[S-GC33] 固体地球化学・惑星化学

[SGC33-04] インドDharwarおよびSinghbhum地塊のコマチアイトの¹⁸²W/¹⁸⁴Wと初期マントルの進化への示唆

*鈴木 勝彦^1,5、Satish-Kumar Madhusoodhan²、Chaudhuri Trisrota³、Jayananda M.⁴ (1.海洋研究開発機構・海底資源センター、2.新潟大学・理学部地質科学、3.コルカタ大学・地質学科、4.ハイデラバード大学・地球宇宙研究センター、5.東北大学・大学院理学研究科地学専攻)

キーワード：マントルの初期進化、タングステン同位体、コマチアイト

Since Hf and W are lithophile and siderophile elements, Hf is likely to have remained in the silicate melt phase and W was preferentially partitioned to the metallic melt phase during the formation of the Earth's core. Such Hf-W fractionation is thought to have occurred before the disappearance of currently distinct ¹⁸²Hf, and the values of μ¹⁸²W (deviations from W standard in ppm) have been reported to show negative deviations from the present-day mantle value (μ¹⁸²W =0) in ocean island basalts such as Hawaii and Samoa (e.g., Mundl et al., 2017; Takamasa et al., 2020). In contrast, most rocks originated at depths and older than 2.5 Ga show relatively uniform μ¹⁸²W values of +10 to +20 (e.g., Willbold et al., 2011, Touboul et al., 2014, Liu et al., 2016, Mundl et al., 2018, Tusch et al., 2019). On the other hand, some komatiites, such as Schapenburg and Komati (both formed at 3.5 Ga), yield negative or have values near 0, respectively (Touboul et al., 2012, Puchtel et al., 2018). In addition, Mei et al. (2019) reported μ¹⁸²W values near 0 for 3.0 Ga Anshan komatiite. Therefore, it is still highly debated on when and how the mantle has reached the present state.

In this study, we analyzed the W isotopes of 3.3 Ga Singhbhum and Dharwar komatiites from India to determine the W isotopic variations in the mantle during the period from 3.5 Ga to 3.0 Ga. The chemical separation of W was carried out using a method modified from Takamasa et al. (2020), and isotope analysis was performed by MC-ICP-MS (Thermofisher Scientific, NEPUTUNE Plus). The μ¹⁸²W values ranged from -0.5 to +5.6 (n=3) and from -1.4 to +5.0 (n=4) for komatiites from the Singhbhum and Dharwar cratons, respectively. These values are also much smaller than the range of uniform μ¹⁸²W values (from +10 to +15) for older rocks above 2.5 Ga, as discussed above. This result suggests that between 3.5 and 3.0 Ga, the primitive mantle with positive μ¹⁸²W values may have been already sufficiently mixed with extraterrestrial materials such as Late Veneer with negative μ¹⁸²W values that some domains of the mantle had the μ¹⁸²W values of the present-day mantle. In other words, mantle convection may have been active as early as 3.5 Ga.