17:15 〜 19:15
[SSS10-P33] Toward frictional experiments at brittle-plastic transition pressures with high-flux synchrotron X-ray
キーワード：脆性塑性転移、高圧実験、摩擦、放射光その場観察、次世代挿入光源、アコースティックエミッション
The frictional properties of rocks at the brittle-plastic transition (P ~1-2 GPa, depths of ~30-70 km) are crucial for understanding faulting processes in subduction zones. Multi-anvil deformation apparatuses can easily achieve high-pressure conditions and have been used to investigate faulting mechanisms by combining X-ray in-situ observations and acoustic emission (AE) measurements (e.g. Ohuchi et al., 2017; Honda et al., 2024). However, the time resolution of data acquisition was low (~102 s) when using bending magnet X-ray sources (BM), making it difficult to capture detailed processes. Recently, advancements in high-resolution (~10-1 s) techniques have been made using high-flux insertion device X-ray sources (ID) (e.g. Ohuchi et al., 2024). In this study, we further developed these methods and applied them to examine the localization and frictional behavior of olivine under the brittle-plastic transition regime.
The experiments were conducted with D-DIA type apparatus at 1-5 GPa and 25-1040°C, with in-situ observation using a 60 keV monochromatic X-ray from BM at SPring-8 BL04B1 and a 100 keV pink beam from ID at BL05XU and BL15XU. The starting material was sintered polycrystalline forsterite (Koizumi et al., 2010). It was processed into thin disks (~0.3 mm) and placed between alumina pistons which were saw-cut at a 45° angle to apply shear deformation. We carried out the deformation at a constant anvil displacement rate (0.2 mm/h) or with velocity stepping (0.2-2 mm/h). X-ray diffraction and radiography were measured to obtain pressure, stress-strain data, and localization behavior. The time resolution was 180-300 s with BM and 0.5 s with ID. Additionally, AE measurement was also conducted during the deformation to detect shear instability.
We first identified the variations in localization behaviors and their conditions using BM. We confirmed localization in all runs through radiography, except for the low-velocity deformation at highest temperature of 1040°C. Unstable slips accompanied by AEs occurred only within a limited range of conditions below 2 GPa and at 150-710°C, which is largely consistent with the conditions of earthquakes in warm slabs. Y-shear and Riedel shear planes developed after yielding, with the latter being particularly prominent in unstable slip involving AEs. Microstructural observation revealed the development of the fault gouge composed of submicron-sized particles, whose morphology varied with temperature. They had rounded shape and mylonitic texture at higher temperature, whereas they formed angular and cataclastic fracture structures at lower temperature. Furthermore, the yield stress exceeded the Goetze criterion (τ/P >0.5), suggesting that localization was initiated by brittle or semi-brittle crack formation. However, once faults formed, the strength decreased below that, transitioning to stable slip without AEs in some samples. In addition, even under identical conditions, variations in the friction coefficient and slip stability were observed, indicating that other factors, such as water content, must also be considered in these behaviors.
We also conducted velocity-step tests to examine the velocity dependence of the friction coefficient in unstable slips with ID. So far, we have successfully identified a trend of velocity weakening correlated with AE, localized deformation, and its onset. Nonetheless, challenges remain, including large measurement errors and stress variations due to changes in the diffraction acquisition position. While improving control precision is necessary, the latter issue also suggests the possibility of capturing the stress distribution within the sample, which could potentially be evaluated quantitatively through high-resolution stress mapping. Although these types of experiments had been previously difficult using the BM method, high-resolution measurements with ID have enabled us to achieve them for the first time using multi-anvil apparatuses.
The experiments were conducted with D-DIA type apparatus at 1-5 GPa and 25-1040°C, with in-situ observation using a 60 keV monochromatic X-ray from BM at SPring-8 BL04B1 and a 100 keV pink beam from ID at BL05XU and BL15XU. The starting material was sintered polycrystalline forsterite (Koizumi et al., 2010). It was processed into thin disks (~0.3 mm) and placed between alumina pistons which were saw-cut at a 45° angle to apply shear deformation. We carried out the deformation at a constant anvil displacement rate (0.2 mm/h) or with velocity stepping (0.2-2 mm/h). X-ray diffraction and radiography were measured to obtain pressure, stress-strain data, and localization behavior. The time resolution was 180-300 s with BM and 0.5 s with ID. Additionally, AE measurement was also conducted during the deformation to detect shear instability.
We first identified the variations in localization behaviors and their conditions using BM. We confirmed localization in all runs through radiography, except for the low-velocity deformation at highest temperature of 1040°C. Unstable slips accompanied by AEs occurred only within a limited range of conditions below 2 GPa and at 150-710°C, which is largely consistent with the conditions of earthquakes in warm slabs. Y-shear and Riedel shear planes developed after yielding, with the latter being particularly prominent in unstable slip involving AEs. Microstructural observation revealed the development of the fault gouge composed of submicron-sized particles, whose morphology varied with temperature. They had rounded shape and mylonitic texture at higher temperature, whereas they formed angular and cataclastic fracture structures at lower temperature. Furthermore, the yield stress exceeded the Goetze criterion (τ/P >0.5), suggesting that localization was initiated by brittle or semi-brittle crack formation. However, once faults formed, the strength decreased below that, transitioning to stable slip without AEs in some samples. In addition, even under identical conditions, variations in the friction coefficient and slip stability were observed, indicating that other factors, such as water content, must also be considered in these behaviors.
We also conducted velocity-step tests to examine the velocity dependence of the friction coefficient in unstable slips with ID. So far, we have successfully identified a trend of velocity weakening correlated with AE, localized deformation, and its onset. Nonetheless, challenges remain, including large measurement errors and stress variations due to changes in the diffraction acquisition position. While improving control precision is necessary, the latter issue also suggests the possibility of capturing the stress distribution within the sample, which could potentially be evaluated quantitatively through high-resolution stress mapping. Although these types of experiments had been previously difficult using the BM method, high-resolution measurements with ID have enabled us to achieve them for the first time using multi-anvil apparatuses.