2:00 PM - 2:15 PM
[SIT16-07] The olivine-ringwoodite transformation under uniaxial stress: Implications for seismicity and rheological weakening in deep slabs
Keywords:deep-focus earthquake, slab weakening, shear instability, olivine-spinel transformation, acoustic emission
In the subducting cold slab, the metastable olivine (Ol) causes non-equilibrium transformation to ringwoodite (Rw) at the lower part of the mantle transition zone (MTZ; 410-670km depth). This has been considered to trigger deep-focus earthquakes (DEQs) based on experimental studies, but they have not been conducted at the pressures of deep MTZ where DEQs most frequently occur (e.g. Kirby et al., 1987; Wang et al., 2017; Ohuchi et al., 2022). In contrast, the deep slab behaves rheologically weak, causing stagnation on the bottom of MTZ (e.g. Fukao and Obayashi, 13). This is also interpreted as the result of the grain-size reduction due to the Ol-Rw transformation. However, there has been no clear demonstration explaining these paradoxical slab behaviors by the same transformation. Here, we report experimental results on the Ol-Rw transformation under uniaxial stress that can trigger both the shear instability and the bulk weakening.
We conducted deformation experiments using the D111-type apparatus combined with KMA-type 8ch acoustic emission (AE) measurement system both at PF-AR NE7A beamline for in-situ X-ray observations and at Kyushu University for the quenching method. The starting material is San Carlos olivine (grain size of 10-100 μm). The samples were deformed at ~20-22GPa and 570-1330°C with an anvil displacement rate of 300 μm/h. We obtained the curves of the stress-strain and reaction kinetics by measuring 2D-XRD and radiography images every 1-5 min using the 60 keV monochromatic X-ray. The quenched samples were observed with the FE-SEM and FE-TEM.
Ol was deformed by the Peierls mechanism at high differential stresses (5-6 GPa), which produces many dislocations forming planar defects (deformation lamellae). Transformation proceeded over 760°C and shear instability only occurred in the limited temperature range of 760-860°C, where stress drops of ~1.5 GPa were repeatedly observed with large AEs during the deformation. In these samples, nano-grain of Rw (~10nm) nucleated on the deformation lamellae forming the nano poly-crystalline lamellae (NPL), which causes the unstable slips with displacements of < 1μm (nano-shear bands; NSBs) and throughgoing faults. The shear instability also occurred at a higher pressure of ~22GPa and 860°C. The shear localization was also confirmed at higher temperatures of ~990°C, but no stress drops and AEs were observed implying the stable slip. We suggest that the shear instability only occurs at the limited temperature range where the thermal feedback between the latent heat release and the shear heating effectively work. This condition is almost consistent with those for DEQs.
At higher temperatures, the grain-size reduction effectively occurs in the bulk sample by the secondary nucleation of Rw on the NPL with incoherent nature. We confirmed that Rw deforms by superplastic flow with smaller stress than Ol, leading to the bulk rock weakening. The single-crystalline topotactic lamellae (STL) partially developed with a crystallographic relationship, but they did not play an important role in the reaction progress and the deformation. Thus, the formation of Rw NPL under differential stress has potentially important roles both for the shear instability in lower temperatures and the bulk rock weakening in higher temperatures.
We conducted deformation experiments using the D111-type apparatus combined with KMA-type 8ch acoustic emission (AE) measurement system both at PF-AR NE7A beamline for in-situ X-ray observations and at Kyushu University for the quenching method. The starting material is San Carlos olivine (grain size of 10-100 μm). The samples were deformed at ~20-22GPa and 570-1330°C with an anvil displacement rate of 300 μm/h. We obtained the curves of the stress-strain and reaction kinetics by measuring 2D-XRD and radiography images every 1-5 min using the 60 keV monochromatic X-ray. The quenched samples were observed with the FE-SEM and FE-TEM.
Ol was deformed by the Peierls mechanism at high differential stresses (5-6 GPa), which produces many dislocations forming planar defects (deformation lamellae). Transformation proceeded over 760°C and shear instability only occurred in the limited temperature range of 760-860°C, where stress drops of ~1.5 GPa were repeatedly observed with large AEs during the deformation. In these samples, nano-grain of Rw (~10nm) nucleated on the deformation lamellae forming the nano poly-crystalline lamellae (NPL), which causes the unstable slips with displacements of < 1μm (nano-shear bands; NSBs) and throughgoing faults. The shear instability also occurred at a higher pressure of ~22GPa and 860°C. The shear localization was also confirmed at higher temperatures of ~990°C, but no stress drops and AEs were observed implying the stable slip. We suggest that the shear instability only occurs at the limited temperature range where the thermal feedback between the latent heat release and the shear heating effectively work. This condition is almost consistent with those for DEQs.
At higher temperatures, the grain-size reduction effectively occurs in the bulk sample by the secondary nucleation of Rw on the NPL with incoherent nature. We confirmed that Rw deforms by superplastic flow with smaller stress than Ol, leading to the bulk rock weakening. The single-crystalline topotactic lamellae (STL) partially developed with a crystallographic relationship, but they did not play an important role in the reaction progress and the deformation. Thus, the formation of Rw NPL under differential stress has potentially important roles both for the shear instability in lower temperatures and the bulk rock weakening in higher temperatures.