[S-CG62_2AM1] 流体と沈み込み帯のダイナミクス

[SCG62-04] ハロゲンと希ガスから明らかになった堆積物中間隙水起源のマントルウェッジ中流体

*小林 真大¹、角野 浩史¹、長尾 敬介¹、石丸 聡子²、荒井 章司³、芳川 雅子⁴、川本 竜彦⁴、熊谷 仁孝⁴、小林 哲夫⁵ (1.東京大学地殻化学実験施設、2.熊本大学理学部地球環境科学講座、3.金沢大学理工学域地球学コース、4.京都大学理学研究科地球熱学、5.鹿児島大学理学部地球環境)

キーワード：水, ハロゲン, 希ガス, 沈み込み帯, マントル, かんらん岩

H₂O plays an important role in mantle processes in subduction zones. Yet its subducting processes to the mantle remain unknown because of scarcity of direct observations of H₂O in mantle-derived materials. Since halogen and noble gas are strongly partitioned into ?uids and they show distinct elemental and/or isotopic ratios depending on their origins, their compositions in mantle rocks can provide complementary constraints on the behavior and origin of H₂O in the mantle. Although only few researches have been conducted, the subduction of halogens and noble gases derived from sedimentary pore fluids (seawater trapped in pores of deep-sea sediments) has been suggested. Pore fluid?like halogens and noble gases were found in mantle wedge peridotites which captured H₂O-rich fluids just above a subducting slab [1]. H₂O-rich fluid inclusions whose salinity is similar to that of pore fluids (salinity of pore fluids is the same level as that of seawater [2]) are found in a mantle xenolith from a subduction zone [3]. We investigated halogen and noble gas compositions of mantle wedge peridotites from subduction zones to better constrain how far the in?uence of subducted sedimentary pore fluids extends into the mantle. The samples studied are harzburgitic xenoliths from the Avacha volcano in Kamchatka and the Pinatubo volcano in the Philippines, and alpine-type peridotite from the Horoman massif in Japan. H₂O-rich ?uid inclusions have been found in olivine of those mantle peridotites [3,4,5]. We applied the noble gas method, in which halogens (Cl, Br, and I) are converted to corresponding isotopes of Ar, Kr, and Xe by neutron irradiation in a nuclear reactor and then the concentrations of noble gas isotopes are determined by noble gas mass spectrometry. Halogen detection limits of this method are from two to ?ve orders of magnitude lower than conventional method, which enable to determine the low halogen abundances in mantle-derived materials. By crushing samples under ultra-high vacuum, noble gases are selectively extracted from H₂O-rich fluid inclusions. Unirradiated peridotites were also analyzed to obtain precise noble gas isotope compositions. The halogens of all peridotites are heavily enriched in I, although the halogen ratios are distinctive in each locality. These high I/Cl ratios show a strong contribution of sedimentary pore fluids [2]. The noble gases except for He have the elemental and isotopic ratios similar to elementally fractionated atmospheric noble gases dissolved in seawater, which is probably equivalent to those dissolved in sedimentary pore fluids. The ³He/⁴He ratios are similar to that of the mantle and distinctly higher than the atmospheric ratio. This indicates that the fluids derived from subducting slabs acquired He from the ambient mantle, where He is much more enriched than in seawater. These pore fluid?like halogen and noble gas signatures are strong evidence that the H₂O-rich fluids in the studied peridotites are derived from sedimentary pore fluids and transported to the mantle.References: [1] Sumino et al. (2010) EPSL 294, 163. [2] e.g. Muramatsu et al. (2007) Appl. Geochem. 22, 534. [3] Kawamoto et al. (2013) PNAS 110, 9663. [4] Ishimaru et al. (2007) J. Petrol. 48, 395. [5] Arai & Hirai (1985) Nature 318, 276.