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

4P1_2I-O19

Room A5

Date/Time: July 24 16:15-16:30

Magma mixing and degassing processes of the 2011 eruption series of Shinmoedake,
Kirishima volcano, based on petrological monitoring and melt inclusion analyses
Genji Saito
Geological Survey of Japan, National Institute of Advance Industrial Science and Technology, Japan
E-mail: saito-g@aist.go.jp

The 2011 eruption activity of Shinmoedake, Kirishima volcanic group, Japan, began with phreatomagmaticeruptions on 19 January. The activity was culminated with the sub-Plinian eruptions on 26-27 January, followed
by lava effusion within the summit crater. Vulcanian explosions and minor ash discharges occurred intermittently
from February to September. Open system behavior was observed by geophysical, geological and geochemicalmonitoring; (1) Continuous inflation of the magma chamber in a year before the eruption (Kawamoto et al., 2011), (2)All tephra samples of the 2011 eruptions observed contained essential material (Oishi et al., 2012), (3) Continuousdegassing activity since January (Mori and Kagoshima Local Meteorological Observatory, 2012), (4) Inflation of themagma chamber from 2 February to November 2011 (Imakiire and Nishimura, 2012; Nakao et al., 2012). In thisstudy, petrological and melt inclusion studies of the 2011 eruption series in order to investigate the eruption anddegassing processes. The bimodal plagioclase core composition, relatively small rims of olivines and pyroxenes,and diffusion profiles of the olivines in the eruptive products of the sub-Plinian eruptions indicate the mixing of mafic
magma and felsic magma in several days before the sub-Plinian eruption. The short time scale is consistent withestimation by diffusion profiles of the magnetite (Tomiya et al., 2012). Melt inclusion analyses indicated that theend members of the magma mixing were basaltic andesite and dacite magmas and its mixing ratio was estimatedto be 0.4 of the basaltic andesite. The eruptive products of the Vulcanian explosions and minor ash discharges inFebruary to August have similar mode composition, chemical compositions of phenocrysts, groundmass minerals
and groundmass and zoning profiles of olivines to those of the sub-Plinian eruptions. These results suggest thatthe same mixing process also occurred before each eruption. The amount of the degassed magma was estimatedbased on sulfur content of melt inclusions of the end member magmas and SO2 flux observation during January
2011 to September 2012. The amount of the degassed magma was larger than that of eruptive products in 2011,indicating the degassing of the magma in the chamber due to magma convection in a conduit. The amount of thedegassed magma during 2 February 2011 to September 2012 (31 Mm3 ) is larger than inflation of the chamber
observed by GPS after 2 February 2011 (10 Mm3 ). However, assuming that the mixing ratio of the magmas (0.4),
the amount of the degassed mafic magma was calculated to 12 Mm3 . This amount is similar to the inflation,suggesting that injection of the mafic magma from a deeper part into the chamber continued after February andcaused the eruptions and degassing activity.

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