5:15 PM - 6:30 PM
[SCG46-P01] Experimental investigation of the pyroxene-garnet and olivine-spinel transformations under shear deformation using synchrotron radiation and acoustic emission techniques
Keywords:Acoustic emission, In situ X-ray observation, Deep-focus earthquake, Pyroxene, Olivine
Detection of acoustic emission (AE) has been recently performed combined with high-pressure deformation apparatus and synchrotron radiation to study the mechanisms of deep-focus earthquakes. To understand effects of large strains on shear localization and instability, deformation experiments in shear is more effective than uniaxial compression. However, the sample volume in shear deformation experiment is generally much smaller than that in uniaxial deformation experiment, which may make it difficult to conduct AE study with required accuracy. In this study, we carried out the pyroxene-garnet and olivine-spinel transformation experiments under shear deformation using AE techniques combined with deformation-DIA (D-DIA) apparatus and synchrotron radiation. Both pyroxene and olivine are major constituent minerals in subducting slabs, but their transformation processes are different. Here we report preliminary results on AE activities induced by these high-pressure transformations in shear.
The high-pressure shear deformation experiments were conducted using a uniaxial press (SPEED Mk. II) with a D-DIA type guide block installed at beamline BL04B1 in SPring-8, Japan. The 60 keV monochromatic X-ray was used to obtain 2D X-ray diffraction pattern and radiograph every 5 min during the experiments. We used MA 6-6 type AE measurement system attached with D-DIA guide block (Iwasato et al., 2020). Three kinds of starting materials of synthesized Al-rich clinopyroxene (Al-rich cpx), natural Al-poor clinopyroxene (Al-poor cpx), and synthesized fayalite were prepared for our study. They were sintered and cut to form polycrystalline disks (300 µm in thick and 1.3 mm in diameter). Each disk was assembled together with strain marker of gold foil between two 45°-cut alumina pistons. The samples were first deformed to shear strains of ~30% at ~10 GPa and 650°C for clinopyroxene and 475°C for fayalite (stage 1), and then heated with a constant rate of 0.05 ℃/sec to cause the phase transformations (stage 2). The shear strain rates and the final shear strains were 1–5 x 10-4 /s and ~70-160%, respectively.
The pyroxene-garnet transformation already started during the deformation at 650°C in Al-rich cpx and proceeded with increasing temperature. In contrast, no transformation was observed in Al-poor cpx up to ~850°C. This is possibly originated from the difference in the Al content. We detected many AEs from the sample region at the stage 2 in the Al-rich cpx run, but no AEs at the stage 2 in the Al-poor cpx run, implying that AEs were induced by the pyroxene-garnet transformation in shear. FE-SEM observations of the recovered sample in the Al-rich cpx run revealed that garnet formed along grain boundary of clinopyroxene, and the shear deformation was localized in some regions. We also detected large AEs from the sample region in the fayalite run during the stage 2 associated with the olivine-spinel transformation. Thus, both the pyroxene-garnet and olivine-spinel transformations caused AE activity in our shear deformation experiments. We recognized two types of waveform of AE: AE with rapid increase in amplitude to the maximum (type 1), and AE with gradual increase in amplitude to the maximum (type 2). The type 2 is dominant in the Al-rich cpx run, whereas AEs in the fayalite run were type 1 only. The estimated locations of AE source were concentrated within the sample region in the case of type 1 AE, whereas those of type 2 AE were widely scattered from the sample to pressure medium. This is probably because the AIC pick is difficult in the type 2 AE. Our study demonstrated that both the pyroxene-garnet and olivine-spinel transformations caused AE activity under shear deformation at ~10 GPa, however further investigations are necessary to understand why the waveform of AEs are rather different between theses transformations.
The high-pressure shear deformation experiments were conducted using a uniaxial press (SPEED Mk. II) with a D-DIA type guide block installed at beamline BL04B1 in SPring-8, Japan. The 60 keV monochromatic X-ray was used to obtain 2D X-ray diffraction pattern and radiograph every 5 min during the experiments. We used MA 6-6 type AE measurement system attached with D-DIA guide block (Iwasato et al., 2020). Three kinds of starting materials of synthesized Al-rich clinopyroxene (Al-rich cpx), natural Al-poor clinopyroxene (Al-poor cpx), and synthesized fayalite were prepared for our study. They were sintered and cut to form polycrystalline disks (300 µm in thick and 1.3 mm in diameter). Each disk was assembled together with strain marker of gold foil between two 45°-cut alumina pistons. The samples were first deformed to shear strains of ~30% at ~10 GPa and 650°C for clinopyroxene and 475°C for fayalite (stage 1), and then heated with a constant rate of 0.05 ℃/sec to cause the phase transformations (stage 2). The shear strain rates and the final shear strains were 1–5 x 10-4 /s and ~70-160%, respectively.
The pyroxene-garnet transformation already started during the deformation at 650°C in Al-rich cpx and proceeded with increasing temperature. In contrast, no transformation was observed in Al-poor cpx up to ~850°C. This is possibly originated from the difference in the Al content. We detected many AEs from the sample region at the stage 2 in the Al-rich cpx run, but no AEs at the stage 2 in the Al-poor cpx run, implying that AEs were induced by the pyroxene-garnet transformation in shear. FE-SEM observations of the recovered sample in the Al-rich cpx run revealed that garnet formed along grain boundary of clinopyroxene, and the shear deformation was localized in some regions. We also detected large AEs from the sample region in the fayalite run during the stage 2 associated with the olivine-spinel transformation. Thus, both the pyroxene-garnet and olivine-spinel transformations caused AE activity in our shear deformation experiments. We recognized two types of waveform of AE: AE with rapid increase in amplitude to the maximum (type 1), and AE with gradual increase in amplitude to the maximum (type 2). The type 2 is dominant in the Al-rich cpx run, whereas AEs in the fayalite run were type 1 only. The estimated locations of AE source were concentrated within the sample region in the case of type 1 AE, whereas those of type 2 AE were widely scattered from the sample to pressure medium. This is probably because the AIC pick is difficult in the type 2 AE. Our study demonstrated that both the pyroxene-garnet and olivine-spinel transformations caused AE activity under shear deformation at ~10 GPa, however further investigations are necessary to understand why the waveform of AEs are rather different between theses transformations.