The two naturally occurring isotopes of helium are 3He and 4He. 3He is much less abundant than 4He; the atmospheric 3He/4He ratio (R a) is 1.39 × 10−6. Nearly all the terrestrial 4He has been produced as alpha-particles from the radioactive decay of 238U, 235U and 232Th over geologic time. By far the most important terrestrial source of 3He is degassing of primor dial volatiles from the Earth's interior. Other sources include auroral precipitation of solar wind, accretion from cosmic rays, and flux of cosmic dust and meteorities. Small amounts of 3He are also produced during radioactive decay of U and Th as a result of neutron interactions with Li according to the reaction 6Li(n,α)3H→3He. The neutrons are produced by alpha-particle interactions on target elements such as Mg, Si and O in the host rock. This results in a low 3He/4He production ratio (typically < 0.02R a), even in crustal rocks that are relatively enriched in Li. Consequently, the terrestrial 3He/4He ratio varies by...
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Cross-references
Cosmic elemental abundances; Cosmogenic nuclides; Earth's atmosphere; Earth's mantle geochemistry; Earth's ocean geochemistry; Elements: noble gases; Geochemical tectonics; Geothermal systems; Helium; Hydrogen isotopes; Isotope fractionation; Mid ocean ridge basalts (MORB); Natural gas; Stable isotopes; Uranium–lead thorium–lead decay system; Volcanic gases; Volcanism
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Graham, D.W. (1998). Helium isotopes . In: Geochemistry. Encyclopedia of Earth Science. Springer, Dordrecht. https://doi.org/10.1007/1-4020-4496-8_154
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