11:00 〜 13:00
[SCG49-P03] Shear Deformation and decomposition reactions of Lawsonite and Glaucophane at high pressures
キーワード：稍深発地震、せん断不安定性、脱水反応
Intermediate-depth earthquakes (IDEQs) occur in cold subducting oceanic crust (MORB) at depths of about 60-300km. Metamorphic reactions in MORB during subduction produce various hydrous minerals involving lawsonite and glaucophane. It has been suggested that dehydration embrittlement of hydrous minerals is one of the important mechanisms for IDEQs. Okazaki and Hirth (2016) conducted deformation experiments on lawsonite using a Griggs-type deformation apparatus at 1 GPa and 300-600℃. They suggested that dehydration of lawsonite can directly trigger unstable fault slip in the subducting oceanic crust although the P-T conditions do not match those for IDEQs. Incel et al. (2017) also investigated the deformation of lawsonite-glaucophane mixtures using a D-DIA apparatus at ~1.5-3.5GPa and ~300-900℃. They showed that dehydration of lawsonite is not directly related, but metamorphic reactions during the transition from lawsonite-blueschist to lawsonite-eclogite can cause faulting accompanied by acoustic emissions (AEs), matching the P-T conditions for the upper plane of double seismic zones of IDEQs. Recently, Shiraishi et al. (2022) conducted deformation experiments on lawsonite using a D-DIA apparatus (2.5-6.0 GPa, 300-800℃). They suggested that fault slip may occur within the stability field of lawsonite under conditions suitable for the generation of IDEQs although AEs were not measured in their study. Thus, the relationship between dehydration of lawsonite and generation of IDEQs is still not clear.
In this study, we conducted shear deformation experiments on lawsonite and glaucophane polycrystals at various pressure and temperature conditions of 1-6 GPa and 400-800℃ using a D-DIA apparatus at the BL04B1 beamline of SPring-8. Monochromatic X-ray (60keV) from synchrotron radiation was used for in-situ observation of stress-strain curves and reaction kinetics. MA6-6 type AE measurement system (Iwasato et al., 2020) were used to monitor AEs. Once the sample reached the target pressure and temperature, it was deformed isothermally for 30 minutes and then further deformed with increasing temperature (0.05℃/s) to cause decomposition reactions. The final shear strains reached to 52-140% with shear strain rates of 1.8-67 x10-5 s-1. After the reaction completed, the sample was quenched and slowly decompressed.
In the lawsonite runs, dehydration reaction occurred near the equilibrium boundary at 1.5 GPa and 470℃, and 2.5 GPa and 620℃. However, at the higher pressure of ~5.5 GPa, the dehydration reaction started at lower temperatures than the equilibrium boundary. In contrast to Okazaki and Hirth (2016), AE activities were not observed at around 1.5 GPa even when dehydration reaction occurred. Instead, we observed AE activities at 5-6 GPa before starting the reaction, and the activities became greater after the dehydration reaction completed at ~800℃. In this sample, slip displacements were observed both in radiography and SEM images, which may indicate shear instability although more detailed analysis is needed. Stress drop occurred especially during the isothermal deformation stage as observed in Shiraishi et al. (2022), but the related AE activities were not detected. Thus, we have not obtained a clear relationship between AE activities and the dehydration reaction and the stress drop in the lawsonite sample so far. We also conducted three runs using glaucophane polycrystal along the similar P-T-t paths as the lawsonite runs. Large shear strains were achieved during the decomposition reaction of glaucophane, however clear AE activities were not detected and the deformation seems to be homogeneous even in large shear strains.
In this study, we conducted shear deformation experiments on lawsonite and glaucophane polycrystals at various pressure and temperature conditions of 1-6 GPa and 400-800℃ using a D-DIA apparatus at the BL04B1 beamline of SPring-8. Monochromatic X-ray (60keV) from synchrotron radiation was used for in-situ observation of stress-strain curves and reaction kinetics. MA6-6 type AE measurement system (Iwasato et al., 2020) were used to monitor AEs. Once the sample reached the target pressure and temperature, it was deformed isothermally for 30 minutes and then further deformed with increasing temperature (0.05℃/s) to cause decomposition reactions. The final shear strains reached to 52-140% with shear strain rates of 1.8-67 x10-5 s-1. After the reaction completed, the sample was quenched and slowly decompressed.
In the lawsonite runs, dehydration reaction occurred near the equilibrium boundary at 1.5 GPa and 470℃, and 2.5 GPa and 620℃. However, at the higher pressure of ~5.5 GPa, the dehydration reaction started at lower temperatures than the equilibrium boundary. In contrast to Okazaki and Hirth (2016), AE activities were not observed at around 1.5 GPa even when dehydration reaction occurred. Instead, we observed AE activities at 5-6 GPa before starting the reaction, and the activities became greater after the dehydration reaction completed at ~800℃. In this sample, slip displacements were observed both in radiography and SEM images, which may indicate shear instability although more detailed analysis is needed. Stress drop occurred especially during the isothermal deformation stage as observed in Shiraishi et al. (2022), but the related AE activities were not detected. Thus, we have not obtained a clear relationship between AE activities and the dehydration reaction and the stress drop in the lawsonite sample so far. We also conducted three runs using glaucophane polycrystal along the similar P-T-t paths as the lawsonite runs. Large shear strains were achieved during the decomposition reaction of glaucophane, however clear AE activities were not detected and the deformation seems to be homogeneous even in large shear strains.