9:45 AM - 10:00 AM
[SCG43-10] The olivine-ringwoodite transformation triggers shear localization and shear instability
Keywords:high-pressure deformation experiments, in-situ X-ray observation, acoustic emission, deep-focus earthquakes, shear instability, olivine-spinel transformation
The non-equilibrium transformation of olivine (Ol) to ringwoodite (Rw) has been proposed as one of the possible mechanisms for the deep-focus earthquakes occurring at depths down to ~680 km depth (~24 GPa) (e.g., Kirby et al., 1987, Green et al., 1991). Previous studies with analogues have demonstrated shear instability associated with the olivine-spinel transformation (e.g. Schubnel et al., 2013), however, the pressure conditions are still low (2-5 GPa) and close to the brittle-plastic transition. Here we report that the transformation of mantle olivine under uniaxial stress causes shear localization and shear instability under MTZ conditions.
We conducted deformation and transformation experiments at MTZ pressures using D-111 type apparatuses both at PF-AR NE7A beamline for in-situ X-ray observations and at Kyushu University for quenching method. The starting material was polycrystalline San Carlos olivine (grain size of ~3-120 µm). This was compressed to ~20 GPa and deformed at ~1183-1673K with constant anvil displacement rate of 300 µm/h. KMA-type 8ch acoustic emission (AE) measurement system was newly developed based on the previous MA 6-6 type system (Iwasato et al., 2020), and attached to the D-111 apparatus to record AEs during deformation. In the case of in-situ X-ray observations, we obtained stress-strain curves and reaction kinetics by measuring 2D-XRD patterns and radiography images every ~1-5 min using 60 keV monochromatic X-ray. The final strains reached ~25-53% with strain rates of 0.4-9.8 x10-5 s-1. Transformation and deformation microstructures of the recovered samples were observed with FE-SEM and FE-TEM.
The Ol-Rw transformation proceeded above ~1200K at ~20 GPa under uniaxial deformation. In contrast to the high strength of olivine (~5 GPa), the newly appeared Rw is very weak (~0.5-1.5 GPa) at ~1400K. The Rw strength is almost constant with increasing temperatures to 1673K, whereas it shows hardening during isothermal deformation. These creep behaviors in Rw can be reasonably interpreted by diffusion creep associated with grain growth (~50-200 nm). Similar results have been reported in the previous study (Mohiuddin et al., 2020).
Rw is produced in the Ol grain as a variety of lamellae and their thickness increase with temperatures. TEM observations revealed the existence of two types of lamellae: one is single-crystalline topotactic lamellae (STL) with a crystallographic orientation relationship of (100)Ol // {111}Rw, and another is nano-polycrystalline lamellae (NPL) with random orientations. NPL has multi-layer textures, in which Rw nano-grains with a grain size of ~20 nm form the center layer, and the outer layers are formed on each side by secondary nucleation and growth especially at higher temperatures. Similar lamellar textures have been reported in shocked meteorites (e.g., Chen et al., 2007).
We observed slip displacements only along the NPL at 1183-1423 K forming nano shear bands (NSBs). AEs were detected from the sample region only at the lowest temperature of 1183K, in which NSBs are very thin less than ~100 nm thick, and orient at ~45° to σ1 direction. No AEs were detected at higher temperatures, where NSBs become thicker and the orientation shifts to higher angles. These observations indicate the transition from the sharp unstable NSBs to the broad stable NSBs with temperatures. The angle of the unstable NSBs (~45°) is consistent with that observed in plastic faulting of ice where shear stress is maximum (Kirby et al., 1991, Renshaw & Schulson, 2017). Thickening of NSBs by grain growth would stabilize the slip and further promote homogeneous deformation of the whole sample, making the angle of inactive NSBs higher.
In this study, we directly demonstrated that the olivine-ringwoodite transformation triggers shear localization and shear instability at MTZ pressures. This is essential to understand the process of deep-focus earthquakes and the rheological weakening of slabs.
We conducted deformation and transformation experiments at MTZ pressures using D-111 type apparatuses both at PF-AR NE7A beamline for in-situ X-ray observations and at Kyushu University for quenching method. The starting material was polycrystalline San Carlos olivine (grain size of ~3-120 µm). This was compressed to ~20 GPa and deformed at ~1183-1673K with constant anvil displacement rate of 300 µm/h. KMA-type 8ch acoustic emission (AE) measurement system was newly developed based on the previous MA 6-6 type system (Iwasato et al., 2020), and attached to the D-111 apparatus to record AEs during deformation. In the case of in-situ X-ray observations, we obtained stress-strain curves and reaction kinetics by measuring 2D-XRD patterns and radiography images every ~1-5 min using 60 keV monochromatic X-ray. The final strains reached ~25-53% with strain rates of 0.4-9.8 x10-5 s-1. Transformation and deformation microstructures of the recovered samples were observed with FE-SEM and FE-TEM.
The Ol-Rw transformation proceeded above ~1200K at ~20 GPa under uniaxial deformation. In contrast to the high strength of olivine (~5 GPa), the newly appeared Rw is very weak (~0.5-1.5 GPa) at ~1400K. The Rw strength is almost constant with increasing temperatures to 1673K, whereas it shows hardening during isothermal deformation. These creep behaviors in Rw can be reasonably interpreted by diffusion creep associated with grain growth (~50-200 nm). Similar results have been reported in the previous study (Mohiuddin et al., 2020).
Rw is produced in the Ol grain as a variety of lamellae and their thickness increase with temperatures. TEM observations revealed the existence of two types of lamellae: one is single-crystalline topotactic lamellae (STL) with a crystallographic orientation relationship of (100)Ol // {111}Rw, and another is nano-polycrystalline lamellae (NPL) with random orientations. NPL has multi-layer textures, in which Rw nano-grains with a grain size of ~20 nm form the center layer, and the outer layers are formed on each side by secondary nucleation and growth especially at higher temperatures. Similar lamellar textures have been reported in shocked meteorites (e.g., Chen et al., 2007).
We observed slip displacements only along the NPL at 1183-1423 K forming nano shear bands (NSBs). AEs were detected from the sample region only at the lowest temperature of 1183K, in which NSBs are very thin less than ~100 nm thick, and orient at ~45° to σ1 direction. No AEs were detected at higher temperatures, where NSBs become thicker and the orientation shifts to higher angles. These observations indicate the transition from the sharp unstable NSBs to the broad stable NSBs with temperatures. The angle of the unstable NSBs (~45°) is consistent with that observed in plastic faulting of ice where shear stress is maximum (Kirby et al., 1991, Renshaw & Schulson, 2017). Thickening of NSBs by grain growth would stabilize the slip and further promote homogeneous deformation of the whole sample, making the angle of inactive NSBs higher.
In this study, we directly demonstrated that the olivine-ringwoodite transformation triggers shear localization and shear instability at MTZ pressures. This is essential to understand the process of deep-focus earthquakes and the rheological weakening of slabs.