[S-CG45] Ocean Floor Geoscience

[SCG45-P15] Geochemical and geochronological study of barite in sediment cores obtained from Higashi Izena hydrothermal field, Okinawa Trough

Yushi Itatani¹, *Jun-ichiro Ishibashi¹, Makiko Tomita², Shin Toyoda², Ryuichi Shinjo³, Kazuno Arai⁴, Taisei Fujiwara⁵ (1.Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu University, 2.Okayama University of Science, 3.Faculty of Science, University of Ryukyus, 4.Center for Advanced Marine Core Research, Kochi University, 5.Institute for Geology and Geochronology)

Keywords:seafloor massive sulfide deposits, strontium isotopic composition, radioactive disequilibrium dating, electron spin resonance dating

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. Scientific drilling performed by J-MARES during JM18-04 Cruise in September 2018 confirmed sulfide and sulfate mineralization 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 (Ishibashi et al., this meeting). We focused on barite in the sediment core and conducted measurement of strontium isotope ratio, gamma ray measurement to determine radium content, and geochronological studies employing ESR dating and 226Ra-210Pb radioactive disequilibrium dating.
Vertical profile of radium content in barite showed monotonic change along the depth, as well as variation of strontium isotope ratio. These results suggest barite formation within the sediment layer by mixing of radium-rich hydrothermal fluid and seawater penetrated through a volcaniclastic sediment layer of high porosity. Results of radioactive disequilibrium dating barite within the sediment core showed similar age to barite obtained from hydrothermal mound on the seafloor where high temperature fluid venting. These results imply barite within the sediment layer is formed by present hydrothermal activity.