15:45 〜 16:00
[SCG55-08] 沖縄トラフ伊江山熱水域から得られた堆積物コアの年代研究
キーワード：海底熱水鉱床、重晶石、ESR年代測定法、放射性炭素年代測定法
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. Topography of Ieyama hydrothermal field is characterized by a series of circular depressions of about 100-300 m diameter located at the slope of a large volcanic body. Active chimneys and mounds composed of massive sulfide deposits were observed along the rim of these crater-like depressions.
Scientific drilling was performed by J-MARES during the SIP Integrated Ocean Resource Surveying System JM18-04 Cruise in September 2018, employing BMS (Boring Machine System) installed on Shincho-maru. Sediment cores obtained from one of crater-like depressions were examined in this study, which were collected with high recovery (>80%) up to 70.7 mbsf (meters below the seafloor). The obtained sediment consisted basically of volcanic clasts such as pumiceous gravel and volcanic breccia. Hydrothermal alteration was recognized only below the depth of 46 mbsf (meters below the seafloor), and minor sulfide and sulfate mineralization was recognized between 53 and 67 mbsf.
We focused on barite in the sediment core, because barite formation is considered related to mixing of the hydrothermal fluid component and seawater penetrated through a volcaniclastic sediment of high porosity. Dating of the barite within the mineralization layer by ESR (Electron Spin Resonance) methods yielded 350-1000 years. It is interesting to notice that radiocarbon dating of planktic foraminifera in the sediment just above the seafloor yielded similar order of ages. Although reliability or accuracy of the age by ESR dating has not been well verified, these results are in accordance with an idea of mineralization within pumiceous sediment layer.
Scientific drilling was performed by J-MARES during the SIP Integrated Ocean Resource Surveying System JM18-04 Cruise in September 2018, employing BMS (Boring Machine System) installed on Shincho-maru. Sediment cores obtained from one of crater-like depressions were examined in this study, which were collected with high recovery (>80%) up to 70.7 mbsf (meters below the seafloor). The obtained sediment consisted basically of volcanic clasts such as pumiceous gravel and volcanic breccia. Hydrothermal alteration was recognized only below the depth of 46 mbsf (meters below the seafloor), and minor sulfide and sulfate mineralization was recognized between 53 and 67 mbsf.
We focused on barite in the sediment core, because barite formation is considered related to mixing of the hydrothermal fluid component and seawater penetrated through a volcaniclastic sediment of high porosity. Dating of the barite within the mineralization layer by ESR (Electron Spin Resonance) methods yielded 350-1000 years. It is interesting to notice that radiocarbon dating of planktic foraminifera in the sediment just above the seafloor yielded similar order of ages. Although reliability or accuracy of the age by ESR dating has not been well verified, these results are in accordance with an idea of mineralization within pumiceous sediment layer.