Name	Institution	Roles
Eiji Ohtani	Tohoku University, Graduate school of Science	Planning of the high pressure and temperature experiments in general
Motohiko Murakami	Tohoku University, Graduate school of Science	Velocity measurements by brillouin scattering and X-ray inelastic scattering
Akio Suzuki	Tohoku University, Graduate school of Science	Experiments for measuring physical properties of metallic melts
Hidenori Terasaki	Tohoku University, Graduate school of Science	High pressure and temperature experiments on core formation
Naohisa Hirao	Japan Synchrotron Radiation Research Institute	In situ high pressure and temperature X-ray diffraction and Mossbauer spectroscopy
Takeshi Sakai	Tohoku University, Tohoku University International Advanced Research and Education Organization	X-ray diffraction and inelastic X-ray scattering experiments at high pressure

　　This project aims to clarify the constitution and evolution of the deep region of the Earth, such as the outer and inner core, the core–mantle boundary (CMB), and the lower mantle. Recent work has clarified several key transitions at the base of the lower mantle, such as a spin crossover in ferropericlase and the post-perovskite transition. There is an intensive debate over the explanation of the seismic anomaly of the ultralow-velocity region at the core–mantle boundary, including proposals such as the accumulation of subducting slabs, partial melting at the CMB, and the existence of the post-perovskite transition. However, there is no consensus regarding an explanation of the seismic anomaly at the CMB. Studies on the outer and inner cores are at the frontier of Earth science, because of the limited data on the sound velocity in the core materials under conditions at the core.

　　We have conducted research to clarify the constitution and evolution of the Earth’s core using a Grant-in-Aid for Scientific Research (S) from the JSPS. In this project, we achieved several challenging targets. We achieved the pressure and temperature conditions at the CMB [1, 2], a pressure of 135 GPa and a temperature of 3500 K. We also achieved a pressure 254 GPa and a temperature of 3500 K based on the NaCl B2 pressure scale of Fei et al. [3], and we confirmed the stability of the hcp FeSi phase under experimental conditions [4]. We also compressed FeNiSi alloy up to 374 GPa, which exceeds the center pressure of the Earth, and found that the hcp phase was stable at such high pressure. We also successfully conducted density measurements on molten Fe-S and Fe-Si alloys up to a pressure of 10 GPa and a temperature of 2000 K using sink–float experiments and X-ray radiography [5]. These density data provide basic information for analysis of the constitution of the outer core.

　　Our previous research was focused on the structure and density of the core and lower mantle materials, and was mainly based on in situ X-ray diffraction experiments at high pressures and temperatures, and we made significant advances in the understanding the central part of the Earth. However, several problems need to be solved to achieve a better understanding of this part of the Earth.

　　Currently, the pressure scale is not accurate enough to be used to qualify quantitative arguments about the Earth’s core. The difference in pressure at 300 GPa among the existing pressure scales exceeds 30 GPa, which corresponds to the pressure difference between the inner core boundary (ICB) and the center of the Earth [3, 6]. To solve this problem requires the construction of a primary pressure scale from simultaneous measurements of the density and sound velocity [7]. Thus, establishing this primary pressure scale under conditions at the core is indispensable to qualify quantitative arguments about the Earth’s core. The most accurate observations from seismology concern the seismic velocity. The density profiles derived from seismology are not accurate enough to constrain a unique model of the core and the lower mantle, although there have been some reports on sound velocity measurements of metallic iron alloys at pressures at the Earth’s core using nuclear resonance inelastic scattering (NRIXS) [8] and inelastic X-ray scattering (IXS) [9]. However, the data are still too limited to model the constitution of the core. Therefore, it is vital to determine the sound velocity of the materials that compose the Earth’s deep interior.

Despite intensive effort to clarify the spin crossover under lower mantle conditions, there is no consensus on the spin states of iron in perovskite and post-perovskite because of the multiple sites available for occupying ferric and ferrous iron in these phases. Some magnetic transitions have been observed in metallic iron alloys at high pressure, such as iron hydride. The effect of these transitions on the density and velocity of the lower mantle and core has not yet been clarified.

Our research project has three major objectives.

　　The first objective is to generate pressure and temperature conditions that cover the center of the　 Earth. We will make simultaneous measurements of the compression and sound velocity of MgO and the B1 and B2 phases of NaCl to establish a primary pressure scale for core conditions. We will also establish routine procedures for the generation of high temperatures exceeding 3000 K at core pressures, and we will perform in situ X-ray observations under these conditions. The measurements will be conducted at the BL10XU beamline at the SPring-8 facility.

　　The second objective is to clarify the nature of the various transitions occurring in the lower mantle and core. These transitions include spin crossovers and the post-perovskite transition in lower mantle minerals, and magnetic transitions in iron alloys at high pressures and temperatures. To clarify these phase transitions, we will introduce X-ray Mossbauer spectroscopy using the nuclear analyzer energy domain method [10], together with the conventional X-ray powder diffraction method at high pressures and temperatures at the BL10XU beamline at the SPring-8 facility. This procedure will make it possible to determine the valence states of the iron, the spin states of the iron in lower mantle silicates, and the magnetic properties of iron alloys in microsamples under extreme conditions. We will expand our measurements up to the pressures and temperatures at the core.

　　The third objective is to clarify the sound velocities of the lower mantle and core materials. A unique mineralogical model has not yet been proposed for the central region of the Earth because of a lack of reliable data on the sound velocity of the deep Earth’s interior. We will perform Brillouin scattering spectroscopy to determine the sound velocities of the lower mantle materials and the effect of the phase transitions existing at the base of the lower mantle using this system at the BL10XU beamline at the SPring-8 facility. We also plan to introduce a system for precise measurements of the sound velocity of single crystals in the laboratory. We will also perform sound velocity measurements at high pressures and temperatures using inelastic X-ray scattering (IXS) spectroscopy of iron alloys and lower mantle minerals. We will clarify the sound velocities of the lower mantle and core materials and the effect of the phase transitions on the sound velocity at high pressures and temperatures at the BL35XU beamline at the SPring-8 facility.
Previous mineralogical models of the Earth’s deep interior have been limited to the density model because of a lack of reliable data on the seismic velocities of the materials in the lower mantle and core.

　　Our goal is to present an advanced model of the lower mantle and core that can explain both the seismic velocities and the density observed in seismology. We can break through the current limit in our understanding of the Earth’s deep interior with the results of this project.