Evolution of rock compositions observed during the 2012–2013 eruptions of the New Tolbachik volcanoes: Online mantle control

The paper presents new isotope geochemical data on the volcanic rocks of the New Tolbachik Fissure Eruption (TFE-50). These data are compared with those of the 1975–1976 Great Tolbachik Fissure Eruption (GTFE) and the Klyuchevskaya Group of Volcanoes (KGV). Two petrogeochemical evolution trends of volcanic rocks have been identified: the calc-alkaline trend for the island-arc rocks and intraplate trend for the subalkaline rocks. The first trend is characterized by different degrees of enriched lithospheric source melting and subsequent fractionation of the melts. The second trend is provided by mixing between an enriched lithospheric source and a fluid–melt component of the mantle diapir. During TFE-50, the transition from the first-cycle basaltic andesites to the second-cycle basalts was accompanied by the increase of the MgO, Ni, Rb, V, Ti, Y, La, and Sr concentrations and 208Pb/204Pb ratio, but decrease of the SiO2, Pb, Nb, Zr, and Hf contents and Sr isotope ratios. This could be explained by an increasing degree of melting of the lithospheric mantle reservoir. Changes in the basalt composition at the final stage of the eruption are accompanied by an increase in the contents of almost all of the fluid- and melt-mobile elements at insignificant variations in the MgO and Ni contents, which is provided by mixing of the melts derived from lithospheric and asthenospheric mantle sources.