14:30 〜 14:45
[SIT14-15] SOR and AE studies on syndeformational olivine transformations up to 25 GPa
キーワード：深発地震、せん断不安定化、オリビン‐スピネル相転移、アコースティックエミッション、高圧実験
The deep-focus earthquakes occur beyond the brittle-plastic transition of rock in the cold subducting slabs. Their foci are distributed whole the mantle transition zone (MTZ; ~300-690 km depth and ~10-24 GPa) and vanish in the lower mantle. Previous studies suggested the exothermic non-equilibrium transformation of olivine (Ol) to wadsleyite (Wds) and ringwoodite (Rwd) as one of the plausible mechanisms of the deep seismicity (e.g., Schubnel et al., 2013; Ohuchi et al., 2022; Honda et al., JpGU2023), and endothermic transformation of Rwd to post-spinel phases (akimotoite or bridgmanite and ferropericlase) may be responsible for the aseismicity of the lower mantle (e.g., Ito and Sato, 1991; Goto et al., JpGU2023). However, direct experimental evidence at MTZ pressures has been rather limited. Our recent deformation experiments at 20 GPa (Honda et al., JpGU2023) demonstrated that the nano-polycrystalline Rwd lamellae (NPL) trigger shear localization and instability due to their superplasticity associated with latent heat release and frictional heating, working as the nano shear bands (NSBs). We believe that the NSB model is an important process for deep seismicity, but it is still unclear whether this also works in a wide pressure range of MTZ, because the transformation textures and grain sizes of new phases possibly change with respect to the overpressure. To address this important issue, we conducted deformation experiments of mantle olivine in the stability fields of Wds, Rwd, and post-spinel phases.
The experiments were conducted with D-111 type high-pressure deformation apparatuses by in-situ X-ray observation method at PF-AR NE7A and SPring-8 BL04B1 beamlines. We used sintered San Carlos olivine polycrystal (grain size of 10-100 μm) as the starting material. It was deformed at 14-25 GPa and 420-1020 °C with a constant anvil displacement rate of 300 μm/h. We measured 2D-XRD patterns and radiography images every 1-5 minutes with 60 keV monochromatic X-ray to monitor the stress-strain curve and transformation kinetics during the deformation. The KMA-type 8-ch acoustic emission (AE) measurement system was also used to detect shear instability. The microstructure of the recovered sample was observed with the FE-SEM and FE-TEM.
In the Wds stability field (~14-16 GPa, 430-910 °C), the transformation occurred above 620 °C. In addition to the nucleation on the grain boundaries, we observed intracrystalline lamellae which are similar to the Rwd NPL produced at 20 GPa. Some NPL had slip displacements suggesting the shear localization occurred. However, no AEs and no clear stress drops were observed in all runs although AEs may have been missed due to the high noise level (~60 mV) at 620 °C. The stress of Wds was measured to be 0.6-1.8 GPa at 910 °C, which can be explained by the diffusion creep with grain size of 19-25 nm (Shimojuku et al., 2009). The strength of the Wds NPL is much weaker than that of Ol (~3 GPa), suggesting the stable NSBs were produced by superplastic flow of Wds at least relatively high temperatures of ~830-910 °C.
In the Rwd stability field, we observed shear instability (AEs and stress drop) associated with NSBs in a wider pressure range of 17-21 GPa and 640-830 °C. This supports that the NSB model confirmed at ~20 GPa is a common process in the Rwd stability field.
In the post-spinel stability field (~25 GPa, 760 °C), Ol metastably reacted to Rwd by ~37 % before deformation, and it further proceeded to ~80 % at the end of the deformation (~20% strain). We did not observe the post-spinel phases from XRD, and the shear instability. The stresses of Ol and Rwd were similar, ~6.5 and ~4.5 GPa, respectively. Rwd is no longer weak at this condition, which may inhibit shear instability.
Our results suggest that the shear localization can be induced by the exothermic transformation of metastable olivine in whole MTZ conditions by the NSB model, but further studies are needed to constrain the process of shear instability outside the Rwd stability field.
The experiments were conducted with D-111 type high-pressure deformation apparatuses by in-situ X-ray observation method at PF-AR NE7A and SPring-8 BL04B1 beamlines. We used sintered San Carlos olivine polycrystal (grain size of 10-100 μm) as the starting material. It was deformed at 14-25 GPa and 420-1020 °C with a constant anvil displacement rate of 300 μm/h. We measured 2D-XRD patterns and radiography images every 1-5 minutes with 60 keV monochromatic X-ray to monitor the stress-strain curve and transformation kinetics during the deformation. The KMA-type 8-ch acoustic emission (AE) measurement system was also used to detect shear instability. The microstructure of the recovered sample was observed with the FE-SEM and FE-TEM.
In the Wds stability field (~14-16 GPa, 430-910 °C), the transformation occurred above 620 °C. In addition to the nucleation on the grain boundaries, we observed intracrystalline lamellae which are similar to the Rwd NPL produced at 20 GPa. Some NPL had slip displacements suggesting the shear localization occurred. However, no AEs and no clear stress drops were observed in all runs although AEs may have been missed due to the high noise level (~60 mV) at 620 °C. The stress of Wds was measured to be 0.6-1.8 GPa at 910 °C, which can be explained by the diffusion creep with grain size of 19-25 nm (Shimojuku et al., 2009). The strength of the Wds NPL is much weaker than that of Ol (~3 GPa), suggesting the stable NSBs were produced by superplastic flow of Wds at least relatively high temperatures of ~830-910 °C.
In the Rwd stability field, we observed shear instability (AEs and stress drop) associated with NSBs in a wider pressure range of 17-21 GPa and 640-830 °C. This supports that the NSB model confirmed at ~20 GPa is a common process in the Rwd stability field.
In the post-spinel stability field (~25 GPa, 760 °C), Ol metastably reacted to Rwd by ~37 % before deformation, and it further proceeded to ~80 % at the end of the deformation (~20% strain). We did not observe the post-spinel phases from XRD, and the shear instability. The stresses of Ol and Rwd were similar, ~6.5 and ~4.5 GPa, respectively. Rwd is no longer weak at this condition, which may inhibit shear instability.
Our results suggest that the shear localization can be induced by the exothermic transformation of metastable olivine in whole MTZ conditions by the NSB model, but further studies are needed to constrain the process of shear instability outside the Rwd stability field.