Isotope composition of helium in ultrabasic xenoliths from volcanic rocks of Kamchatka

The purpose of this work is to refine our knowledge about the nature of helium with a high abundance of the rare isotope³He(³He/⁴He= 10^?5) discovered in terrestrial volcanic gases in 1968.We will discuss here the results of isotope analyses of helium released by step-wise heating of ultrabasic xenoliths and some volcanic rocks. On the basis of these results, possible sources of³He in the earth due to fission and nuclear reactions are considered critically. The most probable source of the high abundance of³He is shown to be due to the capture and trapping of primordial He by the earth during its formation (primordial helium³He/⁴He= 3 × 10^?4), a small but significant fraction of which has been retained to the present time.