17:15 〜 18:45
[SGC33-P04] Hf and W partitioning between liquid metal and silicate melt calculated by using ab initio free energy calculations: implications for the evolution of 182W isotope of the earth.
キーワード：W同位体、第一原理計算、核－マントル相互作用、ハフニウムとタングステンの分配
182Hf decays to 182W with the half life of 6.9 million years. Because Hf and W are lithophile and siderophile elements, respectively, Hf is thought to remain in the silicate melt phase, while W is thought to be distributed in the molten metal in magma ocean. As a result, the primordial mantle is considered to have a high μ182W value (deviation from the present upper mantle value in ppm) derived from 182Hf and the metallic core is considered to have a low μ182W because of a lack of 182Hf in metallic core. The early mantle-derived rocks such as komatiites have positive μ182W from +10 to +20 ppm (e.g., Mei, et al., 2019). Subsequently, a late veneer with low μ182W is thought to have lowered μ182W of the early mantle to the present upper mantle value. The oceanic island basalts originating from the lowermost mantle, such as Hawaii and Samoa, have been reported to show negative μ182W (e.g., Mundl et al., 2017; Rizo et al., 2019).
To understand the 182W variations in the mantle and their evolution, it is necessary to focus on the behavior of Hf and W under high pressure and the influence of extraterrestrial materials such as meteorite impacts. μ182W variations can be attributed to three possible processes. (1) Hf-W fractionation in the mantle. First-principles calculations (Deng and Stixrude, 2021) report that Hf is enriched in bridgmanite; however, the isotopic results are not consistent with the Hf enrichment of bridgmanite subducted into the lower mantle. (2) Hf-W fractionation occurred during core formation. (Mundl et al., 2017; Rizo et al., 2017) (3) Extraterrestrial material with low μ182W fell to the surface by intermittent impact.
It is, thus, important to constrain the partition behavior of Hf and W between silicate and metal for better understanding of the evolution of 182W isotopes. However, it has been difficult to determine the partition coefficients of Hf and W in laboratory experiments because of their low concentration and also the nugget effects of the highly siderophile elements such as W. In this study, we investigated the Hf-W partitioning as HfO2, HfO3 and WO2 between silicate melts and molten iron under high temperature and pressure conditions using ab initio free energy calculations. We also calculated whether the distribution coefficient changes when iron is mixed with silicate and vice versa. The distribution coefficients of W and Hf were not significantly affected by the addition of Fe to the silicate melt, while the mixing of Fe with O decreased and increased the distribution coefficients of W and Hf, respectively. Importantly, however, in both cases, W and Hf remained siderophile and lithophile, respectively, under the calculated conditions, which is consistent with the model in (2). In the future, it is necessary to clarify the evolution of the μ182W while taking into account the models in (2) and (3).
To understand the 182W variations in the mantle and their evolution, it is necessary to focus on the behavior of Hf and W under high pressure and the influence of extraterrestrial materials such as meteorite impacts. μ182W variations can be attributed to three possible processes. (1) Hf-W fractionation in the mantle. First-principles calculations (Deng and Stixrude, 2021) report that Hf is enriched in bridgmanite; however, the isotopic results are not consistent with the Hf enrichment of bridgmanite subducted into the lower mantle. (2) Hf-W fractionation occurred during core formation. (Mundl et al., 2017; Rizo et al., 2017) (3) Extraterrestrial material with low μ182W fell to the surface by intermittent impact.
It is, thus, important to constrain the partition behavior of Hf and W between silicate and metal for better understanding of the evolution of 182W isotopes. However, it has been difficult to determine the partition coefficients of Hf and W in laboratory experiments because of their low concentration and also the nugget effects of the highly siderophile elements such as W. In this study, we investigated the Hf-W partitioning as HfO2, HfO3 and WO2 between silicate melts and molten iron under high temperature and pressure conditions using ab initio free energy calculations. We also calculated whether the distribution coefficient changes when iron is mixed with silicate and vice versa. The distribution coefficients of W and Hf were not significantly affected by the addition of Fe to the silicate melt, while the mixing of Fe with O decreased and increased the distribution coefficients of W and Hf, respectively. Importantly, however, in both cases, W and Hf remained siderophile and lithophile, respectively, under the calculated conditions, which is consistent with the model in (2). In the future, it is necessary to clarify the evolution of the μ182W while taking into account the models in (2) and (3).