High-Pressure Earth Science
Exploring Structures of Materials in Earth's Deep Interior
Observing Rocks under High-pressure, High-temperature Conditions with the “Eye of Light”
Our planet consists of a layered structure from the surface to the center (Fig. 1). The depths and thicknesses of these layers can be determined by analyzing seismic wave propagation utilizing their large density differences. However, this analysis requires information about the structures and properties of Earth's layers, and consequently, the rocks (minerals) that constitute the layers must be investigated. Currently the drilling depth is limited to only ~10 km, which is extremely small compared to Earth's radius of ~6,400 km. Therefore, high-pressure, high-temperature experiments that artificially reproduce the conditions of Earth's interior are very useful in these investigations.
In high-pressure, high-temperature experiments, experimental samples are encapsulated in high-pressure cells covered by heaters and a pressurizing medium. Thus, the actual processes in the cells cannot be directly observed using normal techniques. However, synchrotron radiation, “eye of light”, can make such observations possible. Because synchrotron radiation provides powerful X-rays that can penetrate these materials, information about their crystal structures and volume changes such as density and compressibility can be obtained by analyzing the diffraction of X-rays from the samples. Moreover, the pressure values can be simultaneously determined using a reference material. Therefore, controlling the pressure and temperature in X-ray diffraction experiments allows the state changes in a sample to be observed at a desired depth. Such high-pressure, high-temperature X-ray diffraction experiments were initially conducted in the 1980s at the Photon Factory in Tsukuba, Japan, but are now performed around the globe.
In high-pressure experiments, the sample size typically decreases as the exerted pressure increases. For example, at a pressure of 25 GPa (GPa = 109 Pa = ~10,000 atm), which is equivalent to a mantle transition zone depth of 700 km, the sample size must be ~1 mm3 or smaller. In contrast, at a pressure >100 GPa, which corresponds to the lowest region in the lower mantle, the maximum sample size is <50 μm3. Additionally, the temperature increases with the depth inside the Earth, and it reaches 1,500 °C and 2,000 °C around in the mantle transition zone and lower mantle, respectively. Because maintaining such conditions for long periods of time is extremely tough, X-ray diffraction experiments are difficult to conduct under such conditions. However, the emergence of third generation of synchrotron radiation facilities, including SPring-8, which can provide highly brilliant and highly energetic X-rays, has completely changed the situation. Experiments on small sized samples can now be performed in a relatively short time.
Fig. 1. Interior structure of the Earth
World-First Discoveries at SPring-8
SPring-8 is a pioneer in providing high-pressure, high-temperature experimental conditions, which has allowed researchers to successfully observe the transition processes of olivine. Olivine is the principal mineral of the mantle and undergoes a transition from the spinel structure to the perovskite structure under conditions (24 GPa, 1,600 °C) corresponding to the mantle transition zone (660 km depth). This was the first significant achievement at SPring-8, and the result, which was published in Science (1997), attracted global attention. Since then, the development of high-pressure devices and high-temperature heating technologies using lasers to reproduce conditions equivalent to Earth's deep interior has been energetically pursued around the globe. SPring-8 has developed high-intensity beams to irradiate small samples (<50 μm3) as well as realized higher intensity X-ray sources, which are at least ten times more powerful than previously existing ones by utilizing X-ray collector optics such as KB mirrors1) and refractive lenses. These advances in high-pressure, high-temperature generating technologies along with improved synchrotron radiation technologies allow researchers to reproduce conditions that are equivalent to those of the lowest region of the lower mantle and even deeper core regions.
A collaborative research group from the Tokyo Institute of Technology, the Japan Agency for Marine-Earth Science and Technology, and the Japan Synchrotron Radiation Research Institute (JASRI) has successfully reproduced conditions equivalent to the D” (D double-prime) layer, which is situated between the lower mantle and the outer core, using a diamond anvil device and laser heater. This D” layer has very peculiar properties. The propagation velocities of seismic waves drastically change, depending on the propagation directions; thus, the D” layer remains a mystery. Their research group conducted X-ray diffraction experiments under conditions equivalent to the region near the D” layer (>125 GPa, >2,200 °C), and revealed that the MgSiO3 perovskite phase is converted into a new high-pressure phase, post-perovskite phase. News about the discovery of a post-perovskite phase, which consistently explains the mysteries of the discontinuities and anisotropy of seismic wave velocities, made a striking impact on Earth scientists worldwide. Their remarkable accomplishment was immediately published in Science and Nature in 2004, and has been acknowledged as a significant breakthrough in science history. Additionally, their research group has further developed high-pressure, high-temperature technologies and successfully conducted experiments on quartz crystal under unprecedented conditions of 300 GPa and 2,000 °C. They demonstrated for the first time that a quartz crystal changes into a new dice-shaped pyrite-type mineral under pressure conditions exceeding 270 GPa. This pyrite-type mineral has received attention as a potential candidate for the principal mineral in the cores of Uranus and Neptune.
High-pressure, high-temperature experimental technologies and synchrotron radiation technologies have been used to investigate the crystal structures in X-ray diffraction experiments as well as in research on the physical properties such as viscosity, density, and elastic wave velocity. In addition to providing insight on Earth's origin, experimentally measuring the viscosity and density of magma, which stagnates at depths of several hundred kilometers, can provide information about its ascent, transportation velocities, and production mechanism. Additionally, comparing the experimentally measured elastic wave velocity to the actual seismic wave velocity can provide clues about the materials in Earth's interior. In recent years, the techniques to analyze seismic waves have remarkably progressed.
Image analysis, called seismic wave tomography, which utilizes large-scale computers to analyze data collected from seismometers placed worldwide, has become very popular. Seismic wave tomography has revealed the state of the subducted plates directly under the Japanese islands, which exist near the mantle transition zone. However, because the seismic wave velocity of the mantle transition zone and that of each plate material are unknown, seismic wave analyses could not identify specific rocks (minerals).
A research group at Ehime University, Japan and JASRI developed techniques to measure seismic wave velocities using elastic waves. They successfully carried out experiments in the mantle transition zone at a depth of 660 km. They measured the elastic wave velocities, and revealed that a major component of the mantle transition zone is olivine. Moreover, they discovered that a form of olivine, which is termed harzburgite, is a major material of the plate in the loweret region of the mantle transition zone at depth of 660 km. Their result that harzburgite stalls at depth of ~660 km due to its relatively low density is consistent with the result from seismic wave tomography. Their research results were published in Nature (February 2008).
Additionally, high-pressure earth science research that integrates high-pressure, high-temperature experimental technologies and synchrotron radiation technologies has been steadily advancing. Technology developed at SPring-8 will continue to lead the world in the high-pressure earth science research.
1) Kirkpatrick and Baez mirrors can provide a highly efficient and energy-tunable focusing of X-rays.