Genesis of Cenozoic basalts in Central Asia and northern Mongolia (from helium isotope and petrochemical data)

Isotopic compositions of He (27 samples) and Ar (7 samples) as well as major and trace elements have been determined in Cenozoic effusive rocks and hosted olivine and pyroxene phenocrysts from northern and central Mongolia. The R = ³He/⁴He values are within (0.1–13)?10^?6. Abnormally high R value, 13?10^?6, atypical of Cenozoic basaltic-volcanism areas in Mongolia, has been first revealed at one of the sites of the Hangayn upland. In composition the rocks under study correspond to tephrites, trachybasalts, and subalkalic andesite-basalts. Analysis of their REE patterns and spidergrams shows that the elements participating in the formation of basalt fields of the Hangayn upland were supplied from the enriched mantle (EM1). These patterns are similar to the OIB ones; (La/Yb)_n = 9–53. The R values in the olivine phenocrysts are higher as compared with the pyroxene phenocrysts and the bulk rock compositions of the same samples. Based on the elemental composition of the rocks, their contents of radiogenic ⁴He and ³He were calculated. The rate of ³He formation is 5.65?10^?2 at/(g?year). The calculated and measured R values in the rock samples point to the presence of trapped mantle helium.