Deep-focus earthquakes occur at the depth from 440 km to 660 km in the slabs. By the geophysical observations (e.g. Zhan et al., 2014) and deformation experiments (e.g. Green and Burnley, 1989), the phase transformational faulting mechanism is presumed as the precursor of deep-focus earthquakes. This mechanism is that shear instability occurs at fine-grained spinel nucleated by the phase transformation from metastable olivine in the subducting slab. This shear instability depends on grain size, but previous studies have not referred to the grain size dependency of metastable olivine on the phase transformational faulting mechanism (e.g. Burnley et al., 1991). Therefore, I'm conducting deformation experiments of germanate olivine with various grain sizes.
Olivine is main constituent of the upper mantle and the phase transformation affects the rheology of subduction zone. Therefore, it is important to reveal the kinetics of olivine-spinel phase transformation under differential stress. It is difficult to conduct the deformation experiments of silicate olivine under high pressure. Therefore, I conducted the Phase Field simulation. This can simulate microstructure evolution with complex morphological features.
Serpentinazation is important to understand the fulid transportation in the subduction zone because serpentine is hydrous mineral and the dehydration affects the mantle hydration and formation of volcanic arc. However, the amount of antigorite is uncertain because the amount of water infiltrated into the oceanic lithosphere is still debated. To reveal whether antigorite can form under small amount of water, I conducted deformation experiments of germanate olivine with a small amount of water. As a result, antigorite widely was formed in the samples deformed at temperature lower than 600 ℃. Thus, partly hydrated peridotite in the oceanic lithosphere can become serpentine under slight water infiltration due to high strain accumulated by the subduction.