日本地球惑星科学連合2018年大会

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[EE] Eveningポスター発表

セッション記号 S (固体地球科学) » S-IT 地球内部科学・地球惑星テクトニクス

[S-IT22] 核-マントルの相互作用と共進化

2018年5月22日(火) 17:15 〜 18:30 ポスター会場 (幕張メッセ国際展示場 7ホール)

コンビーナ:飯塚 毅(東京大学)、渋谷 秀敏(熊本大学大学院先端科学研究部基礎科学部門地球環境科学分野)、土屋 卓久(愛媛大学地球深部ダイナミクス研究センター、共同)、太田 健二(東京工業大学大学院理工学研究科地球惑星科学専攻)

[SIT22-P30] High precision 182W/183W isotopic compositions of terrestrial samples

賞雅 朝子1深海 雄介1、*鈴木 勝彦1,2 (1.国立研究開発法人海洋研究開発機構・海底資源研究開発センター、2.東北大学・大学院理学研究科・地学専攻)

キーワード:核-マントル相互作用、タングステン同位体、海洋島玄武岩、高精度同位体分析

Tungsten (W) has five isotopes (M = 180, 182, 183, 184, 186), and 182W isotope is a beta-decay product of derived from 182Hf with the short half life of 8.9 m.y. Both Hf and W are highly refractory elements and are accumulated in the early stage of the proto-earth. As Hf and W are a lithophile and is a siderophile elements, respectively, 182Hf-182W system could give constraints on metal-silicate (core-mantle) differentiation especially core segregation in the very early Earth system because of its large fractionation between metal-silicate and the short half life of 182Hf. Improvement of analytical techniques of W isotope analyses allows us to obtain highly precise 182W/183W ratios of vocanic rocks, which leads to findings of W isotope anomalies (mostly positive) in old komatiites (2.4 – 3.8 Ga) and young volcanic rocks with positive anomalies of 12 Ma Ontong Java Plateau and 6 Ma Baffin Bay (Rizo et al., 2016) and with negative anamalies of those such as the Loihi basalt.

In our study, high-precision W isotope ratio measurement with MC-ICP-MS (Thermo co. Ltd., NEPTUNE PLUS) has been developed. We have measured W standard solution (SRM 3163) and obtained the isotopic compositions with a enough high precision of ± 5ppm. However, the standard solution, which separated by cation or anion exchange resin, has systematical 183W/184W drift of -5ppm. These phenomena was also reported by Willbold et al. (2011). Therefore, we corrected the measured W isotope ratios of samples with the standard solution processed by the same method as that of the samples. This technique leads to the reproducible W isotopic compositions with reproducibility of several ppm. We have obtained the negative 182W/183W for the basalts with the high 3He/4He isotopic composition from the Loihi, Hawaii, through the developed analytical method. This result is consitent with that of Mundl et al., (2017). As negative anomaly of 182W/183W could be created by the early earth core segregation, it is probably a signature of core-mantle interaction.