Japan Geoscience Union Meeting 2016

Presentation information


Symbol S (Solid Earth Sciences) » S-VC Volcanology

[S-VC48] Volcanic and igneous activities, and these long-term forecasting

Sun. May 22, 2016 3:30 PM - 5:00 PM Convention Hall A (2F)

Convener:*Teruki Oikawa(Institute of Earthquake and Volcano Geology, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology), Takeshi Hasegawa(Department of Earth Sciences, College of Science, Ibaraki University), Daisuke MIURA(Geosphere Sciences, Civil Engineering Research Laboratory, Central Research Institute of Electric Power Industry), Yoshihiro Ishizuka(Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology), Nobuo Geshi(Geological Survey of Japan, The National Institute of Advanced Industrial Science and Technology), Chair:Koji Uno(Graduate School of Education, Okayama University), Fukashi Maeno(Earthquake Research Institute, University of Tokyo)

4:00 PM - 4:15 PM

[SVC48-17] Petrological characteristics of Aso-ABCD tephra which erupted before Aso-4 pyroclastic eruption

Fumiko Sugiyama1, *Toshiaki Hasenaka1, Atsushi Yasuda2, Natsumi Hokanishi2, Yasushi Mori3 (1.Graduate School of Science and Technology, Kumamoto University, 2.Earthquake Research Institute, University of Tokyo, 3.Kitakyushu Museum of Natural History and Human History)

Keywords:Aso, caldera-forming eruption, melt inclusion, Aso-ABCD tephra, Aso-4 pyroclastic eruption

Several tephra layers such as Aso-A, B, C, D, , , M, N, α, β, , , η, between Aso-4 (89 ka) and Aso-3 (123 ka) pyroclastic eruptions have been described by Ono et al. (1977). Among them, Aso-ABCD is located at the top of all, and represent a series of continuous eruption events. Nagahashi et al. (2007) estimated their age as 97.7 ka. Ono et al. (1977) estimated the eruption source to be the south of present central cones from the isopach maps. Machida and Arai (1992) estimated their volume to be 3.5 km3. Just before the eruption of Aso-4 event occurred the formation of Omine pyroclastic cone and associated Takayubaru lava flow. This lava flow is overlain by Aso-4 tephra with no intercalated soil. The volume estimate is 2 km3. The most voluminous felsic eruption after Aso-4 event was that of Kusasenri-ga-hama volcano with 1.4 km3 volume (Miyabuchi, 2003). Thus, Aso-ABCD and Omine volcanoes represent voluminous precursory eruptions before caldera-forming Aso-4.
Pumice and volcanic ash was sampled 20 km east of Aso caldera, where the thickness of Aso-ABCD tephra in total is 3 m. The samples were analyzed for XRF bulk-rock chemical analysis, EPMA mineral analyses, FT-IR analyses of melt inclusions. Phenocryst assemblage is plagioclase, clinopyroxene, orthopyroxene, and magnetite, with no hornblende which is common in Aso-4 products. Bulk composition of pumice ranges 63-66 wt. % silica, and mostly plot on Aso-3 trend, and not on Aso-4 trend of Kaneko et al. (2007, 2015). Melt inclusions in plagioclase and pyroxenes also show compositions similar to glass of Aso-3 with silica range mostly in 70-72 wt. %. Compositional range of phenocryst cores are An40-64 for plagioclase, Mg# =70-74 for orthopyroxene, and Mg#=74-81 for clinopyroxene. Water content was estimated to be 1.0-4.8 wt.% for melt inclusions in host minerals of An40-64 plagioclase, Mg#=70-74 orthopyroxene, and Mg#=74-81 clinopyroxene.
Equilibrium relationship between melt inclusion and host clinopyroxene provides temperature estimate of 860-950 ℃ and pressure estimate of 1.1-2.7 kbar (Putirka, 2008). The pressure corresponds to the depth of 3-9 km, comparable to the estimated depth (6 km) of present Kusasenri-ga-hama magma reservoir. Furukawa et al. (2006) showed a gradual change of tephra composition, estimated temperature, estimated water content, and estimated oxygen fugacity from Aso-3 to Aso-4. However, our study showed similarity of magma composition between Aso-ABCD tephra and Aso-3 products. Aso-4 magma reservoir was not yet prepared 9000 years before Aso-4 eruption. Or Aso-4 reservoir at that time was independent from, and had no interaction with, other magma supply system then.