Deep Earth Materials Science

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KUBO Tomoaki, Professor: Understanding the Earth as one of the rocky planets in the Solar System requires knowledge of its origin, internal structure, and dynamic evolution. Our group focuses on the kinetics and rheology of deep Earth and planetary materials under extreme conditions, with particular emphasis on earthquakes and mantle convection. Multi-anvil type high-pressure deformation apparatuses are used to reproduce the dynamic environments of planetary interiors, while synchrotron X-ray experiments at SPring-8 and KEK enable in-situ observations under extreme conditions. Through experimental studies on rocks and minerals, we aim to understand the dynamic processes operating within the Earth and other planetary bodies.

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.