[S-GC36] 固体地球化学・惑星化学

[SGC36-02] ¹⁸²W/¹⁸⁴W of the komatiites from Dharwar and Singhbhum cratons, India: implications for the ¹⁸²W isotope evolution of the mantle

*鈴木 勝彦¹、Satish-Kumar Madhusoodhan²、Chaudhuri Trisrota³、Jayananda M.⁴ (1.国立研究開発法人海洋研究開発機構・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)

キーワード：核－マントル相互作用、タングステン同位体、地球の初期進化

¹⁸²Hf decays to ¹⁸²W with a geologically short half-life of 8.9 million years. Fractionation between Hf and W occurring during the life of ¹⁸²Hf leads to variation in ¹⁸²W isotopes Since Hf and W are lithophile and siderophile elements, Hf is likely to have remained in the silicate melt phase, and W is preferentially removed from the silicate and partitioned to the metallic melt phase during the formation of the Earth's core. This fractionation of Hf-W is believed to have occurred before the disappearance of the ¹⁸²Hf, and the negative values of μ¹⁸²W (deviations of the present-day upper mantle value in ppm) have been reported to in basalts of ocean islands 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). However, some komatiites, such as Schapenburg and Komati (both with 3.5 Ga), yield negative or have values unresolvable from 0, respectively (Touboul et al., 2012; Puchtel et al., 2018). Furthermore, Mei et al. (2019) reported μ¹⁸²W values of ¹⁸²W close to 0 for 3.0 Ga Anshan komatiite. Therefore, it is still highly debated as to when and how the mantle has reached the present state.

Here we report 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. As mentioned above it is a period when the komatiites with the low ¹⁸²W isotopic composition were found. The chemical separation of W is the 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 the Singhbhum and Dharwar komatiites, respectively. These values are much lower than the range of uniform μ¹⁸²W values (from +10 to +15) for older rocks than 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 and/or Late Heavy Bombardment with negative μ¹⁸²W values and 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.