Deep Earth Materials Science

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KUBO Tomoaki, Professor TSUBOKAWA Yumiko, Assistant Professor: Studies on the properties of the Earth and planetary constituent minerals provide the certain proof of the elementary process operated in the evolution of the planets. Our group focus the dynamic properties (rheology, faulting, diffusion and reaction kinetics) under high pressure. Multi-anvil system and diamond anvil cell are used to generate the high pressure and temperature conditions of the planetary interiors. Synchrotron X-ray sources at Spring-8 and KEK are essential to in-situ observation method. The former and future research topics are given below.

1. Phase transition and rheology of deep Earth materials

Figure 1. We conduct simultaneous observations of reaction kinetics, creep behavior, and acoustic emissions by using high-pressure deformation apparatus combined with synchrotron X-ray to investigate the reaction–deformation coupling in deep Earth materials.

High-pressure phase transition caused internal stratified structure of the terrestrial and icy planetary bodies. The heat and material transport by solid state convection in such regime controls characters of the surface tectonics. In case of the Earth, subduction of the cold plate into the mantle carries substantial amounts of water and the components with low melting temperature to the deep layers. The volcanic and seismic activities in the deep Earth origin are related to dehydration, melting, and phase transitions of the minerals in the subducting plate. Especially, we try to solve the problems; On what conditions subducted plate could penetrate or be stagnated at the upper and lower mantle boundary? Why deep earthquakes occur beyond brittle–ductile transition exclusively inside the plate? How does convective mixing of the chemically differentiated plate occur in the lower mantle and D″ layer? By the experimental approach consisting of plastic deformation and acoustic emission measurement, we study coupling phenomena of phase transition and rheology at high pressure (Fig. 1).

2. Shock metamorphism in meteorites

We also study formation process of high-pressure minerals in shocked meteorites to investigate collisional history of asteroids and formation process of planets in early sola system. In general, short period of impact caused metastable transition of minerals and non-equilibrium texture, which are preserved in shocked meteorites. Reproduction of such characters by high-pressure experiments (Fig. 2) enables us to evaluate the impact conditions (pressure, temperature, and duration and consequently impact velocity and size of impactor).

Figure 2. Seifertite is the dense polymorph of silica found in lunar and Martian shocked meteorites. We demonstrated that seifertite, thermodynamically stable at more than ~100 GPa (blue), metastably appears at pressures as low as ~11 GPa (red) by in-situ X-ray observations. This finding can be used as a unique shock indicator for understanding the collisional and formation process of planets in the early solar system.

3. Rheology of planetary ices

Figure 3. Deformation experiments of ice II (left) and ice VII (right), major constituents in large icy moons such as Ganymede and Calisto, were conducted at ~200 MPa and ~200 K by gas apparatus, and at ~5 GPa and ~300 K by multi-anvil type deformation apparatus with synchrotron radiation, respectively.

Various types of icy moons have been found in the outer solar system. Icy super-Earths have also been detected as exoplanets. Thermal convection in ice shells and mantles of these icy bodies is critical to understanding their thermal histories, internal dynamics, surface tectonic activities, and survival of internal oceans. Ice viscosity is one of the most important parameters for occurrence and style of internal convection. We experimentally investigate rheology, polycrystalline kinetics, and diffusion of planetary ices such as water ice, high-pressure ices, and non-water ices (Fig. 3). They are shown to unexpectedly be weak under low-stress condition in the interior of icy bodies, and aggregation with small amount of CO₂ ice has a profound effect to further decrease the strength and viscosity. These results can be used to constrain tectonic activities and internal convection of icy moons and planets in extremely low temperature environment.

4. Melting phase relation

Figure 4. Element partitioning experiments between silicate melt and the lower-mantle mineral bridgmanite under the condition for the magma ocean in early Earth.

Terrestrial planets had been formed from the solid precursors with various sizes (planetesimals and proto-planet) by impact accretion at 4.6 billion years ago in relatively short period of time (less than 100 million years). In such scenario, hot origin of the terrestrial planets and magma ocean at the surface layer are inevitable consequences with rapid core formation. Understanding of extensive mantle differentiation at the early stage is important to study the chemical evolution and heterogeneities of the planets. The melting phase relation and element partitioning in the magnesian silicates and iron are clarified by the high pressure experiments (Fig. 4). These results are indispensable to study comparative planetology of the solar and extra-solar system, which could have been realized with the observational data from planetary exploration and astronomy.

5. Mineral diversity

Figure 5. High-resolution image of chrysotile fibers formed by the water–rock reaction. We observe nano-scale microstructure of minerals by transmission and scanning electron microscopy.

The mineral species observed at the planetary surface are multiplied and complicated by presence and activity of water and volatiles. Fine nano-scale particles of clay minerals (Fig. 5) are useful resources to industry, and great interest has been paid to the relation between crystal structures and properties. The acidic to pegmatitic melt in the Earth's crust are generated by extreme fractionation in the hydous magma and high concentrations of the rare elements result in thousands of unique minerals. We adopt conventional and advanced methods of descriptive mineralogy; X-ray diffractometry, and electron microscopies, together with geologic field work, to clarify origin and formation process of the minerals.