Isotope geochemistry is an aspect of Geology based upon study of the relative and absolute concentrations of the elements and their isotopes in the Earth.
Magmatic isotopes
Helium-3
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.
Due to similiarities in helium and carbon in magma chemistry, outgassing of helium requires bubbling of carbon dioxide, which happens at depths of less than 60 km.
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
Ground water isotopes
Tritium/Helium-3
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