17:15 〜 19:15
[SCG62-P05] Shear deformation of single-crystal olivine under the pressure and temperature conditions of deep subducted slabs
キーワード：深発地震、カンラン石相転移、キンクバンド
Seismicity decreases at the greater depths due to the positive pressure dependency 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 Mg2GeO4 germanite olivine and showed that the phase transition of olivine polymorphs could trigger the earthquake in the MTZ. This hypothesis 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). Ohuchi et al. (2022) have conducted in situ deformation experiments on Mg1.8Fe0.2SiO4 mantle olivine under the conditions of slabs subducted to the MTZ (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. Recently, Shi et al. (2024) conducted deformation experiments on Mn2GeO4 (transforming to the modified spinel phase at 3−5 GPa), and showed that olivine-wadsleyite transition preferentially proceeded on the kink-band boundaries, leading unstable slips. This suggests that the formation of kink bands promotes shear localization. However, the detailed process of the phase transition of olivine via kinking is still unclear.
In this study, we conducted shear deformation experiments on single-crystal olivine samples at a pressure of 14.5 GPa and temperatures of 1020-1220 K with a constant strain rate using a deformation-DIA apparatus at GRC, Ehime Univ. An oriented single-crystal olivine samples were deformed on the shear plane of either (100), (010) or (001). The deformed microstructures in the recovered samples were examined by using a scanning electron microscope equipped with an electron back-scattered diffraction camera and a transmission electron microscope.
Total shear strains of the recovered samples were in the range of 0.07-0.80 and were lower at higher temperatures. The formation of olivine kink bands parallel to the shear direction was commonly observed. At higher temperatures (1120-1220 K), olivine grain boundaries were dominant nucleation sites for wadsleyite particles (~1 μm in a diameter). In contrast, at a lower temperature (1020 K), grain boundary nucleation of wadsleyite was hardly observed. In the case of the shear plane of (100) and the shear direction of [010], intracrystalline nucleation of thin wadsleyite lamellae (~500 nm in a thickness) parallel to the olivine kink bands was commonly observed at 1020-1120 K. This observation suggests a pseudo-martensitic transition of olivine to wadsleyite via poirierite as a result of kinking (i.e., shearing) of olivine by the (100)[010] slip system (Madon and Poirier, 1983). Some wadsleyite lamellae consist of nanograins of wadsleyite (10-50 nm), suggesting that the coalescence of numerous lamellae results in the formation of thin weak layers, which induces shear localization (i.e., faulting). Our findings imply that a “diffusionless” pseudo-martensitic olivine-wadsleyite transition may provide a natural explanation for the cause of deep earthquakes in bending regions of cold subducted slabs.
In this study, we conducted shear deformation experiments on single-crystal olivine samples at a pressure of 14.5 GPa and temperatures of 1020-1220 K with a constant strain rate using a deformation-DIA apparatus at GRC, Ehime Univ. An oriented single-crystal olivine samples were deformed on the shear plane of either (100), (010) or (001). The deformed microstructures in the recovered samples were examined by using a scanning electron microscope equipped with an electron back-scattered diffraction camera and a transmission electron microscope.
Total shear strains of the recovered samples were in the range of 0.07-0.80 and were lower at higher temperatures. The formation of olivine kink bands parallel to the shear direction was commonly observed. At higher temperatures (1120-1220 K), olivine grain boundaries were dominant nucleation sites for wadsleyite particles (~1 μm in a diameter). In contrast, at a lower temperature (1020 K), grain boundary nucleation of wadsleyite was hardly observed. In the case of the shear plane of (100) and the shear direction of [010], intracrystalline nucleation of thin wadsleyite lamellae (~500 nm in a thickness) parallel to the olivine kink bands was commonly observed at 1020-1120 K. This observation suggests a pseudo-martensitic transition of olivine to wadsleyite via poirierite as a result of kinking (i.e., shearing) of olivine by the (100)[010] slip system (Madon and Poirier, 1983). Some wadsleyite lamellae consist of nanograins of wadsleyite (10-50 nm), suggesting that the coalescence of numerous lamellae results in the formation of thin weak layers, which induces shear localization (i.e., faulting). Our findings imply that a “diffusionless” pseudo-martensitic olivine-wadsleyite transition may provide a natural explanation for the cause of deep earthquakes in bending regions of cold subducted slabs.