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

3A2_2G-O7

Room B2

Date/Time: July 23 10:45-11:00

Volatile evolution of magma plumbing system of the 2000 eruption at Miyakejima volcano
deduced from melt inclusion analyses
Genji Saito, Yuichi Morishita, Hiroshi Shinohara
Geological Survey of Japan, National Institute of Advance Industrial Science and Technology, Japan
E-mail: saito-g@aist.go.jp

The 2000 eruption of Miyakejima volcano, Japan, is characterized by intrusion of a dyke and a submarine
eruption in June, subsidence of the summit area with phreatic and phreatomagmatic explosions in July to August,and intense and continuous degassing from the summit after the major eruptions (e.g., Nakada et al., 2005).
Geophysical and petrological observations suggested that shallow andesitic and deeper basaltic magma chambers
existed beneath the volcano before the 2000 eruption (e.g., Amma-Miyasaka et al., 2005). In this study, petrologicaland melt inclusion studies were carried out in order to know volatile evolution of the magma plumbing system. We
can consider three component magmas, "A" magma in the shallow magma chamber erupted at the submarine
eruption, and "B" and "C" magmas erupted at the summit eruptions. The C magma represents the deeper-sourced
basaltic magma and the B magma was produced by 40 wt% fractional crystallization of the C magma. The bimodal
olivine core composition and diffusion profiles of the olivines in the ejecta of the summit eruptions indicate the mixingof B and C magmas in two months before the eruption. The melt inclusions in Mg-rich olivines derived from theC magma have slightly higher H2 O (1.9-3.5 wt%) and S (0.06-0.21 wt%) contents, and a lower Cl (0.04-0.07 wt%)
content than the inclusions in Mg-poor olivines from the B magma, while they have similar CO2 (0.003-0.025 wt%)
contents. The wide range of CO2 contents can be explained by magma degassing with a decrease in pressure.
The initial CO2 content of the deep basaltic magma (0.15 wt%) was estimated from the H2 O and S contents of
the inclusion in the Mg-rich olivine and the volcanic gas composition. Based on the variation in the chemicalcomposition of the melt inclusions, we propose volatile evolution of the magma plumbing system as follows, In the
deep magma chamber at depth of about 10 km, 40 wt% fractional crystallization of C magma having initial volatile
content of 3 wt% H2 O and 0.15 wt% CO2 made B magma having bulk volatile content of 5 wt% H2 O and 0.25 wt%
CO2 . This B magma ascended from the deep magma chamber to the shallow magma chamber (3-5 km depth),causing degassing of the B magma with pressure decrease. The decompression degassing decreased the volatile
content of the B magma to 3.9 wt% H2 O and 0.008 wt% CO2 . Before the 2000 eruption, the addition of CO2 -richgas to the B magma occurred in the shallow magma chamber and caused a change in the volatile content of themelt to 2.8 wt% H2 O and 0.035 wt% CO2 . In addition, the C magma ascended from the deep magma chamber andwas injected into the B magma. The mixture of the B and C magmas ascended to a depth shallower than 3 km,
accompanied by magma degassing with the pressure decrease.

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