[S-CG55] 沈み込み帯へのインプット：海洋プレート誕生から沈み込み帯まで

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

★招待講演

*朴 進午¹、高畑 直人¹、Jamali Hondori Ehsan¹、山口 飛鳥¹、鹿児島 渉悟²、鶴 哲郎³、藤江 剛⁴、孫 岳¹、芦 寿一郎¹、山野 誠⁵、佐野 有司¹ (1.東京大学　大気海洋研究所、2.富山大学　理学部、3.東京海洋大学　海洋資源環境学部、4.海洋研究開発機構　海域地震火山部門、5.東京大学　地震研究所)

キーワード：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>