Physical properties and dynamics in the Earth's interiors.
Rock is a polycrystalline aggregate of minerals, and at high temperatures and pressures such as those found in the Earth's interior, macroscopic properties are affected by the behavior of atomic-scale defects (dislocations, grain boundaries, etc.). In order to understand the macroscopic mechanical properties of rocks, it is essential to elucidate microscopic processes.
1, Analog experiments
The reason why it is difficult to elucidate the physical properties of rocks lies in the complexity of polycrystalline materials and the difficulty of high-temperature, high-pressure experiments. However, if we use appropriate analog polycrystals with low melting points, it is possible to reproduce the elementary physical processes common to the Earth's interior near room temperature and ambient pressure. Using this unique method, we realized various experiments that cannot be performed on rocks and obtained highly accurate experimental data.
2, Theory linking micro and macro
In order to apply the results of analog experiments to the Earth's interior, it is important to clarify universal mechanisms that are independent of individual materials through theoretical models. We are developing a theory to derive macroscopic mechanical constitutive laws of a polycrystalline material based on microscopic elementary processes.
3. Viscoelastic constitutive law and grain boundary phase transition
Viscoelastic constitutive laws of rocks are frequently used to seamlessly describe long-timescale phenomena treated in geodesy and short-timescale phenomena such as seismic wave propagation, but the microscopic mechanisms of rock viscoelasticity have not been well understood. Only recently has it become clear that atomic diffusion and sliding at grain boundaries play an important role across all time scales from viscous to elastic. Although grain boundaries are very thin regions with a thickness of a few nanometers, they also undergo phase transitions (grain boundary melting) depending on temperature, pressure, and chemical composition. Experimental data on analog polycrystalline materials have revealed that the occurrence of grain boundary melting in the upper mantle is the key to understanding the seismic structure (existence of low-velocity regions) and viscosity structure (existence of the asthenosphere).
1. Takei Y. 2017, Effects of partial melting on seismic velocity and attenuation: A new insight from experiments, Annu. Rev. Earth Planet. Sci. 45:447–70. https://doi.org/10.1146/annurev-earth-063016- 015820
2. Yamauchi, H., and Takei, Y. 2020, Application of a premelting model to the lithosphere-asthenosphere boundary. G-cubed, 21, https://doi.org/10.1029/2020GC009338
3. Takei Y. 2022, Effect of impurities on polycrystal anelasticity, JGR solid Earth, in press, https://doi.org/10.1029/2021JB023224