17:15 〜 18:30
[SCG46-P03] SOR and AE study on the olivine-spinel transformation in (Mg,Fe)2SiO4 under uniaxial deformation with D111-type apparatus
キーワード：高圧変形実験、放射光その場観察、アコースティックエミッション、スラブ軟化、深発地震、オリビン-スピネル相転移
In cold subducting slabs, it has been pointed out that olivine remains in metastable phase beyond the equilibrium boundary. The metastable olivine transforms into its high-pressure phases, such as wadsleyite or ringwoodite under non-equilibrium conditions. It has been suggested that this phenomenon leads to rheological weakening and shear instability causing large deformation and deep-focus earthquakes in subducting slabs. For example, Schubnel et al. (2013) reported AE activity following the olivine-spinel transformation with in-situ X-ray observation using D-DIA apparatus. However, they used olivine analogue, Mg2GeO4, which transformed to spinel phase at low pressures (~2 GPa). In the present study, we conducted deformation experiments with San Carlos olivine polycrystal under the mantle transition zone (MTZ) conditions using D111-type apparatus combined with synchrotron radiation to observe creep behavior during the phase transformation. In addition, we developed 8-channel Kawai-type AE measurement system attached with D111-type apparatus to observe shear instability at MTZ pressures. Here we report some preliminary results on these experimental studies.
We carried out deformation and transformation experiments with synchrotron X-ray using a D111-type apparatus (MAX-III) installed at PF-AR NE7 beamline. The starting material was polycrystalline San Carlos olivine sintered at 1323 K (grain size of ~100 µm, Run oldt11) and 1223 K (grain size of ~30 µm, Run oldt12). The sample was first uniaxially deformed to ~1-3 % strain with a constant anvil displacement rate of 300 µm/h at ~20 GPa and 773 K / 873 K, and then heated with a constant rate of 0.09 K/s to cause transformation during deformation. We obtained 2D-XRD patterns and radiography image every 5 minutes using a 60 keV monochromatic X-ray to measure stress-strain curves and reaction kinetics.
Transition to ringwoodite started at ~20 GPa and ~1000K with ~5% strain, and finally proceeded more than 90% transformation at 1273K with the final strain of ~20-35%. The strain rate was 0.4-7.5e-5 s-1. The stress of olivine reached to ~5 GPa at most before the transition and gradually decreased to ~2-3 GPa with the increase in temperature and transformed fraction. In contrast, the stress of ringwoodite newly appeared was very small less than 1 GPa and subtly increased. The similar results have been reported in the RDA study (Mohiuddin et al., 2020), however, the strength of ringwoodite kept small in our experiments. The temperature dependence of the olivine stress was almost consistent with the flow law of Peierls mechanism constructed by Mei et al. (2010) at the initial stage, and the olivine got more weakened as the transformation proceeded especially in Run oldt11. These creep behaviors suggest that reaction-induced weakening occur, but not significant. It is unlikely that the interconnected weak layer was formed at the initial stage of the transformation. In the recovered samples, intracrystalline lamellae filled with fine ringwoodite grains were widely observed. Similar textures have been reported in shocked meteorite (Chen et al., 2004) and the syn-deformational olivine-spinel transition of fayalite (Mori et al., 2020).
We also have set up 8-channel Kawai-type AE measurement system attached with another D111-type apparatus (QDES2) newly installed at Kyushu University. This is almost similar to 6-channel MA6-6 type AE measurement system with D-DIA apparatus (Iwasato et al., 2020) for lower-pressure experiments. After the verification of the performance by conducting cold compression runs using grass beads and single crystalline San Carlos olivine, we have started syn-deformational transformation experiment in San Carlos olivine under the MTZ conditions with the AE system. We have recorded AE from the sample during transformation, however no distinct faulting structure was observed so far except for lenses and thin lamellae filled with fine-grained ringwoodite.
We carried out deformation and transformation experiments with synchrotron X-ray using a D111-type apparatus (MAX-III) installed at PF-AR NE7 beamline. The starting material was polycrystalline San Carlos olivine sintered at 1323 K (grain size of ~100 µm, Run oldt11) and 1223 K (grain size of ~30 µm, Run oldt12). The sample was first uniaxially deformed to ~1-3 % strain with a constant anvil displacement rate of 300 µm/h at ~20 GPa and 773 K / 873 K, and then heated with a constant rate of 0.09 K/s to cause transformation during deformation. We obtained 2D-XRD patterns and radiography image every 5 minutes using a 60 keV monochromatic X-ray to measure stress-strain curves and reaction kinetics.
Transition to ringwoodite started at ~20 GPa and ~1000K with ~5% strain, and finally proceeded more than 90% transformation at 1273K with the final strain of ~20-35%. The strain rate was 0.4-7.5e-5 s-1. The stress of olivine reached to ~5 GPa at most before the transition and gradually decreased to ~2-3 GPa with the increase in temperature and transformed fraction. In contrast, the stress of ringwoodite newly appeared was very small less than 1 GPa and subtly increased. The similar results have been reported in the RDA study (Mohiuddin et al., 2020), however, the strength of ringwoodite kept small in our experiments. The temperature dependence of the olivine stress was almost consistent with the flow law of Peierls mechanism constructed by Mei et al. (2010) at the initial stage, and the olivine got more weakened as the transformation proceeded especially in Run oldt11. These creep behaviors suggest that reaction-induced weakening occur, but not significant. It is unlikely that the interconnected weak layer was formed at the initial stage of the transformation. In the recovered samples, intracrystalline lamellae filled with fine ringwoodite grains were widely observed. Similar textures have been reported in shocked meteorite (Chen et al., 2004) and the syn-deformational olivine-spinel transition of fayalite (Mori et al., 2020).
We also have set up 8-channel Kawai-type AE measurement system attached with another D111-type apparatus (QDES2) newly installed at Kyushu University. This is almost similar to 6-channel MA6-6 type AE measurement system with D-DIA apparatus (Iwasato et al., 2020) for lower-pressure experiments. After the verification of the performance by conducting cold compression runs using grass beads and single crystalline San Carlos olivine, we have started syn-deformational transformation experiment in San Carlos olivine under the MTZ conditions with the AE system. We have recorded AE from the sample during transformation, however no distinct faulting structure was observed so far except for lenses and thin lamellae filled with fine-grained ringwoodite.