10:45 〜 11:00
[SIT17-07] In situ observation of faulting in olivine under the pressure-temperature conditions of subducting slabs using a high-flux synchrotron X-ray
★Invited Papers
キーワード：スラブ内地震、カンラン石、アコースティック・エミッション、その場観察、X線CT
The subduction zone produces a major fraction of the Earth’s seismic activity. The mechanisms of intraslab earthquakes at depths of > 40 km are fundamentally different from those of shallow earthquakes. This is because the frictional strength of silicate rocks is proportional to the confining pressure and it exceeds the upper limit of the stress level in the upper mantle. To understand the process triggering intraslab earthquakes, many experimental studies on faulting of slab-forming rocks have been conducted at upper mantle pressures using a D-DIA apparatus or a Griggs rig. Previous studies have revealed that shear localization induced by dehydration of hydrous minerals (e.g., Okazaki & Hirth, 2016; Ferrand et al., 2017) or adiabatic shear heating (e.g., Kelemen & Hirth, 2007) is essential for the occurrence of faulting at high pressures. Although acoustic emission (AE) monitoring technique for D-DIA apparatuses enabled us to discuss the process of microcracking at high pressures, mechanical behavior at the onset of faulting is still unclear due to low time-resolution stress/strain measurements using a synchrotron X-ray. The cause of bottleneck in stress/strain measurements is a long exposure time required for the acquisition of a two-dimensional X-ray diffraction pattern of minerals. Considering that the timescale of stress drop associating faulting is on the order of 0.01 sec (e.g., Okazaki & Katayama, 2016), a significant improvement for time resolution of stress/strain measurements is required for the observations of precursors occurring prior to the faulting. To improve the time resolution of stress/strain measurements, we installed a series of new devices at BL15XU, SPring-8. In this talk, we will report recent progresses on in situ measurements for faulting in rocks at high pressures.
We conducted in situ triaxial deformation experiments on as-is olivine aggregates at pressures 1-3 GPa and temperatures 700-1250 K using a deformation-DIA apparatus, installed at BL15XU, SPring-8. Constant strain-rate deformation runs were performed. Two-dimensional radial diffraction patterns and X-ray radiographic images were alternately acquired by adjusting sizes of the incident slit and operating a cadmium telluride imaging detector and a CCD camera using a high-flux pink beam (energy 100 keV) from an undulator source with a double multilayer monochromator (0.4 s of exposure time for each). Pressure and differential stress were determined from the d-spacing of olivine. Strain of a deforming sample was evaluated from the distance between platinum strain markers. AEs were recorded continuously on six sensors, and three-dimensional AE source location were determined. The three-dimensional microstructures of the recovered samples were observed using fast X-ray computed micro-tomography (XCT) at BL20B2/SPring-8.
Stress increased with strain at the beginning of sample deformation, and it reached the yielding point at strains of ~0.1 or less. AEs from the deforming sample were detected when stress exceeded ~1 GPa and the amplitude of AE is positively correlated with the magnitude of stress. At strains higher than 0.1 (i.e., beyond the yielding point), both softening (i.e., decrease in stress and/or increase in strain rate) and a decrease in AE rate were observed prior to the occurrence of faulting. Faulting was followed by unstable slips. AE hypocenters were dispersed in the deforming sample at the beginning of deformation. XCT observations revealed that many AE hypocenters were located around the fault planes in the late deformation stage. Occurrence of large AEs repeated around a limited part of the fault after the occurrence of faulting, probably due to unstable slips. Our observations suggest that the detected AEs are related to the onset of formation of faults (i.e., localization of damage).
We conducted in situ triaxial deformation experiments on as-is olivine aggregates at pressures 1-3 GPa and temperatures 700-1250 K using a deformation-DIA apparatus, installed at BL15XU, SPring-8. Constant strain-rate deformation runs were performed. Two-dimensional radial diffraction patterns and X-ray radiographic images were alternately acquired by adjusting sizes of the incident slit and operating a cadmium telluride imaging detector and a CCD camera using a high-flux pink beam (energy 100 keV) from an undulator source with a double multilayer monochromator (0.4 s of exposure time for each). Pressure and differential stress were determined from the d-spacing of olivine. Strain of a deforming sample was evaluated from the distance between platinum strain markers. AEs were recorded continuously on six sensors, and three-dimensional AE source location were determined. The three-dimensional microstructures of the recovered samples were observed using fast X-ray computed micro-tomography (XCT) at BL20B2/SPring-8.
Stress increased with strain at the beginning of sample deformation, and it reached the yielding point at strains of ~0.1 or less. AEs from the deforming sample were detected when stress exceeded ~1 GPa and the amplitude of AE is positively correlated with the magnitude of stress. At strains higher than 0.1 (i.e., beyond the yielding point), both softening (i.e., decrease in stress and/or increase in strain rate) and a decrease in AE rate were observed prior to the occurrence of faulting. Faulting was followed by unstable slips. AE hypocenters were dispersed in the deforming sample at the beginning of deformation. XCT observations revealed that many AE hypocenters were located around the fault planes in the late deformation stage. Occurrence of large AEs repeated around a limited part of the fault after the occurrence of faulting, probably due to unstable slips. Our observations suggest that the detected AEs are related to the onset of formation of faults (i.e., localization of damage).