[S-CG55] Oceanic plate as inputs to subduction zone: from oceanic spreading ridge to subduction trench

[SCG55-10] Mantle-derived helium released through the Japan Trench bend-faults

★Invited Papers

*Jin-Oh Park¹, Naoto Takahata¹, Ehsan Jamali Hondori¹, Asuka Yamaguchi¹, Takanori Kagoshima², Tetsuro Tsuru³, Gou Fujie⁴, Yue Sun¹, Juichiro Ashi¹, Makoto Yamano⁵, Yuji Sano¹ (1.Atmosphere and Ocean Research Institute, University of Tokyo, 2.Department of Environmental Biology and Chemistry, University of Toyama, 3.Department of Marine Resources and Energy, Tokyo University of Marine Science and Technology, 4.Japan Agency for Marine-Earth Science and Technology, 5.Earthquake Research Institute, University of Tokyo)

Keywords:Mantle-derived helium, bend-faults, Japan Trench

Plate bending-related normal faults (i.e. bend-faults) develop at the outer trench-slope of the oceanic plate incoming into the subduction zone. Numerous geophysical and numerical simulation studies suggest that bend-faults play a key role by providing pathways for seawater to flow into the oceanic crust and upper mantle, thereby promoting hydration of the oceanic plate. However, deep penetration of seawater along bend-faults remains controversial because fluids that have percolated down into the mantle are difficult to detect. In this talk we presents anomalously high helium isotope (3He/4He) ratios and seismic reflection data which support fluid infiltration into the mantle and subsequent outflow through bend-faults across the outer slope of the Japan Trench. Almost constant 3He/4He and 4He/20Ne ratios with depth at sites near trench bend-faults suggest local seawater inflow. Our findings provide the first reported evidence for a potentially large-scale active hydrothermal circulation system through bend-faults across the Moho (crust-mantle boundary) in and out of the oceanic lithospheric mantle. Mantle fluids expelled from the oceanic plate before subduction, if found to be pervasive across most of the ocean trench slope, would necessitate reconsideration of the water budget between Earth’s surface and its interior in terms of the global water cycle. As a future study, time-series geochemical observations at additional sites are required to clarify detailed fluid flux and the water circulation pattern. Ocean drilling of the bend-faults, which enables us to define mechanical and hydrological properties of the faults and to observe the mantle fluids at deeper depths, would be useful for more comprehensive understanding of the mantle fluid flow.<gdiv></gdiv><gdiv></gdiv><gdiv></gdiv>