16:00 〜 16:15
[SGC38-03] 第一原理計算による溶融鉄-ケイ酸塩メルト間でのHf-W分配と182W同位体進化
キーワード：W同位体、第一原理計算、核－マントル相互作用
182Hf has a geologically short half-life of 8.9 million years, and the fractionation of Hf and W that occurred while 182Hf remains in the earth results in 182W isotopic variations. As Hf and W are lithophile and siderophile elements, respectively, Hf is thought to remain in the silicate melt phase while W is distributed in the metal melt phase during earth core formation. As a result, the metallic core should have low 182W compared with the mantle. Ocean island basalts from Hawaii and Samoa, whose source would be the lowermost mantle, show low μ182W (deviation from present-day upper mantle values in ppm), and these are considered evidence for the inclusion of core material. On the other hand, the μ182W values of Hedean and Archean rocks, such as komatiites, are positive, ranging from +10 to +20 ppm. However, some komatiites and diamictites, such as Schapenburg and Komati, have negative or zero μ182W values, respectively. To understand what these 182W isotopes indicate in the evolution of the earth, it is necessary to look at the behavior of Hf and W under high pressure and the effects of meteorite impact. The high and partly low μ182W values of many rocks of this age can be attributed to the following three main processes. (1) Hf-W fractionation in the mantle. Although it has been reported that Hf is enriched in bridgmanite based on first-principles calculations, this is not consistent with the isotopic results because the bridgmanite that sinks in the lower mantle is more enriched in Hf. (2) Material from the core with low μ182W entered the source material of komatiites originating from the deep mantle. For this to occur, Hf-W fractionation must occur between the core and mantle. (3) Extraterrestrial materials with low W isotopic ratios descended to the Earth's surface by late veneer and intermittent collisions, and these materials were incorporated into volcanic rocks and diamictites.
In this study, we investigated the partitioning of Hf-W between silicate melts and molten iron using first-principles free energy calculations under high-temperature and high-pressure conditions, which is difficult to achieve by laboratory experiments. We also calculated whether the partition coefficient changes when iron is mixed with silicate or vice versa. As a result, it was confirmed that W is distributed in iron and Hf is distributed in silicate melt in all cases. The partition coefficients of W and Hf were not largely affected by the addition of Fe to the silicate melt, while those of W and Hf were decreased and increased, respectively, when Fe was mixed with O. This proves that Hf-W partitioning can occur due to partitioning between core and mantle, resulting in μ182W variations.
The reason why many rocks from the deep mantle in the early Earth have high μ182W values while some rocks have low μ182W values is because the core material with low μ182W values in (2), or the core material containing extraterrestrial material in (2), or extraterrestrial material in (2), has high μ182W values while komatiites and other rocks originating in the early mantle formed in (2) have high μ182W values. The μ182W values suggest that the core material with lower μ182W values in (2), or the core material containing extraterrestrial material in (2), or diamictite containing extraterrestrial material existed, and that they were not well mixed up to about 2.5 Ga.
In this study, we investigated the partitioning of Hf-W between silicate melts and molten iron using first-principles free energy calculations under high-temperature and high-pressure conditions, which is difficult to achieve by laboratory experiments. We also calculated whether the partition coefficient changes when iron is mixed with silicate or vice versa. As a result, it was confirmed that W is distributed in iron and Hf is distributed in silicate melt in all cases. The partition coefficients of W and Hf were not largely affected by the addition of Fe to the silicate melt, while those of W and Hf were decreased and increased, respectively, when Fe was mixed with O. This proves that Hf-W partitioning can occur due to partitioning between core and mantle, resulting in μ182W variations.
The reason why many rocks from the deep mantle in the early Earth have high μ182W values while some rocks have low μ182W values is because the core material with low μ182W values in (2), or the core material containing extraterrestrial material in (2), or extraterrestrial material in (2), has high μ182W values while komatiites and other rocks originating in the early mantle formed in (2) have high μ182W values. The μ182W values suggest that the core material with lower μ182W values in (2), or the core material containing extraterrestrial material in (2), or diamictite containing extraterrestrial material existed, and that they were not well mixed up to about 2.5 Ga.