[S-CG45] Ocean Floor Geoscience

[SCG45-P14] Mineralogical and geochemical study of sediment cores collected from the Ieyama hydrothermal field in the Okinawa Trough

*Jun-ichiro Ishibashi¹, Takashi Miyamoto^1,3, Shuhei Totsuka^1,2, Kazuhiko Shimada¹, Toru Yamasaki², Ken Ikehara², Toshiro Nagase⁴, Yutaro Takaya⁵, Hideaki Machiyama⁶, Koichi Iijima⁶, Hirofumi Yamamoto⁶, Hidenori Kumagai⁶ (1.Department of Earth and Planetary Sciences, School of Science, Kyushu University, 2.National Institute of Advanced Industrial Science and Technology, 3.School of science, University of Tokyo, 4.Tohoku University Museum, 5.Waseda Research Institute for Science and Engineering, 6.Japan Agency for Marine-Earth Science and Technology)

Keywords:seafloor massive sulfide deposits, sulfide and sulfate mineralization, subseafloor of hydrothermal field, scientific drilling using BMS(Boring Machine System)

Ieyama hydrothermal field is located at the seafloor around a water depth of 1050-1100 mbsl (meters below the sea level) in the mid-Okinawa Trough. Semi-detailed exploration survey during YK17-18C cruise using AUV (Autonomous underwater vehicle) identified negative self-potential anomalies probably related to mineralization on and beneath the seafloor in this area (Kasaya et al., 2020). Moreover, ROV (Remotely operated vehicle) surveys conducted in the same and later durations observed and accessed present hydrothermal activities represented by high temperature fluid venting from mound structure consisted of sulfide and sulfate mineral deposits. Scientific drilling was performed by J-MARES during the SIP Integrated Ocean Resource Surveying System JM18-04 Cruise in September 2018. Sediment cores were obtained from 3 sites in the Ieyama field, employing BMS (Boring Machine System) installed on Shincho-maru. Lithology and mineralogy of the obtained sediment cores were studied based on VCD (Visual Core Description), measurements of physical properties, XRD (X-ray diffractometer) analysis, and electron probe microanalyzer (EPMA) analysis.

Sulfide mineralization was recognized in the layer from 50 to 65 mbsf (meters below the seafloor) of the sediment core collected from a bottom of circular depression of about 100 m diameter. This layer was overlaid by thick pile of pumice and pumiceous tuff, and underlaid by tuffaceous breccia. Mineralogical and chemical studies of sulfide minerals revealed several similarities to sulfide minerals collected from chimney and mound structures found in the vicinity of the drilled site.

Acknowledgements:

This study was conducted under the framework of the Next-Generation Technology for Ocean Resources Exploration Project, Cross-ministerial Strategic Innovation Promotion Program (SIP) funded by CSTI, Cabinet Office, Government of Japan. We are thankful to J-MARES (Research and Development Partnership for Next Generation Technology of Marine Resources Survey) for providing samples used in this study.