JpGU-AGU Joint Meeting 2020

Presentation information

[E] Oral

S (Solid Earth Sciences ) » S-IT Science of the Earth's Interior & Techtonophysics

[S-IT23] Property and role of liquids inside terrestrial planets

convener:Tatsuya Sakamaki(Department of Earth Science, Tohoku University), Yoichi Nakajima(Kumamoto University, Priority Organization for Innovation and Excellence)

[SIT23-06] Equation of State for Liquid Iron under Extreme Conditions

★Invited Papers

*Yasuhiro Kuwayama1, Guillaume Morard2, Yoichi Nakajima3,4, Kei Hirose1,5, Alfred Q. R. Baron4, Saori I Kawaguchi6, Taku Tsuchiya7, Daisuke Ishikawa6,4, Naohisa Hirao6, Yasuo Ohishi6 (1.The University of Tokyo, 2.Laboratoire ISTERRE, Université Grenoble Alpes, 3.Kumamoto University, 4.RIKEN MDR, 5.Tokyo Institute of Technology, 6.JASRI, 7.Ehime University)

Keywords:liquid iron, equation of state, density, sound velocity, high pressure

The Earth’s core is composed mainly of iron and almost molten. Likewise, Mercury and Mars are also expected to have a partially molten iron core. Since density (ρ) and longitudinal sound velocity (VP) are the primary observables of the Earth’s liquid outer core and possibly for other planets in future, laboratory measurements on these properties at relevant high-pressure conditions are of great importance to understand the core composition and dynamics of Earth and other terrestrial planets. Our knowledge for those properties of liquid iron under high-pressure and -temperature (P-T) conditions is, however, completely limited because of the experimental difficulties. In this study, we have determined the ρ and VP of liquid iron up to 116 GPa and 45 GPa, respectively, via static compression in a laser-heated diamond-anvil cell (LH-DAC).
We determined the ρ of liquid iron up to 116 GPa and 4350 K based on in-situ x-ray diffraction measurements at BL10XU, SPring-8 [1]. A new analytical method was applied to derive ρ from the diffuse x-ray scattering signals from the liquid. We also obtained the VP of liquid iron up to 45 GPa by inelastic x-ray scattering (IXS) measurements in the LH-DAC at BL43LXU, SPring-8 [2]. From our new data combined with previous shock-wave data, we obtained the P–TρVPγ relation for the Earth’s entire outer core conditions. Compared to the ρ, VP, and adiabatic bulk modulus (KS) of liquid iron calculated along the isentrope with TICB (the temperature at the inner core boundary) = 5400 K, the Earth’s outer core have 7.5–7.6% lower ρ, 3.7–4.4% higher VP but an almost identical KS.
Seismology gives the density difference between the liquid and solid core at the ICB; ΔρICB = 0.55–0.82 g/cm3 (e.g., [3]). Our results show that liquid iron is less dense than hexagonal-close-packed (hcp) iron [4] by = 0.32 g/cm3 at 330 GPa and its melting point of 6230 K [5]. This is approximately half of the observed ΔρICB, indicating that the remaining 0.23–0.50 g/cm3 (corresponding to 1.9–4.1% of the outer core density at the ICB) should be attributed to a compositional difference between the outer and inner core.
[1] N. Hirao, S. I. Kawaguchi, K. Hirose, K. Shimizu, E. Ohtani, and Y. Ohishi, Matter Radiat. Extrem. 5, 018403 (2020).
[2] A. Q. R. Baron, SPring-8 Inf. Newsl. 15, 14 (2010).
[3] G. Masters and D. Gubbins, Phys. Earth Planet. Inter. 140, 159 (2003).
[4] A. Dewaele, P. Loubeyre, F. Occelli, M. Mezouar, P. I. Dorogokupets, and M. Torrent, Phys. Rev. Lett. 97, 1 (2006).
[5] S. Anzellini, A. Dewaele, M. Mezouar, P. Loubeyre, and G. Morard, Science 340, 464 (2013).