Identification of mantle plumes based on studying the composition of volcanic rocks and their isotopic-geochemical characteristics

New author's and literature data are employed in discussing the role of a diversity of factors con trolling variations in the composition and isotopic-geochemical characteristics of plume-related volcanics. There are three currently known mantle sources that can be reliably identified based on a complex of isotopic- geochemical indicators: PHEM, MORB, and BSE, with the leading role in the recognition of the lower mantle (i.e., plume per se) component belonging to He isotopes. Variations in the isotopic ratios of Sr, Nd, Pb, Hf, and Os in mantle-plume volcanic rocks are controlled by the following three main factors: the heterogeneity of the plume itself (in its different parts), plume contamination with lithospheric material, and the mixing of melts in magmatic chambers. The composition of fluids in basalts and mantle xenoliths in the areas of plume magmatism is discussed, and the conclusion is drawn that the fluid regime of plumes is determined by CO2, H-2, and hydrocarbons. The evolution of plume magmatism is not related to lithospheric extension and precedes rifting. An increase in the Earth's crust thickness (underplating) is the most important consequence of plume activity. The study of plumes in basalts testifies to a petrological (chemical) nature of the Mohorovicic discontinuity.