Science Fair Project Encyclopedia
Helium-3 was trapped in the planet when it was created. Some He-3 is being added by meteoric dust, primarily collecting on the bottom of oceans (although due to subduction, all oceanic tectonic plates are younger than continental plates).
It has been observed that He-3 is present in volcano emissions and oceanic ridge samples. How He-3 is stored in the planet is under investigation, but it is associated with the mantle and is used as a marker of material of deep origin.
Helium-4 is created by radiogenic production (by decay of Uranium/Thorium-series elements). Due to crystallization of U/Th-series elements, the continental crust has become enriched with those elements and more He-4 is produced in the crust than in the mantle. The mantle is believed to have a lower ratio of He-3 to He-4 than the crust.
The ratio (R) of He-3 to He-4 is often used to represent He-3 content. R usually is given as a multiple of the present atmospheric ratio (Ra).
Common values for R/Ra:
- Continental plate rocks: less than 1
- mid-ocean ridge basalt (MORB): 7 to 9
- Spreading ridge rocks: 9.1 plus or minus 3.6
- Hotspot rocks: 5 to 42
- (U-Th)/He dating of apatite as a thermal history tool
- USGS: Helium Discharge at Mammoth Mountain Fumarole (MMF)
Ground water isotopes
Tritium was released to the atmosphere during atmospheric testing of nuclear bombs. Radioactive decay of tritium produces the noble gas helium-3. Comparing the ratio of tritium to helium-3 allows estimation of the age of recent ground waters.
General online stable isotope references
- USGS: Stable Isotopes and Mineral Resource Investigations in the United States
- USGS: Fundamentals of Stable Isotope Geochemistry
- Environmental Isotopes
- Fundamentals of Isotope Geochemistry
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