[S-CG55] Dynamics in mobile belts

[SCG55-06] Evolution of magma plumbing system beneath a submarine lava dome after the 7.3-ka caldera-forming Kikai-Akahoya eruption: An update

★Invited Papers

*Morihisa Hamada¹, Takeshi Hanyu¹, Iona M. McIntosh¹, Maria Luisa G. Tejada¹, Qing Chang¹, Katsuya Kaneko², Jun-Ichi Kimura¹, Koji Kiyosugi², Takashi Miyazaki¹, Reina NAKAOKA², Kimihiro Nishimura², Tomoki Sato¹, Nobukazu Seama², Keiko Suzuki-Kamata², Satoru Tanaka¹, Yoshiyuki Tatsumi², Kenta Ueki¹, Bogdan S. Vaglarov¹, Kenta Yoshida¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.Kobe University)

Keywords:Kikai caldera, Akahoya eruption, Post-caldera volcanism

Caldera-forming eruption is one of geological events which characterizes mobile belts, such as the Japanese island arc. Kikai caldera is a submarine volcano which has undergone repeated caldera-forming eruptions. Recent submarine surveys have revealed that a 600 m high central lava dome grew inside the Kikai caldera, with a volume of 32 km³. Several satellite volcanic cones formed to the east and the west of this central lava dome. In addition, the SW topographic features are extending toward the outside of the outer caldera. These features would postdate the 7.3-ka Akahoya eruption. However, the geochemical characteristics and the formation mechanism of these submarine volcanic edifices are yet to be well understood. Here we present the whole-rock and mineral chemistry of dredged rocks from the 22 dredge sites. The results are used to constrain magmatic conditions that formed these submarine volcanic edifices. Most rhyolites from the central lava dome, the western satellite volcanic cones, and the SW topographic features are compositionally distinct from those of the Akahoya eruption and the old Iwo-dake volcanic stage (5.2–3.9 ka) rhyolites. Instead, the chemical compositions of these submarine rhyolites overlap with those of the volcanic products of the young Iwo-dake, which is active since 2.2 ka. The compositional similarity suggests that the central lava dome and the topographic features extending toward the outside tapped the same magma supplying the young Iwo-dake volcanism. By contrast, the dredge samples from the eastern satellite volcanic cones and the outer caldera wall exhibit binary chemical trends: one group having chemical composition that overlaps with those of the young Iwo-dake rhyolites; the other group being chemically similar to the rhyolites from the old Iwo-dake and the Akahoya eruption. The magma temperature estimated from Fe-Ti oxide thermometry (900–940 ℃ at the central lava dome and 870–970 ℃ at the SW topographic features) is almost equal to those estimated for subaerially erupted rhyolites at Iwo-dake during the young Iwo-dake volcanic stage and the Showa-Iwojima eruption in 1934–1935. The oxygen fugacity estimated from Fe-Ti oxygen barometry is 0.3–0.8 log unit higher than Ni-NiO buffer. A magmatic hygrometer estimated H₂O content of the melt ranging 2.2–3.5 wt% at the central lava dome and 1.5–2.2 wt% at the SW topographic features. Polybaric crystallization from H₂O-saturated melts may have occurred at depths of 2–4 km beneath the submarine lava dome and at shallower level toward outside of caldera.

In addition to results of major element analysis, we are now analyzing trace elements and isotopes of these rock samples to better constrain their origin. We also anticipate geophysical imaging of magma stored beneath the submarine caldera to strengthen the geochemical constraints.