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

4W_1D-P7

Date/Time: July 24 Poster

Evolution of magma plumbing system of Sakurajima volcano in the last 50 years:
Frequent vulcanian eruptions controlled by basalt injection
Kaho Ebihara1 , Mitsuhiro Nakagawa1 , Akiko Matsumoto1 , Mizuho Amma-Miyasaka1 , Masato Iguchi2
1

Hokkaido University, Japan, 2 Kyoto University, Japan
E-mail: shrimp@mail.sci.hokudai.ac.jp

Sakurajimna volcano, a post-caldera volcano of Aira caldera in South Kyushu, has repeated plinian eruptions
three times since 1471 and effused lava in 1946. Vulcanian eruptions have continued since 1955 until now.
Nakagawa et al. (2011) concluded that three end-member magma mixing have occurred in 1914, 1946, 1958
and 1990; basaltic magma (B) inject into mixed magma of silicic (S) and andesitic (A) just before eruption. We
develop petrological study of the vulcanian eruptions in the last 50 years to investigate temporal change of magma
plumbing system in detail. Whole-rock SiO2 content of erupted materials in the last 50 years are 58-64% (andesitic).
Although these rocks have gradually changed to become more mafic with time since 1914, their compositional
trends in SiO2 -variation diagrams are consistent with those of 1914 and 1946. All the ejecta contain plagioclase,
orthopyroxene, clinopyroxene and magnetite as phenocrysts, and sometimes accompanying olivine. Plagioclase
phenocrysts show compositionally bimodal distributions (An60 and An80), and sometimes there exist compositional
peaks of An90. Orthopyroxene phenocrysts show unimodal or bimodal distribution in the range of Mg#65-75,
and clinopyroxene phenocrysts similarly in Mg#70-80. Furthermore, normally and reversely zoned phenocrystsof plagioclase and pyroxene usually coexist in a single sample. Olivine phenocrysts are mainly divided into twotypes, one is Fo70 (surrounded by thick pyroxenes), and another is Fo80 (surrounded by microlites, have noreaction rims). Bimodal distribution of plagioclase phenocrysts, coexistence of normally and reversely zoned
phenocrysts and presence of Mg-rich olivines that compositionally disequilibrium with pyroxenes suggested thatmagma mixing also occurred in the last 50 years. S-magma: plagioclase (An=46-64), orthopyroxene (Mg#=60-68),
clinopyroxene (Mg#=66-72) and magnetite, and A-magma: plagioclase (An=64-86), orthopyroxene (Mg#=68-76),
clinopyroxene (Mg#=72-79) and magnetite, and B-magma: plagioclase (An=86-94) and olivine (Fo=75-82). The
end-member magmas of these vulcanian eruptions are similar to those of Nakagawa et al.(2011). The relationships
between ratios of these phenocrysts and whole-rock SiO2 suggests that whole-rock compositional variations ofjuvenile materials since 20th century should be formed by injection of B-magma into the mixed magma of S and A(Nakagawa et al., 2011). The level of eruptive activity would correlate to whole-rock chemistry of juvenile materials.
During the periods, in 1914 and from the late 1970’s to 1980’s, which the level of the activity increased, whole-rock
SiO2 contents of the juveniles had also decreased. This fact suggests that the scale and/or frequency of theinjection of B-magma would control the level of eruptive activity. In other word, we suggest that monitoring of theB-magma might be essential to forecast future eruptive activity in Sakurajima volcano.

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