[S-IT17] Property and role of liquids inside terrestrial planets

[SIT17-06] Density and Sound Velocity of Liquid Fe and FeO at High Pressure

*Yasuhiro Kuwayama¹, Kei Hirose^1,2, Yoichi Nakajima^3,4, Yuki Adachi³, Alfred Q.R. Baron⁴, Guillaume Morard⁵, Saori I. Kawaguchi⁶, Daisuke Ishikawa^6,4, Naohisa Hirao⁶, Yasuo Ohishi⁶ (1.The University of Tokyo, 2.Tokyo Institute of Technology, 3.Kumamoto University, 4.RIKEN MDR, 5.Laboratoire ISTERRE, Université Grenoble Alpes, 6.JASRI)

Keywords:liquid, density, sound velocity, high-pressure, core

The Earth's core is almost molten. While the dominant component of the core is believed to be iron, a large amount of lighter impurities can be dissolved into the core. Oxygen is a possible candidate for the light impurity in the liquid outer core. Seismological observations exhibit the presence of a low-velocity layer (LVL) at the top of the Earth's core, which might be attributed to the enrichment of oxygen [1,2]. Furthermore, the enrichment of the FeO component and/or partial melting are considered to be the origin of the ultra-low velocity zone (ULVZ) at the base of the lower mantle (e.g. [3]). Therefore, the density (ρ) and longitudinal sound velocity (V_P) of Fe-O liquids under relevant high-pressure and -temperature conditions are of great importance to understand those seismological anomalies. Here we present those elastic properties of liquid Fe and FeO under high-pressure and -temperature (P-T) conditions.

We have determined the ρ of liquid Fe up to 116 GPa based on in-situ x-ray diffraction measurements at BL10XU, SPring-8, using a laser-heated diamond-anvil cell (LH-DAC) [4]. The V_P of liquid Fe was obtained up to 45 GPa by inelastic x-ray scattering (IXS) measurements in the LH-DAC at BL43LXU, SPring-8 [5]. From these results combined with previous shock-wave data, we obtained the P–T–ρ–V_P–γ relation for the Earth's entire outer core conditions [6]. Also, we determined the V_P of liquid FeO up to 82 GPa using the IXS technique. The present results show that the V_P of liquid FeO is faster than that of liquid Fe by ~15% at the condition of the Earth's core-mantle boundary (CMB). Since the FeO component increases the P-wave velocity of liquid iron alloy, the enrichment of FeO cannot be the origin of the low-velocity anomaly at the top of the outer core. On the other hand, the V_P of liquid FeO is slower by ~35% relative to lower mantle minerals at the base of the lower mantle, so that the ULVZ can be attributed to a tiny amount of liquid FeO component.

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[6] Y. Kuwayama, G. Morard, Y. Nakajima, K. Hirose, A. Q. R. Baron, S. I. Kawaguchi, T. Tsuchiya, D. Ishikawa, N. Hirao, and Y. Ohishi, Phys. Rev. Lett. 124, 165701 (2020).