演題概要

3C-O1-1027

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中性子照射と希ガス質量分析によるマントル由来岩石の極微量ハロゲン分析で明らかにされた地球内部の揮発性成分循環

(¹東大先端研・ ²東大院総合文化)
^o角野浩史¹・ 任杰²

Halogens (Cl, Br, and I) and noble gases (He, Ne, Ar, Kr, and Xe) are highly concentrated in surface geochemical reservoirs. Since elemental and isotope ratios of halogens and noble gases vary widely depending on the geochemical reservoirs, they are expected to be good tracers of the material exchange between the Earth's surface and mantle, especially the water recycling into the mantle. Halogens in mantle materials have been challenging to analyze by conventional methods due to their low concentrations, but recently, highly sensitive analysis has become possible by a combination of neutron irradiation and noble gas mass spectrometry.
The halogen and noble gas compositions of slab-derived fluids preserved in exhumed metamorphic rocks from the Sanbagawa metamorphic belt and mantle-derived rocks from subduction zones are similar to those of sedimentary pore fluids. The slab fluids' halogen and noble gas signatures also resemble those of forearc and seafloor serpentinites. These suggest that serpentine supplies a significant amount of water to the mantle wedge beneath the volcanic front and that this water is not strongly decoupled from these two groups of elements. On the other hand, halogen compositions of the slab fluids are different from those in mantle-derived xenoliths and diamonds from continental regions far from subduction and those of MORB- and OIB-source mantle, suggesting that the pore-fluid-like halogens would not subduct beyond subduction zones.