) are not primordial, as they must be constantly regenerated by stellar radiation (in the case of carbon and tritium) or geochemical transmutation (in the case of plutonium).
Helium is unusual; most of it would have escaped Earth's gravitational pull as it was forming. Most helium is mined and comes as a result of radioactive decay in the core of the earth, so it is difficult to characterize helium as primordial.
In astrochemistry, primordial elements are chemical elements which were synthesized in the first round of nuclear synthesis after the big bang. These are believed to include only hydrogen, helium, lithium, and beryllium. To produce heavier atoms, stellar nuclear synthesis is required. For this reason it is known that the sun is not a first-generation star.