Editing Helium-3 (section)

== Natural abundance ==

=== Terrestrial abundance ===
{{Main|Isotope geochemistry}}

<sup>3</sup>He is a primordial substance in the Earth's [[mantle (geology)|mantle]], thought to have become entrapped in the Earth during planetary formation. The ratio of <sup>3</sup>He to <sup>4</sup>He within the Earth's crust and mantle is less than that of estimates of solar disk composition as obtained from meteorite and lunar samples, with terrestrial materials generally containing lower <sup>3</sup>He/<sup>4</sup>He ratios due to production of <sup>4</sup>He from radioactive decay.

<sup>3</sup>He has a cosmological ratio of 300 atoms per million atoms of <sup>4</sup>He (at. ppm),<ref name=Witt>[[#Witt|Wittenberg 1994]]</ref> leading to the assumption that the original ratio of these primordial gases in the mantle was around 200-300 ppm when Earth was formed. Over Earth's history alpha-particle decay of uranium, thorium and other radioactive isotopes has generated significant amounts of <sup>4</sup>He, such that only around 7% of the helium now in the mantle is primordial helium,<ref name=Witt /> lowering the total <sup>3</sup>He/<sup>4</sup>He ratio to around 20 ppm. Ratios of <sup>3</sup>He/<sup>4</sup>He in excess of atmospheric are indicative of a contribution of <sup>3</sup>He from the mantle. Crustal sources are dominated by the [[helium-4|<sup>4</sup>He]] produced by radioactive decay.

The ratio of helium-3 to helium-4 in natural Earth-bound sources varies greatly.<ref name=Aldrich>Aldrich, L.T.; Nier, Alfred O. Phys. Rev. 74, 1590 – 1594 (1948). The Occurrence of He3 in Natural Sources of Helium. Page 1592, Tables I and II.</ref><ref name=Holden>Holden, Normen E. 1993. Helium Isotopic Abundance Variation in Nature. [http://www.osti.gov/bridge/servlets/purl/10183304-ds0WIi/10183304.PDF copy of paper BNL-49331] "Table II. <sup>3</sup>He Abundance of Natural Gas ... <sup>3</sup>He in ppm ... Aldrich 0.05 – 0.5 ... Sano 0.46 – 22.7", "Table V. ... of Water ... <sup>3</sup>He in ppm ... 1.6 – 1.8 East Pacific ... 0.006 – 1.5 Manitoba Chalk River ... 164 Japan Sea" (Aldrich measured Helium from US wells, Sano that of Taiwan gas: {{Cite journal| doi = 10.1038/323055a0| issn = 1476-4687| volume = 323| issue = 6083| pages = 55–57| last1 = Sano| first1 = Yuji| last2 = Wakita| first2 = Hiroshi| last3 = Huang| first3 = Chin-Wang| title = Helium flux in a continental land area estimated from <sup>3</sup>He/<sup>4</sup>He ratio in northern Taiwan| journal = Nature| date = September 1986| bibcode = 1986Natur.323...55S| s2cid = 4358031}})</ref> Samples of the [[lithium]] ore [[spodumene]] from Edison Mine, South Dakota were found to contain 12 parts of helium-3 to a million parts of helium-4. Samples from other mines showed 2 parts per million.<ref name=Aldrich/>

Helium is also present as up to 7% of some natural gas sources,<ref>[http://www.webelements.com/webelements/elements/text/He/key.html WebElements Periodic Table: Professional Edition: Helium: key information] {{Webarchive|url=https://web.archive.org/web/20080509100431/http://www.webelements.com/webelements/elements/text/He/key.html |date=2008-05-09 }}. Webelements.com. Retrieved on 2011-11-08.</ref> and large sources have over 0.5% (above 0.2% makes it viable to extract).<ref name=SmithDM>[[#Smith|Smith, D.M.]] "any concentration of helium above approximately 0.2 percent is considered worthwhile examining" ... "U.S. government still owns approximately 1 billion nm<sup>3</sup> of helium inventory", "Middle East and North Africa ... many very large, helium-rich (up to 0.5 percent) natural gas fields" (Smith uses nm<sup>3</sup> to mean "normal [[cubic metre]]", elsewhere called "cubic metre at [[normal temperature and pressure|NTP]])</ref> The fraction of <sup>3</sup>He in helium separated from natural gas in the U.S. was found to range from 70 to 242 parts per billion.<ref name=CRS/><ref name="BoM/DoI">{{cite report| first1=Thomas A. | last1=Davidson | first2=David E. | last2=Emerson| publisher=[[United States Bureau of Mines|Bureau of Mines]], [[US Department of the Interior]] | title= Method and Apparatus for Direct Determination of Helium-3 in Natural Gas and Helium | id=Report of Investigations 9302 | date=1990}}</ref> Hence the US 2002 stockpile of 1 billion normal m<sup>3</sup><ref name=SmithDM/> would have contained about {{convert|12 to 43|kg}} of helium-3.  According to American physicist [[Richard Garwin]], about {{convert|26|m3}} or almost {{convert|5|kg}} of <sup>3</sup>He is available annually for separation from the US natural gas stream.  If the process of separating out the <sup>3</sup>He could employ as feedstock the liquefied helium typically used to transport and store bulk quantities, estimates for the incremental energy cost range from {{Convert|34 to 300|$/l}} NTP, excluding the cost of infrastructure and equipment.<ref name=CRS/> Algeria's annual gas production is assumed to contain 100 million normal cubic metres<ref name=SmithDM/> and this would contain between {{convert|7 and 24| m3}} of helium-3 (about {{convert|1 to 4|kg}}) assuming a similar <sup>3</sup>He fraction.

<sup>3</sup>He is also present in the [[Earth's atmosphere]]. The natural abundance of <sup>3</sup>He in naturally occurring helium gas is 1.38{{e|-6}} (1.38 parts per million). The partial pressure of helium in the Earth's atmosphere is about {{convert|0.52|Pa}}, and thus helium accounts for 5.2 parts per million of the total pressure (101325 Pa) in the Earth's atmosphere, and <sup>3</sup>He thus accounts for 7.2 parts per trillion of the atmosphere. Since the atmosphere of the Earth has a mass of about {{convert|5.14e18|kg}},<ref>{{Cite journal|doi = 10.1175/JCLI-3299.1|title = The Mass of the Atmosphere: A Constraint on Global Analyses|year = 2005|last1 = Smith|first1 = Lesley|last2 = Trenberth|first2 = Kevin E.|journal = Journal of Climate|volume = 18|issue = 6|pages = 864–875|bibcode = 2005JCli...18..864T| s2cid=16754900 |doi-access = free}}</ref> the mass of <sup>3</sup>He in the Earth's atmosphere is the product of these numbers, or about {{convert|37,000|t}} of <sup>3</sup>He. (In fact the effective figure is ten times smaller, since the above ppm are ppmv and not ppmw. One must multiply by 3 (the molecular mass of helium-3) and divide by 29 (the mean molecular mass of the atmosphere), resulting in {{convert|3,828|t}} of helium-3 in the earth's atmosphere.)

<sup>3</sup>He is produced on Earth from three sources: lithium [[spallation]], [[cosmic rays]], and beta decay of tritium (<sup>3</sup>H). The contribution from cosmic rays is negligible within all except the oldest regolith materials, and lithium spallation reactions are a lesser contributor than the production of <sup>4</sup>He by [[alpha particle]] emissions.

The total amount of helium-3 in the mantle may be in the range of {{convert|0.1–1|Mt}}. Most mantle is not directly accessible. Some helium-3 leaks up through deep-sourced [[Hotspot (geology)|hotspot]] volcanoes such as those of the [[Hawaiian Islands]], but only {{convert|300|g}} per year is emitted to the atmosphere. [[Mid-ocean ridge]]s emit another {{convert|3|kg/year|g/day}}. Around [[subduction|subduction zone]]s, various sources produce helium-3 in [[natural gas]] deposits which possibly contain a thousand tonnes of helium-3 (although there may be 25 thousand tonnes if all ancient subduction zones have such deposits). Wittenberg estimated that United States crustal natural gas sources may have only half a tonne total.<ref>[[#Witt|Wittenberg 1994]] p. 3, Table 1; p. 9.</ref> Wittenberg cited Anderson's estimate of another {{convert|1200|t}} in [[interplanetary dust]] particles on the ocean floors.<ref>[[#Witt|Wittenberg 1994]] Page A-1 citing Anderson 1993, "1200 metric tonne"</ref> In the 1994 study, extracting helium-3 from these sources consumes more energy than fusion would release.<ref>[[#Witt|Wittenberg 1994]] Page A-4 "1 kg (<sup>3</sup>He), pumping power would be 1.13{{e|6}} MWyr ... fusion power derived ... 19 MWyr"</ref>

=== Lunar surface ===
See [[#Extraterrestrial mining|Extraterrestrial mining]] or [[Lunar resources#Helium-3|Lunar resources]]

=== Solar nebula (primordial) abundance ===
One early estimate of the primordial ratio of <sup>3</sup>He to <sup>4</sup>He in the solar nebula has been the measurement of their ratio in the atmosphere of Jupiter, measured by the mass spectrometer of the Galileo atmospheric entry probe. This ratio is about 1:10,000,<ref>{{Cite journal | bibcode = 1996Sci...272..846N | title = The Galileo Probe Mass Spectrometer: Composition of Jupiter's Atmosphere | last1 = Niemann | first1 = Hasso B. | last2 = Atreya | first2 = Sushil K. | last3 = Carignan | first3 = George R. | last4 = Donahue | first4 = Thomas M. | last5 = Haberman | first5 = John A. | last6 = Harpold | first6 = Dan N. | last7 = Hartle | first7 = Richard E. | last8 = Hunten | first8 = Donald M. | last9 = Kasprzak | first9 = Wayne T. | display-authors = 8| volume = 272 | date = 1996 | pages = 846–9 | journal = Science | doi = 10.1126/science.272.5263.846 | pmid = 8629016 | issue = 5263| s2cid = 3242002 }}</ref> or 100 parts of <sup>3</sup>He per million parts of <sup>4</sup>He. This is roughly the same ratio of the isotopes as in [[lunar regolith]], which contains 28&nbsp;ppm helium-4 and 2.8&nbsp;ppb helium-3 (which is at the lower end of actual sample measurements, which vary from about 1.4 to 15&nbsp;ppb). Terrestrial ratios of the isotopes are lower by a factor of 100, mainly due to enrichment of helium-4 stocks in the mantle by billions of years of [[alpha decay]] from [[uranium]], [[thorium]] as well as their [[decay product]]s and [[extinct radionuclide]]s.