First-principles calculations of the Hf-W partitioning between molten iron and silicate melt and its implications for 182W isotopes in Ethiopian basalts

Tungsten-182 is produced by beta-decay of the currently extinct nuclide 182Hf and has a relatively short half-life of 8.9 million years. Negative anomalies of 182W (parts per million deviations relative to the current mantle value of 0) have been reported for ocean island basalts from Hawaii and Samoan islands (e.g., Mundl et al., 2017; Takamasa et al., 2020). The most plausible process among the models proposed so far for such 182W variation is as follows. The lithophilic Hf remains in the silicate mantle while W has an affinity for metal, causing Hf-W fractionation when the core separates from the mantle. This process lowers the 182W of the metal core (negative 182W), suggesting that core-derived W contributed to the mantle source of the oceanic island basalts. In this study, we attempt to constrain the Hf-W fractionation under the lowermost mantle conditions at the core separation in the early Earth. First-principles free energy calculations show that even under core-mantle boundary P-T conditions, W in both metal and oxides species is strongly partitioned into the liquid metal phase, while Hf oxide remains in the silicate melt. This Hf-W fractionation results in a core having a low 182Hf/182W ratio and thus a low 182W. It has been shown by high-temperature and high-pressure experiments that W in the core can migrate to the mantle by diffusion at mineral boundary (Yoshino et al., 2019). In this study, we further analyzed the W isotopic composition of the Ethiopian basalt, which is considered to be generated by the Afar mantle plume. We used a chemical procedure modified from the method of Takamasa et al. (2020). The results show that the Ethiopian basalt has a slightly negative 182W compared to the present-day average mantle. This result, combined with the low 182W isotope in the core from the ab initio calculations above, suggests that the Afar mantle plume that produces the Ethiopian basalt is likely derived from the lowermost mantle and contains core components with low 182W isotope values.