[S-VC25] International Volcanology

[SVC25-P06] Tracing the Kikai volcano evolution from geochemical surveys of subaerial and submarine deposits

*Maria Luisa Tejada¹、Takeshi Hanyu¹、Takashi Miyazaki¹、Morihisa Hamada¹、Qing Chang¹、Tomoki Sato¹、Iona McIntosh¹、Kenta Ueki¹、Bogdan Vaglarov¹、Nobukazu Seama^1,2、Katsuya Kaneko²、Reina Nakaoka²、Koji Kiyosugi²、Keiko Suzuki-Kamata²、Yoshi Tatsumi²、Osamu Ishizuka³ (1.Volcano and Earth Interior Research Center, Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology、2.Kobe Ocean Bottom Exploration Center, Kobe University、3.Geological Survey of Japan)

キーワード：Kikai caldera, magmatic evolution, calderagenic eruptions

Previous studies on Kikai volcano offshore SW Kyushu revealed multiple caldera-forming eruptions at 140 ka (Koabi, KKb), 95 ka (Tozurahara, KTz), and 7.3 ka (Akahoya, KAh). However, information about the behavior and magmatic evolution of the volcano is limited because it is largely underwater. A comprehensive geochemical study of both subaerial and submarine rocks, recovered during JAMSTEC cruises from 2019 to 2020, was conducted to better understand its magmatic evolution. The recovered pumice and denser volcanic rocks range in composition from andesite to rhyolite (SiO2 = 56.82-74.95 wt%) that contrast with the apparent bimodal composition of volcanic products sampled in the islands, suggesting a more complete record of magmatic evolution for the Kikai volcano. ⁴⁰Ar-³⁹Ar age dating of samples from the eastern outer caldera wall yielded 205 ka and 235 ka plateau ages, further indicating that they represent pre-Koabi and pre-Tozurahara volcanic edifice. These results allow us to investigate the pre-, syn-, and post-caldera stage volcanism of the Kikai caldera.
Statistical and comparative analyses of the chemical and isotopic variation among the Kikai volcanic ejecta point to the existence of up to three magmatic trends, namely, pre-KKb, KTz-KKb and KAh from oldest to youngest, consistent with at least three magmatic evolution stages. The first magmatic stage could represent the ~700 ka initiation and formation of the Kikai volcano or the pre-KKb stage volcanism, based on their distinctive isotopic signatures. The second magmatic stage possibly built the 235-205 ka andesitic cone and may have evolved to silicic volcanism leading to the first and second calderagenic KKb and KTz eruptions, respectively. This stage includes the subaerial rhyolitic Akazaki flows, which share the same isotopic signatures with the ~140 ka Koabi and ~95 ka Tozurahara deposits. The third magmatic cycle is marked by a small but distinct shift in isotopic compositions to Akahoya signature. This stage may have started with the bimodal eruption of andesitic lava flows (Takahira- and Magome- yama, Uze) and rhyolitic obsidian flows and pumices (Yakuroze, Uze pumice). Their isotopic affinities to the 7.3 Akahoya ejecta suggest derivation from the same magma source. Trends in Pb-Pb isotope plots are transverse to those of the Ryukyu rear arc and Central Kyushu volcanic front volcanoes, possibly indicating tectonic control on magmatic evolution. In both Akahoya and Tozurahara-Koabi cases, the data for the calderagenic eruptions define the most mantle-like ends of the arrays, suggesting that fresh inputs of mantle-derived magma into the fractionating crustal magma chambers, may have shifted the isotopic ratios toward mantle compositions, and triggered the calderagenic eruptions of the Kikai volcano.