Helium-3 is much rarer than helium-4; less than 0.00014% of Earth's helium is helium-3), and most of that is of extraterrestrial origin in the form of meteoritic dust. The estimated helium-3 flux from the mantle is only a few kilograms per year. No additional heat is produced by this helium-3, since it is a stable isotope.
Source(s):
http://en.wikipedia.org/wiki/Helium-3#Terrestrial_abundance

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