[S-CG55] 海洋底地球科学

[SCG55-P03] 伊豆・小笠原弧東青ヶ島海丘カルデラ海底熱水活動域における高分解能磁気探査

*北田 数也¹、笠谷 貴史²、岩本 久則³、野木 義史⁴ (1.国立研究開発法人海洋研究開発機構　超先鋭研究開発部門（X-star）、2.国立研究開発法人海洋研究開発機構　海洋機能利用部門海底資源センター、3.日本海洋事業株式会社、4.国立極地研究所)

キーワード：海底熱水系、磁気異常、磁気探査、伊豆・小笠原弧、東青ヶ島海丘

Higashi-Aogashima Knoll Caldera hydrothermal field in the Izu-Ogasawara arc, which was newly discovered in 2015 (Katase et al., 2016; Iizasa et al., 2019), has three hydrothermal sites (Central Cone Site, Southeast Site, and East Site). Among them, anomalous gold enrichment was observed only at the Central Cone Site (Iizasa et al., 2019). However, the cause and accumulation process of anomalous gold enrichment are still unclear. During the KS-21-20 and KM23-01 cruises by the R/V Shinsei Maru and R/V Kaimei, we conducted high-resolution magnetic surveys near the seafloor using the Remotely Operated Vehicles (ROVs) Hyper-Dolphin and KM-ROV to investigate magnetic anomalies associated with submarine geological structures and hydrothermal deposits around the submarine hydrothermal sites. Four ROV dives were conducted on the two hydrothermal sites (Central Cone Site and East Site) in Higashi-Aogashima Knoll Caldera. During these dives, we successfully collected high-resolution magnetic data near the seafloor. The geomagnetic total intensity was measured by an Overhauser-type scalar magnetometer (SeaSPY2, Marine Magnetics Corp.) towed ~15m below the ROVs with a survey line spacing of ~20 m or less over the hydrothermal sites. The altitudes of the magnetometer and vehicle speed were ~10 m and ~0.2 knots, respectively. Based on the observed magnetic data, we estimated high-resolution magnetization intensities. The Central Cone Site is characterized by dipole-like magnetic anomalies with an amplitude of ∼250 nT, likely caused by the hydrothermal mound. The low magnetization zone was identified over the hydrothermal mound, suggesting the effect of demagnetization due to hydrothermal alteration. On the other hand, the magnetic anomalies in the East Site exhibit the E-W trending magnetic lineation with an amplitude of ∼150 nT, implying a two-dimensional subseafloor geological structure. The high-resolution magnetization intensities obtained from these near-seafloor magnetic surveys using ROVs will contribute to a better understanding of the subseafloor processes involved in hydrothermal system development within the Higashi-Aogashima Knoll Caldera.