[S-CG55] Ocean Floor Geoscience

[SCG55-P03] Exploration of submarine hydrothermal systems based on high-resolution magnetic anomalies in the Higashi-Aogashima Knoll Caldera, Izu-Ogasawara arc, Japan

*Kazuya Kitada¹, Takafumi Kasaya², Hisanori Iwamoto³, Yoshifumi Nogi⁴ (1.Institute for Extra-cutting-edge Science and Technology Avant-garde Research, Japan Agency for Marine-Earth Science and Technology, 2.Submarine Resources Research Center, Research Institute for Marine Resources Utilization, Japan Agency for Marine-Earth Science and Technology, 3.Nippon Marine Enterprises, Ltd., 4.National Institute of Polar Research)

Keywords:Hydrothermal systems, Magnetic anomalies, Magnetic survey, Izu-Ogasawara arc, Higashi-Aogashima Knoll

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.