[S-IT15] Study of the Earth's Deep Interior - Interaction and Coevolution of the Core and Mantle

[SIT15-05] Hf and W partitioning between liquid metal and liquid silicate and resultant core-mantle interaction signature on the ¹⁸²W isotope of the Afar plume

*Katsuhiko Suzuki¹, Asako Takamasa^2,1, Taku Tsuchiya³, Yusuke Fukami⁴, Yuji Orihashi⁵, Ryuichi Shinjo⁶ (1.Submarine Resources Research Center, Japan Agency for Marine-Earth Science and Technology, 2.JAPAN NUS Co., Ltd, 3.Geodynamics Research Center, Ehime University, 4.Department of Chemistry, Gakushuin University, 5.Department of Earth and Environmental Sciences, Hirosaki University, 6.Department of Physics and Earth Sciences, University of the Ryukyus)

Keywords:core-mantle interaction, partitioning of Hf and W, W isotope

Low ¹⁸²W/¹⁸⁴W relative to the average value of modern mantle have been found in certain modern ocean island basalts such as the ones from Hawaii and Samoa (i.e, Rizo et al., 2019; Mundl-Petermeier et al., 2020). The most plausible process among the proposed models for such ¹⁸²W variations is the one including the fractionation of Hf-W in core separation from the mantle due to the affinity of W to metal. This process leads to a low ¹⁸²W of the metallic core, which may have contributed to the source of the ocean island basalt.
Here we report the evidence of the Hf-W fractionation under the lowermost mantle condition in core segregation in the early Earth. First-principles free energy calculations reveal that W is strongly partitioned into the liquid metal phase, while Hf remains in the molten silicate even at the core-mantle boundary P,T condition. Such Hf-W fractionation imparts a low ¹⁸²W property to the core.
We present slightly negative ¹⁸²W anomalies relative to the average present-day mantle for the Ethiopian basalts, which is closely related to the Afar mantle plume and non-resolvable ¹⁸²W isotope for the Aden Bay MORB. The basalt samples were analyzed following the modified procedure from Takamasa et al. (2020). The low ¹⁸²W isotope values indicated the Afar mantle plume generating the Ethiopian basalts were originated from the lowermost mantle and likely contained the core component with low ¹⁸²W isotope values, which was likely to be diffused from the core (Yoshino et al., 2019).