Title

G.A. SHATKOV, O.YU. LEBEDEVA, A.V. ANTONOV, N.G. BEREZHNAYA, P.A. LVOV, YU.S. BALASHOVA, S.A. SERGEEV (VSEGEI)

Volcanic ashes of Magadan Region: petrological and geochemical features and age

Composition, geochemistry and age of Khasynsky and Uptarsky volcanic ash deposit were clarified. An ash sampling original technique has been offered. Using electron probe microanalysis and scanning microscopy, the texture and microstructure of ashes, were studied and documented. We suppose that ashfall and precipitation took place directly from atmosphere, without material transfer by water. Thus, the ash deposits above should be considered as autochthnous sedimentary ones. The deposits studied are composed of isotopic glass particles, free of impurities. 70% of particles are of 0.1–0.04 mm in size. Each particle has the same composition as a whole deposit. Chemically, volcanic ashes are matching to trachyte riodacite. It seems that for such distinctive rocks, forming unique deposits, the name «khasynit» may be proposed. Eleven accessory zircon crystals were separated and analysed (SIMS SHRIMP), 16 data spots in total). Its U­Pb isotopic age is of 140–350 Ka, corresponding to the Midneopleistocene time.
Keywords: volcanic ash, sampling technique, electron microprobe, trachyte ryodacite, molybdenum, isotopic age, Neopleistocene, paleovolcano, Okhotsk sea.