IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1A1_3B-O3

Room A5

Date/Time: July 21 9:15-9:30

Petrology and geochemistry of the New Tolbachik Fissure Eruption volcanic rocks andtheir evolution during the first two weeks of eruption
Anna O. Volynets1 , Dmitry Melnikov1 , Anton Yakushev2 , Maria Tolstykh3
1

Institute of volcanology and seismology FED RAS, Petropavlovsk-Kamchatsky, Russia, 2 Institute ofGeology of Ore Deposits, Petrography, Mineralogy and Geochemistry RAS, Moscow, Russia,
3Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Russia
E-mail: a.volynets@gmail.com

The New Tolbachik Fissure Eruption started on 27.11.2012. Plosky Tolbachik is a stratovolcano, active in Holocene;
it is intersected by Tolbachinsky Dol - the monogenetic lava field, formed by numerous cinder cones and lava flows
eruptions which repeatedly took place for the last 10 Ka. Most of all cinder cones are concentrated in the narrow
axial zone responding to the fault zone location (Fedotov et al., 1984). The last big fissure eruption in this region
occurred 37 years ago and lasted for 2 years; the volume of erupted material was 2.2 km3, lava flows covered thearea of 45 km2; magnesian, alumina and transitional basalts were erupted. Besides, a new caldera was formed atthe summit crater of Plosky Tolbachik (Fedotov et al., 1984).
The New Tolbachik Fissure Eruption began from the effusion and explosion of high-K Ti-rich aluminous
trachy-basaltic andesites, black, sub-aphyric, with rare phenocrysts of Pl, Ol and Cpx. Microlites of the groundmass
with pilotaxitic and hyalopilitic structures are mainly Pl, dovetail shaped, and, to a lesser extent, Ol and Cpx; Mt
is abundant; the areas with hyaline groundmass compositionally similar to the whole rocks are confined to thehighly-porous parts of the rock. The petrography of the volcanic rocks of the first days of eruption attests very
rapid cooling, high rates of eruption and high mobility of the lava. Composition of Ol varies from Fo64 to 80, Pl
is An41 to 62; Cpx are Augites and Salites. The average content (wt.percent) of SiO2 in the whole rock fromthe first days of eruption is 54.57, Al2O3 16.6, K2O 2.5, MgO 3.36, TiO2 1.8 (for comparison, the eruption in1975 started from high-Mg medium-K basalts and was continued by high-Al high-K basalts, which are close incomposition to the newly erupted basaltic andesites, but have lower content of silica, alkalis and TiO2 at higher
MgO and CaO). During the next two weeks bigger plagioclase crystals appeared as phenocrysts (up to 1 cm indiameter); also Ol became more abundant and bigger in size (up to 3 mm in diameter) in comparison to the lavasof the beginning of this eruption. The composition of rocks gradually changed to more mafic: after 10-14 days
silica content (wt.percent) dropped to 52.79, while MgO increased to 4.08, TiO2 to 2.03, Al2O3 decreased to 15.8
and K2O remained roughly the same. The trace elements content also changes regularly with the shifting of themain oxides content. Overall, the incompatible trace elements content in lavas of the first portions of new eruption
is higher than in all previously studied volcanic rocks of Tolbachinsky Dol; the distribution of trace elements on
spidergrams and REE on REE-diagrams forms sub-parallel trends, indicating possible derivation of these rocks
through fractional differentiation processes of parent magmas, responsible for the formation of basalts of the Northand South Vents of the Great Fissure Tolbachik Eruption in 1975-76.
This work is supported by FED RAS grant 12-III-A-08-165.

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