日本地球惑星科学連合2022年大会

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[E] ポスター発表

セッション記号 S (固体地球科学) » S-IT 地球内部科学・地球惑星テクトニクス

[S-IT17] 地球型惑星内部での液体の特性とその役割

2022年5月30日(月) 11:00 〜 13:00 オンラインポスターZoom会場 (21) (Ch.21)

コンビーナ:坂巻 竜也(東北大学大学院理学研究科)、コンビーナ:中島 陽一(熊本大学大学院先導機構)、座長:坂巻 竜也(東北大学大学院理学研究科)、中島 陽一(熊本大学大学院先導機構)

11:00 〜 13:00

[SIT17-P03] Sound velocity of liquid Fe-S at high-pressure

*中島 陽一1,2岩本 朝希1、上野 雄生1桑山 靖弘3廣瀬 敬3,4Morard Guillaume5、石川 大介2,6、Baron Alfred2,6 (1.熊本大学、2.理化学研究所物質ダイナミクス研究グループ、3.東京大学、4.東京工業大学 地球生命研究所、5.Grenoble Univ. Alps、6.高輝度光科学研究センター)

キーワード:コア軽元素、液体鉄合金、P波速度、高圧

The Earth’s liquid outer core consists of iron alloy with ~5 wt.% nickel and ~10 wt.% lighter elements [1]. Sulfur (S) is one of the candidates for the core light-elements. Iron-sulfides are commonly found in meteorites, and the concentrations of S are relatively high as 2-6 wt% S in a variety of chondrites [2]. On the other hand, S is depleted in Earth’s mantle [3]. A recent study on the element partitioning between magma ocean and core-forming metal proposed an upper limit of S in the core to be 6.3 wt% [4]. To examine the possibility of sulfur in the core, we have measured the P-wave velocity of liquid Fe-S alloy up to 100 GPa and 3000 K using inelastic X-ray scattering (IXS) from a sample in a laser-heated diamond-anvil cell (LH-DAC).
We carried out IXS measurements at the RIKEN Quantum NanoDynamics beamline BL43LXU of SPring-8 [5]. The starting specimen was a foil of Fe83S17, which was loaded in LH-DAC together with single-crystal Al2O3 discs acting as the thermal- and chemical-insulator. The sample melting was confirmed upon laser-heating before and after each IXS measurement based on X-ray diffraction patterns. The IXS spectra were collected in a momentum transfer range of 3-5.7 nm-1 with an energy resolution of ~2.8 meV at 17.79 keV. The P-wave velocity of liquid Fe83S17 was determined from the dispersion relation of the longitudinal acoustic phonon mode of the liquid. Sulfur was found to increase the P-wave velocity of liquid Fe [6], while the effect is gentle. Using the present P-wave data-set, we constructed an equation of state (EoS) for liquid Fe83S17 and estimated the P-wave velocity and density under the core P-T conditions. Comparing with seismological observations of the outer core, we found liquid Fe alloying with 6-7 wt% S to satisfy both P-wave velocity and density of the outer core simultaneously, which is similar to the S content based on previous element-partitioning experiments [4]. The present results imply that S can be the dominant light-element in the Earth’s outer core.

[1] Stevenson, Science 214, 611-619 (1981).
[2] Wasson and Kallemeyn, Phil. Trans. R. Soc. Lond. A 325, 535-544 (1988).
[3] McDonough. Treatise on Geochemistry (Second Edition), 3, 559–577 (2014).
[4] Mahan et al. Geohim. Cosmochim. Ata 196, 252-270 (2017).
[5] Baron, SPring-8 Inf. Newsl. 15, 14-19 (2010).
[6] Kuwayama et al. Phys. Rev. Lett. 124(16), 165701 (2020).