5:15 PM - 7:15 PM
[SGC38-P03] First principle calculation of W and Hf partitioning and resultant 182W isotope evolution
Keywords:W isotope, Partitioning of Hf and W, Core-mantle interaction, first principle calculation
The 182W isotopes are considered to be one of the few chemical information carriers of information about the Earth's core. Therefore, analytical techniques have been developed in anticipation of 182W isotope anomalies as traces of core-mantle interactions in oceanic island basalts and flood basalts originating from the core-mantle boundary.
The anomalies in 182W are due to differences in the core-mantle partitioning of Hf and W in early Earth's magma oceans. Because Hf is the lithophile element and W is the siderophile element, it is thought that Hf is partitioned into silicate melts and W is partitioned into the molten metal core during the formation of the Earth's core. This partitioning occurs before the disappearance of 182Hf with a short half-life (8.9 million years: Vockenhuber et al., 2004), resulting in isotopic variations in the 182W isotope. As a result, the early mantle has a high μ182W value (difference from the present-day upper mantle value in ppm) and the metallic core has a low μ182W. Some time after core formation, the late veneer material with the chemical composition of the so-called meteorite, i.e., low 182W, lowers the W isotopic ratios of the early mantle to values of the present upper mantle. In rocks derived from the early mantle, such as komatiites, μ182W values are positive, ranging from +10 to +20 ppm (see, e.g., Mei et al., 2019), which is the value of the primordial mantle. However, some oceanic island basalts (OIB) originating from the lowest mantle of the present day, such as Hawaii and Samoa, have been reported to show negative μ182W (e.g. Mundl et al., 2017; Rizo et al., 2019), and a model has been proposed that these are evidence for the presence of core material in the OIB.
To test this model and to understand the evolution of 182W in the Earth's deep interior, attention should be paid to the behavior of Hf and W under high pressure and temperature and the influence of extraterrestrial materials such as meteorite impacts. However, high-temperature and high-pressure experiments are not easy, especially for trace siderophilic elements such as W because they tend to form nuggets in rocks and cannot be homogeneous. Therefore, in this study, the partition of Hf-W between silicate melts and molten iron under high pressure and pressure conditions, which is difficult in laboratory experiments, was investigated using ab initio free energy calculations on the partitioning of HfO2 and WO2. Since it was pointed out that Hf in the mantle exists as HfO3, we further calculated the partitioning of HfO3 in the present study. We also calculated whether the distribution coefficient changes when a small amount of iron is added to silicate and vice versa. In any case, W remained siderophilic and Hf lithophilic under the calculated conditions. The partition coefficients of W and Hf were not significantly affected by the addition of iron to the silicate melt. The partition coefficients of W and Hf to the molten iron decreased and increased, respectively, when oxygen was added to the molten iron. The most important aspect of the present results is that W and Hf are partitioned into molten core and silicate melt, respectively, which is consistent with the model described above. Furthermore, calculations of the structures of Hf and W suggest that both W-Fe and W-Hf in molten iron are similar to the Fe-Fe distance, suggesting that both are replacing sites of iron in molten iron. The distribution behavior of Hf and W to iron is very different, whereas the sites they contain are the same. It is interesting to note that the sites contain.
In the future, the evolution of the isotopic ratios of W should be clarified based on the calculated results, compared to the analysis of μ182W values of rocks from different periods.
The anomalies in 182W are due to differences in the core-mantle partitioning of Hf and W in early Earth's magma oceans. Because Hf is the lithophile element and W is the siderophile element, it is thought that Hf is partitioned into silicate melts and W is partitioned into the molten metal core during the formation of the Earth's core. This partitioning occurs before the disappearance of 182Hf with a short half-life (8.9 million years: Vockenhuber et al., 2004), resulting in isotopic variations in the 182W isotope. As a result, the early mantle has a high μ182W value (difference from the present-day upper mantle value in ppm) and the metallic core has a low μ182W. Some time after core formation, the late veneer material with the chemical composition of the so-called meteorite, i.e., low 182W, lowers the W isotopic ratios of the early mantle to values of the present upper mantle. In rocks derived from the early mantle, such as komatiites, μ182W values are positive, ranging from +10 to +20 ppm (see, e.g., Mei et al., 2019), which is the value of the primordial mantle. However, some oceanic island basalts (OIB) originating from the lowest mantle of the present day, such as Hawaii and Samoa, have been reported to show negative μ182W (e.g. Mundl et al., 2017; Rizo et al., 2019), and a model has been proposed that these are evidence for the presence of core material in the OIB.
To test this model and to understand the evolution of 182W in the Earth's deep interior, attention should be paid to the behavior of Hf and W under high pressure and temperature and the influence of extraterrestrial materials such as meteorite impacts. However, high-temperature and high-pressure experiments are not easy, especially for trace siderophilic elements such as W because they tend to form nuggets in rocks and cannot be homogeneous. Therefore, in this study, the partition of Hf-W between silicate melts and molten iron under high pressure and pressure conditions, which is difficult in laboratory experiments, was investigated using ab initio free energy calculations on the partitioning of HfO2 and WO2. Since it was pointed out that Hf in the mantle exists as HfO3, we further calculated the partitioning of HfO3 in the present study. We also calculated whether the distribution coefficient changes when a small amount of iron is added to silicate and vice versa. In any case, W remained siderophilic and Hf lithophilic under the calculated conditions. The partition coefficients of W and Hf were not significantly affected by the addition of iron to the silicate melt. The partition coefficients of W and Hf to the molten iron decreased and increased, respectively, when oxygen was added to the molten iron. The most important aspect of the present results is that W and Hf are partitioned into molten core and silicate melt, respectively, which is consistent with the model described above. Furthermore, calculations of the structures of Hf and W suggest that both W-Fe and W-Hf in molten iron are similar to the Fe-Fe distance, suggesting that both are replacing sites of iron in molten iron. The distribution behavior of Hf and W to iron is very different, whereas the sites they contain are the same. It is interesting to note that the sites contain.
In the future, the evolution of the isotopic ratios of W should be clarified based on the calculated results, compared to the analysis of μ182W values of rocks from different periods.