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

[SIT21-01] Sound velocity of hcp-iron to the Earth's inner core pressure: Implication for light elements in the core

*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, Inner core, High pressure, Inelastic x-ray scattering

The Earth's core has supposed to be mainly composed iron with some light elements. However, the only way we can directly observe the Earth's interior is through seismic observations, which show its density and velocity profile as a function of depth known as the preliminary reference Earth model (PREM) (ref. 1). Therefore, the physical properties of iron under high-pressure by lab-based high-pressure experiments are important knowledge to discuss the inner core in detail. To constrain the inner core, sound velocities and/or densities of iron and its alloys at high pressures has been measured and discussed comparing with PREM (e.g., refs. 2-5), though the discussion has been based on extrapolation of experimental results around 100-150 GPa due to experimental difficulties. Recently, we advanced the sound velocity measurement of hexagonal close-packed (hcp) iron to 13.2 g/cm³, over the PREM inner core densities (12.8-13.1 g/cm³), but its pressure was 250-270 GPa (ref. 6). In this study, we have conducted sound velocity measurements for hcp-iron to inner core pressure with IXS method by using a newly designed diamond anvil to stable high-pressure generations. The experimental density of hcp-iron reached up to 13.87 g/cm³, the maximum pressure is above 3 megabar (310-330 GPa) close to the pressure of inner core boundary. The relation between density and sound velocity shows linear relation known as Birch's law even under such extreme pressure conditions. Through combining present result with previous sound velocity measurements (e.g., refs. 3-4), we re-evaluated the pressure and temperature dependence of sound velocity for hcp-iron and its alloys. We discuss the estimation for the amount of light elements in the inner core to account both compressional and shear wave velocities of the PREM.

References:
[1] Dziewonski & Anderson. Phys. Earth Planet. Inter. 25, 297-356 (1981).
[2] Kamada et al. Am. Mineral. 99, 98-101 (2014).
[3] Sakamaki et al. Sci. Adv. 2, e1500802 (2016).
[4] Sakairi et al. Am. Min. 103, 85-90 (2018).
[5] Ikuta et al. Commun. Earth Environ. 2, 225 (2021).
[6] Ikuta et al. (2021, May 30-June 6). JpGU Meeting 2021, Online, Japan.