[S-IT18] Planetary cores: Structure, formation, and evolution

[SIT18-P01] Sound velocity of hcp-iron at extreme pressure by inelastic x-ray scattering measurement

*Daijo Ikuta¹, Eiji Ohtani¹, Hiroshi Fukui^2,3, Daisuke Ishikawa³, Alfred Q. R. Baron³ (1.Department of Earth Science, Tohoku University, 2.Department of Material Science, University of Hyogo, 3.RIKEN)

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

The Earth's core has supposed to be composed mostly of iron (over about 85 wt%), therefore the physical properties of iron at extreme conditions is one of the most important knowledge to the discussion of Earth's core. Since we cannot directly reach the Earth's core, the most reliable knowledge of the Earth's core comes from seismological observations, which show its density and velocity profile as a function of depth known as the preliminary reference Earth model (PREM). To constrain the Earth's core, sound velocities of iron and iron alloys at high pressures has been measured and compared with seismological observation (e.g., refs. 1-4). So far, the discussion has been based on extrapolation of experimental results around 100-150 GPa, for more detailed constraints, higher pressure experiments without extrapolation are necessary. Recent advances of high-pressure generation techniques using diamond anvil cells (DAC) and strong x-ray by synchrotron radiation have made it possible to perform the inelastic x-ray scattering (IXS) experiments to measure the sound velocity of hexagonal close-packed (hcp) iron at core pressures (e.g., ref. 1). However, a new problem has arisen as the pressure became closer to the core's pressure. That is, the scattering from diamond, which is used for high pressure generation, overlaps the scattering from samples at extreme conditions, thus it is hard to distinguish the IXS signals of the sample from those of the diamond, and the diamond becomes a wall of the IXS measurement limit (e.g., ref. 1). In this study, we have conducted sound velocity measurements for hcp-iron with IXS method by using a newly designed the solar screen system (ref. 5) that reduced noise as much as possible, and challenged the present limitation of sound velocity measurements of hcp-iron close to the "diamond's wall". The experimental density of hcp-iron reached to >12.8 g/cm³ (exceeding the inner core density at inner-core boundary) and 250-270 GPa, and we succeeded to obtained clear IXS peaks from hcp-iron and derived sound velocity at extreme pressures. We discuss the composition of the Earth's core based on the present sound velocity data by comparing with the PREM.

References:
[1] Sakamaki et al., Sci. Adv. 2, e1500802 (2016).
[2] Sakairi et al., Am. Min. 103, 85-90 (2018).
[3] Dominijanni et al., JpGU-AGU joint meeting 2020.
[4] Ikuta et al., JpGU-AGU joint meeting 2020.
[5] Baron et al., AIP Conf. Proc. 2054, 020002 (2019).