16:30 〜 16:45
[SCG44-05] Deformation of metastable olivine and faulting under pressure and temperature conditions of slabs subducted into the mantle transition zone
キーワード：深発地震、準安定カンラン石、キンクバンド、その場観察
Seismicity decreases at the greater depths due to the positive pressure dependence of frictional strength (i.e., Byerlee’s rule). However, seismicity increases with depths in slabs subducted into the mantle transition zone (MTZ) (i.e., depths of 410−600 km: Florich, 1989), implying that the process controlling the occurrence of deep earthquakes is significantly different from that of shallower earthquakes. Green and Burnley (1989) conducted deformation experiments on germanite olivine and showed that the phase transformations of olivine polymorphs could trigger the earthquakes in the MTZ. This hypothesis, so called the transformational faulting model, has been tested by many deformation experiments using olivine analogues such as Mg2GeO4 olivine (transforming to the spinel phase at 1−2 GPa: Schubnel et al., 2013) and Fe2SiO4 fayalite (at 4−9 GPa: Officer and Secco, 2020). However, these materials might not be suitable for the following two points; (ⅰ) faulting can easily occur in deformation experiments using germanite olivine because confining pressure is comparable to differential stress, and (ⅱ) the germanite olivine and fayalite directly transform to a spinel phase (i.e., absence of modified spinel phase). Recently, Ohuchi et al. (2022) have conducted the deformation experiments on Mg1.8Fe0.2SiO4 mantle olivine under the conditions of subducted slabs (11−17 GPa and 960−1350 K). They reported that deep earthquakes could be triggered by the formation of weak gouge layers filled with nanocrystalline metastable olivine and wadsleyite (and/or ringwoodite) around the surface of the metastable olivine wedge. However, the detailed process of the formation of weak gouge layers via the phase transformation of metastable olivine is still unclear.
In this study, we conducted in situ uniaxial deformation experiments on olivine aggregates at pressures of 15−20 GPa and temperatures of 970−1120 K corresponding to the conditions in the slabs subducted into the mantle transition zone, using a deformation-DIA apparatus at BL04B1/SPring-8. Pressure, stress, and strain were determined by using X-ray diffraction patterns and radiographs. Acoustic emissions (AEs) were also recorded by using six sensors glued on the sides of the second-stage anvils. The recovered samples of experiments were examined by scanning electron microscope and transmission electron microscope (TEM). During the deformation stage, the yield strength was typically ~3 GPa. We observed the throughgoing faulting (fault displacement ~50 μm) at temperatures of 970−1120 K and pressures of ~17 GPa. A sudden increase in strain followed by a softening (i.e., decrease in stress and/or increase in strain rate) associating a few AEs were observed before the faulting. TEM observations of the recovered samples showed the formation of gouge layers (thickness of 50−800 nm) filled with nanograins of olivine polymorphs (diameter of 20−50 nm) and platinum blobs (products of melting of the strain markers), suggesting the occurrence of unstable fault slip along the gouge layers aided by the adiabatic shear heating (up to 2500 K). Formation of the kink bands sub-parallel to the gouge layers was also observed. Some mode-Ⅱ cracks associating kink bands were found at 1120 K and 18−20 GPa. The kink-band boundaries were often filled with nanograins of olivine polymorphs (10−20 nm), suggesting that the occurrence of dynamics recrystallization of olivine and preferential nucleation of wadsleyite and/or ringwoodite. The absence of slips on the mode-Ⅱ cracks implies that the nucleation of wadsleyite and/or ringwoodite is needed to promote the unstable slip. Our results suggest that the formation of mode-Ⅱ cracks associating kink bands in olivine is the first step for the occurrence of faulting in the deep subducted slabs.
In this study, we conducted in situ uniaxial deformation experiments on olivine aggregates at pressures of 15−20 GPa and temperatures of 970−1120 K corresponding to the conditions in the slabs subducted into the mantle transition zone, using a deformation-DIA apparatus at BL04B1/SPring-8. Pressure, stress, and strain were determined by using X-ray diffraction patterns and radiographs. Acoustic emissions (AEs) were also recorded by using six sensors glued on the sides of the second-stage anvils. The recovered samples of experiments were examined by scanning electron microscope and transmission electron microscope (TEM). During the deformation stage, the yield strength was typically ~3 GPa. We observed the throughgoing faulting (fault displacement ~50 μm) at temperatures of 970−1120 K and pressures of ~17 GPa. A sudden increase in strain followed by a softening (i.e., decrease in stress and/or increase in strain rate) associating a few AEs were observed before the faulting. TEM observations of the recovered samples showed the formation of gouge layers (thickness of 50−800 nm) filled with nanograins of olivine polymorphs (diameter of 20−50 nm) and platinum blobs (products of melting of the strain markers), suggesting the occurrence of unstable fault slip along the gouge layers aided by the adiabatic shear heating (up to 2500 K). Formation of the kink bands sub-parallel to the gouge layers was also observed. Some mode-Ⅱ cracks associating kink bands were found at 1120 K and 18−20 GPa. The kink-band boundaries were often filled with nanograins of olivine polymorphs (10−20 nm), suggesting that the occurrence of dynamics recrystallization of olivine and preferential nucleation of wadsleyite and/or ringwoodite. The absence of slips on the mode-Ⅱ cracks implies that the nucleation of wadsleyite and/or ringwoodite is needed to promote the unstable slip. Our results suggest that the formation of mode-Ⅱ cracks associating kink bands in olivine is the first step for the occurrence of faulting in the deep subducted slabs.