eruptions of Bezymianny volcano, Kamchatka: Petrological snapshots of the compositionally changing magma system Pavel Izbekov Alaska Volcano Observatory, Geophysical Institute, University of Alaska Fairbanks and PIRE team ( and PIRE team ( December 11, 2007

Introduction Volcanoes, which erupt frequently / continuously, are perfect targets to study processes in active magma systems. Each individual eruption is a snapshot of magma system at a particular time. The sequence of such snapshots show compositional changes in magma system as a function of time throughout the entire period of eruptive activity. By looking at changes of magma composition we might be able to answer What processes are behind compositional variations?What processes are behind compositional variations? How composition of magma affects eruptive behavior?How composition of magma affects eruptive behavior? How fast minerals crystallize in a particular magma system?How fast minerals crystallize in a particular magma system?

Geological background and eruptive history RUSSIA ALASKA Bezymianny

Bezymianny volcano is part of the Kluchevskoy group of volcanoes, located in the Northern part of the Central Kamchatkan Depression Shiveluch Ushkovsky Kluchevskoy Kamen Bezymianny Tolbachik View to the North from the Int. Space Station, NASA

Geological background and eruptive history Years BC Years AD The oldest part of the stratovolcano was built by lavas, pyroclastic flows, and ash-fall deposits from 10,000-11,000 yr. BP to ca. 6,900 yr. BP. The activity resumed at the same location in 4,700 yr. BP. Since then, the stratocone of the modern Bezymianny was built by intermittent eruptive activity, separated by long periods of dormancy (Braitseva et. al., 1991). The most recent eruptive cycle of Bezymianny started in October 1955 after ca years of quiescence.

Post-1956 trend in eruptive style photo by Gorshkov 1946 photo by Piip 1946

Bezymianny dome in August 2007

Trend in whole rock composition

Changes in mineral assemblage Hornblende core in a OPx-CPx-Mt aggregate of the May 9, 2006 andesite Hb 0.4 mm Juvenile clast from the pyroclastic flow of the. Hornblende is exceptionally rare, however it’s remnants abaund. 5/9/ mm

BB AAAA 90 C C D D E F E F Plagioclase texture and composition 1956 magma

Plagioclase texture and composition Relative distance, core to rim, microns A n, m o l. % A B A A B B A A B B December 24, 2006 May 12, 2007

Glass composition SiO 2, wt.% CaO, wt.% May 9, 2006 Open symbols – inclusions Filled symbols – matrix glass Dec. 24, 2006 May 12, 2007

May 9, 2006 Open symbols – inclusions Filled symbols – matrix glass Dec. 24, 2006 May 12, 2007 Glass composition

Time of eruption, calendar years SiO 2, wt. % Trend in whole rock composition

Summary of observations and conclusion Since 1956 Bezymianny erupts magma, which becomes progressively more mafic with time. On a smaller scale, there are periods, during which the erupted products become more silicic. These short-term variations are superimposed on the general trend. The sequence of products may represent one of such examples, which is corroborated by our glass data. The composition of melt inclusions in the outermost “dusty” zones of plagioclase phenocrysts and the composition of matrix glass form a linear trend with time becoming more silicic. This suggests that the outermost “dusty” zones of plagioclase phenocrysts form immediately prior to each individual eruption. These observations are consistent with a view that Bezymianny magma system is frequently replenished by mafic inputs, which change overall composition of the magma system and may serve as eruption triggers.