Abstract
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. The compiled data including those we obtained 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.
