Helium and argon isotopes in basalts and ultrabasic xenolithes of the Baikal rift zone

Helium isotope composition has been studied and apparent K-Ar age of the collection of alkali basalt samples and ultrabasic xenolithes containing in them, which are considered as representatives of the upper mantle substance according to the complex of petrologic and geochemical data, has been determined. The considerable difference in helium isotope composition of the ultrabasic xenolithes of the Khamar-Daban and Udokan ridges has been obtained. Volcanites of the latter contain helium contaminated to a considerable extent by a crustal component. Probably, the source of volcanites of the Udokan ridge is characterized by less ratio 3He/(U+Th) than the Khamar-Daban ridge. The ultrabasic xenolithes of the latter contain helium with the isotope composition similar to that accepted for the depleted mantle of the MORB-type. The apparent K-Ar age of basalts of both regions is similar and constitutes 5-7 mln years. The impurity of excess argon in the basalts studied is shown to be small and have practically no influence on age data