[S-IT21] Planetary cores: Structure, formation, and evolution

[SIT21-P01] Sound velocity measurement of hcp-iron above 3 megabar with a new type diamond anvil cell

*Daijo Ikuta¹, Eiji Ohtani¹, Hiroshi Fukui², Takeshi Sakai³, Daisuke Ishikawa^2,4, Alfred Q. R. Baron^2,4 (1.Department of Earth Science, Tohoku University, 2.JASRI, 3.Geodynamics Research Center, Ehime University, 4.RIKEN)

Keywords:Sound velocity, Iron, High-pressure generation, Inelastic x-ray scattering

The Earth's inner core is considered to be iron alloys with some light elements that is under pressures above 3 megabar. To elucidate the property of the inner core precisely without extrapolations, many lab-based high-pressure experiments toward the inner core conditions had been attempted with diamond anvil cells. However, even with diamond, the hardest material we know, there are still many difficulties in stable pressure generation and in-situ observation experiments over multi-megabar regions. To solve these problems, several types of improving the shape of diamond anvils have been proposed, such as double-stage anvils and/or toroidal-shape anvils (e.g., ref. 1-3). Recently, sound velocity of hexagonal close-packed (hcp) iron has been measured up to 2.5 megabar by using a double beveled diamond anvil, but further high-pressure generations and measure nets were quite difficult due to some issues like damage to diamonds (ref. 4). In this study, we have conducted sound velocity measurements for hcp-iron with inelastic x-ray scattering (IXS) method to the Earth's inner core pressure above 3 megabars, by using a newly designed diamond anvil for extreme pressure generations, and the Soller screen system (ref. 5) reducing noises as much as possible. The experiments were succeeded to obtained reasonable IXS peaks from hcp-iron (though it is quite close to the IXS peaks from diamond that is cause of annoyance for high-pressure IXS measurements). The experimental density of hcp-iron reached up to 13.87 g/cm³, the maximum pressure is above 3 megabar (310-330 GPa, depends on pressure scales), and we succeeded to derive sound velocity of hcp-iron at the Earth's inner core pressures. Here we report the high-pressure sound velocity measurements of hcp-iron with newly developed diamond anvil cell above 3 megabar.

References:
[1] Sakai et al. High Press. Res. 38, 107-119 (2018).
[2] Dewaele et al. Nat. Commun. 9, 2913 (2018).
[3] Yagi et al. High Press. Res. 40, 148-161 (2020).
[4] Ikuta et al. (2021, May 30-June 6). JpGU Meeting 2021, Online, Japan.
[5] Baron et al. AIP Conf. Proc. 2054, 020002 (2019).