title: Constraints on Earth's inner core composition inferred from measurements of the sound velocity of hcp-iron in extreme conditions.
abstract: Hexagonal close-packed iron (hcp-Fe) is a main component of Earth's inner core. The difference in density between hcp-Fe and the inner core in the Preliminary Reference Earth Model (PREM) shows a density deficit, which implies an existence of light elements in the core. Sound velocities then provide an important constraint on the amount and kind of light elements in the core. Although seismological observations provide density-sound velocity data of Earth's core, there are few measurements in controlled laboratory conditions for comparison. We report the compressional sound velocity (V P) of hcp-Fe up to 163 GPa and 3000 K using inelastic x-ray scattering from a laser-heated sample in a diamond anvil cell. We propose a new high-temperature Birch's law for hcp-Fe, which gives us the V P of pure hcp-Fe up to core conditions. We find that Earth's inner core has a 4 to 5% smaller density and a 4 to 10% smaller V P than hcp-Fe. Our results demonstrate that components other than Fe in Earth's core are required to explain Earth's core density and velocity deficits compared to hcp-Fe. Assuming that the temperature effects on iron alloys are the same as those on hcp-Fe, we narrow down light elements in the inner core in terms of the velocity deficit. Hydrogen is a good candidate; thus, Earth's core may be a hidden hydrogen reservoir. Silicon and sulfur are also possible candidates and could show good agreement with PREM if we consider the presence of some melt in the inner core, anelasticity, and/or a premelting effect.
authors.affiliation: Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai, Miyagi 980-8578, Japan.
authors.forename: Tatsuya
authors.initials: T
authors.lastname: Sakamaki
authors.affiliation: Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai, Miyagi 980-8578, Japan.; V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090, Russia.
authors.forename: Eiji
authors.initials: E
authors.lastname: Ohtani
authors.affiliation: Center for Novel Material Science under Multi-Extreme Conditions, Graduate School of Material Science, University of Hyogo, Kamigori, Hyogo 678-1297, Japan.; Materials Dynamics Laboratory, RIKEN SPring-8 Center, Sayo, Hyogo 679-5148, Japan.
authors.forename: Hiroshi
authors.identifier: 0000-0002-7880-635X
authors.initials: H
authors.lastname: Fukui
authors.affiliation: Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai, Miyagi 980-8578, Japan.; Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai Miyagi 980-8578, Japan.
authors.forename: Seiji
authors.initials: S
authors.lastname: Kamada
authors.affiliation: Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai, Miyagi 980-8578, Japan.
authors.forename: Suguru
authors.initials: S
authors.lastname: Takahashi
authors.affiliation: Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai, Miyagi 980-8578, Japan.
authors.forename: Takanori
authors.initials: T
authors.lastname: Sakairi
authors.affiliation: Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai, Miyagi 980-8578, Japan.
authors.forename: Akihiro
authors.initials: A
authors.lastname: Takahata
authors.affiliation: Geodynamics Research Center, Ehime University, Matsuyama, Ehime 790-8577, Japan.
authors.forename: Takeshi
authors.initials: T
authors.lastname: Sakai
authors.affiliation: Research and Utilization Division, Japan Synchrotron Radiation Research Institute, SPring-8, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Tsutsui
authors.affiliation: Research and Utilization Division, Japan Synchrotron Radiation Research Institute, SPring-8, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Ishikawa
authors.affiliation: Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai, Miyagi 980-8578, Japan.
authors.forename: Rei
authors.initials: R
authors.lastname: Shiraishi
authors.affiliation: Department of Earth and Planetary Sciences, Kobe University, Kobe 657-8501, Japan.
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Seto
authors.affiliation: Geodynamics Research Center, Ehime University, Matsuyama, Ehime 790-8577, Japan.
authors.forename: Taku
authors.identifier: 0000-0002-6042-3692
authors.initials: T
authors.lastname: Tsuchiya
authors.affiliation: Materials Dynamics Laboratory, RIKEN SPring-8 Center, Sayo, Hyogo 679-5148, Japan.; Research and Utilization Division, Japan Synchrotron Radiation Research Institute, SPring-8, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan.
authors.forename: Alfred Q R
authors.initials: AQ
authors.lastname: Baron
keywords: Earth’s core; High Temperature; density; hexagonal close-packed iron; high pressure; inelastic x-ray scattering; laser heated diamond anvil cell; sound velocity; velocity dificit