Japan Geoscience Union Meeting 2018

Presentation information

[EE] Poster

S (Solid Earth Sciences) » S-VC Volcanology

[S-VC39] Pre-eruptive magmatic processes: petrologic analyses, experimental simulations and dynamics modeling

Thu. May 24, 2018 10:45 AM - 12:15 PM Poster Hall (International Exhibition Hall7, Makuhari Messe)

convener:Michihiko Nakamura(Division of Earth and Planetary Materials Science, Department of Earth Science, Graduate School of Science, Tohoku University), Akihiko Tomiya(Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology), Shanaka L de Silva (共同), Fidel Costa(Earth Observatory of Singapore, Nanynag Technological University)

[SVC39-P11] Magmatic conditions prior to the 2000 eruption of Usu volcano, Japan

Satoka Ohnishi1, *Akihiko Tomiya1 (1.National Institute of Advanced Industrial Science and Technology)

Usu volcano is one of the most active volcanoes in Japan, which is in the Shikotsu-Toya National Park, Hokkaido. There have been eight recorded eruptions since AD 1663 at a relatively short time interval (ca. 30-50 y). We here focus on the most recent AD 2000 eruption to understand the present conditions of the Usu magma-feeding system, which is important to prepare for the next eruption. The AD 2000 eruption started with a phreatomagmatic (small phreatoplinian) eruption, producing dacitic pumice (Us-2000pm; Tomiya et al., 2001), followed by many small phreatic explosions. Petrographical studies (Tomiya and Miyagi, 2002; Tomiya and Takahashi, 2005) and melting experiments under high pressure and temperature conditions (Tomiya, 2002; Suzuki et al., 2007) of the AD 2000 volcanic products (Us-2000pm) have been performed. However, petrological constraints are still limited.
In this study, we measured the trace element concentration in plagioclase of the AD 2000 volcanic products to reveal the change in magmatic conditions (e.g., temperature, pressure, melt composition) prior to the eruption, using LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) at the GSJ-Lab (New Wave Research NWR213 + Agilent 7700x) with the method by Yamasaki et al. (2015). We found that the Mg, Ti, Fe contents and Sr content in plagioclase were increased and decreased, respectively, from AD 1663 to 2000 eruption, which were consistent with our SIMS (Secondary Ion Mass Spectrometry) analyses for plagioclase in the former products (e.g., AD 1977, AD 1943-45; Tomiya et al., 1998). We consider that the change was caused by mixing of mafic components.
We are also conducting melting experiments of Us-2000pm to estimate the pressure and temperature of the pre-eruptive magmatic conditions, using an internally heated pressure vessel at GSJ (HARM-200; Tomiya and Miyagi, 2001). At 147 MPa and 950°C, we observed only magnetite and apatite in the experimental product. On the other hand, magnetite, apatite and plagioclase (An=58±5) were formed at 98 MPa, 940°C. Considering that Us-2000pm contains plagioclase (An56-60), orthopyroxene, and magnetite (±apatite) (Tomiya and Miyagi, 2002), the magmatic conditions may be close to 98 MPa and 940°C, but slightly lower temperature.
We will discuss the petrological understanding of Usu volcano, combining these results and previous studies.