[S-IT16] Planetary cores: Structure, formation, and evolution

[SIT16-09] Sound velocity measurement for high-pressure phase of iron sulfide compounds at high pressure and high temperature

*Daijo IKUTA¹, Eiji OHTANI², Hiroshi FUKUI^3,4, Tatsuya SAKAMAKI², Daisuke ISHIKAWA^4,3, Alfred Q. R. BARON^4,3 (1.Institute for Planetary Materials, Okayama University, 2. Department of Earth Science, Tohoku University, 3.JASRI, 4.RIKEN)

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

The most reliable knowledge of the Earth's interior is based on seismic observations. The Preliminary Reference Earth Model (PREM) [1], a seismic model, provides the profile of sound velocity and density as a function of depth. Comparing the model with laboratory high-pressure experiments, pure iron cannot account for the sound velocity and density of the PREM inner core [2]. Geochemical and meteorite studies [3] suggest that the Earth's inner core is composed of an alloy of iron and nickel containing some light elements such as sulfur, oxygen, carbon, hydrogen, and silicon. Of these, sulfur is one of the most likely candidates for light elements, both from its cosmochemical abundance and from studies of its chemical reactions in the evolution of the Earth's core. Therefore, sound velocity measurements in the Fe-S system are important for understanding the properties of the Earth's core and constraining its chemical composition. However, sound velocity measurements in the Fe-S system under high pressure and temperature conditions have not been well studied [4].
In the Fe-S system, Fe2S is the stable phase of iron sulfide under Earth's inner core pressure [5]. Therefore, experimental knowledge of the sound velocity of this phase is important for understanding and constraining the composition of Earth's inner core. In this study, we have measured the sound velocity for the first time and density of Fe2S up to 130 GPa and 2300 K using inelastic x-ray scattering (IXS) and x-ray diffraction (XRD) methods at BL43LXU of SPring-8. The IXS intensities were measured using a total of 16 analyzer crystals installed in BL43LXU, and a Soller screen system [6] to reduce noise as much as possible. The sound velocity was derived from the relation between the change in momentum transfer and the change in energy. The XRD patterns were measured under the same conditions using the flat panel detector, and the density of the sample was calculated from the patterns. These results provide us the knowledge of the relations between sound velocity and density and its temperature dependence.
We discuss the light elements contained in the Earth's inner core by comparing our results on the sound velocity and its temperature dependence of iron sulfide at high pressure and high temperature conditions with PREM.

References:
[1] Dziewonski and Anderson, Phys. Earth Planet. Inter. 25, 297-356 (1981).
[2] Ikuta et al., Nat. Commun. 13, 7211 (2022).
[3] McDonough, Treatise on Geochemistry 2nd Ed. 3, 559-577 (2014).
[4] Kamada et al., Am. Mineral. 99, 98-101 (2014).
[5] Tateno et al., Geophys. Res.Lett. 46, 11944-11949 (2019).
[6] Baron et al., AIP Conf. Proc. 2054, 020002 (2019).