HIGH-HE-3 PLUME ORIGIN AND TEMPORAL-SPATIAL EVOLUTION OF THE SIBERIANFLOOD BASALTS

An olivine nephelinite from the lower part of a thick alkalic ultrabas ic and mafic sequence of volcanic rocks of the northeastern part of th e Siberian flood basalt province (SFBP) yielded a Ar-40/Ar-39 plateau age of 253.3 +/- 2.6 million years, distinctly older than the main tho leiitic pulse of the SFBP at 250.0 million years. Olivine phenocrysts of this rock showed He-3/He-4 ratios up to 12.7 times the atmospheric ratio; these values suggest a lower mantle plume origin. The neodymium and strontium isotopes, rare earth element concentration patterns, an d cerium/lead ratios of the associated rocks were also consistent with their derivation from a near-chondritic, primitive plume. Geochemical data from the 250-million-year-old volcanic rocks higher up in the se quence indicate interaction of this high-He-3 SFBP plume with a suboce anic-type upper mantle beneath Siberia.