11:30 〜 11:45
[SCG49-16] Uniaxial deformation of ringwoodite under hydrous conditions
キーワード：水、リングウッダイト、変形実験、X線その場観察
It has been suggested that hydrous minerals in subducting slabs transport water to the mantle transition zone (410-660 km depth). Ringwoodite is a major olivine polymorph in the lower part of the mantle transition zone and can contain water up to 3.3 wt.% H2O (e.g., Kohlstedt et al., 1996). Water is known to promote plastic deformation of minerals, and the strength of slabs in a hydrous environment may be weaker than in dry conditions. It has been reported that the water content in nominally anhydrous minerals increases after the dehydration of hydrous minerals (e.g., Ishii and Ohtani., 2021), but its effect on the plastic deformation of high-pressure minerals has not been well investigated.
In this study, we conducted high pressure deformation experiments using the D-111 type apparatus combined with synchrotron radiation under mantle transition zone conditions (18-20 GPa at 1073K) to investigate the plastic strength of ringwoodite under hydrous conditions. The deformation experiments were carried out at the NE-7 beamline of PFAR. We put nominally dry polycrystalline ringwoodite into the sample capsules made of hydrous minerals such as antigorite and talc, and deform it under uniaxial stress after the dehydration of the capsule at high pressures. Dehydration reactions in the sample capsule and stress-strain curves in ringwoodite were observed by in-situ X-ray observations. The final strains reached to 20-27% with strain rates of 1.5-4.9 x10-5 s-1. Temperature was measured by using thermocouples located near the sample capsule, however finally estimated from the input power because the thermocouple became unstable. In situ X-ray diffraction measurements and analysis of the recovered samples indicate that the talc capsule used in Run hrwdf11 decomposed into Phase D + stishovite + ringwoodite, and the antigorite capsule used in Run hrwdf12 decomposed into Phase D + superhydrous phase B (ShB) + H2O when increasing temperature to 1073K at ~18-20 GPa. The latter is a dehydration reaction, and the sample part is expected to be under hydrous condition. The flow stress of ringwoodite in this Run (hrwdf12) is 1/3 of that in Run hrwdf11, implying the water weakening in ringwoodite. In-situ X-ray and SEM observations indicate that grain growth and dynamic recrystallization occurred in Run hrwdf12. These processes may also be enhanced by water. However, since there were no significant differences in water contents of recovered ringwoodite samples between these runs (~560 wt. ppm H2O), water might be escaped from the sample during the deformation stage. The weakening in ringwoodite in Run hrwdf12 may be derived from the differences not only in hydrous conditions but also in temperatures and grain sizes.
In this study, we conducted high pressure deformation experiments using the D-111 type apparatus combined with synchrotron radiation under mantle transition zone conditions (18-20 GPa at 1073K) to investigate the plastic strength of ringwoodite under hydrous conditions. The deformation experiments were carried out at the NE-7 beamline of PFAR. We put nominally dry polycrystalline ringwoodite into the sample capsules made of hydrous minerals such as antigorite and talc, and deform it under uniaxial stress after the dehydration of the capsule at high pressures. Dehydration reactions in the sample capsule and stress-strain curves in ringwoodite were observed by in-situ X-ray observations. The final strains reached to 20-27% with strain rates of 1.5-4.9 x10-5 s-1. Temperature was measured by using thermocouples located near the sample capsule, however finally estimated from the input power because the thermocouple became unstable. In situ X-ray diffraction measurements and analysis of the recovered samples indicate that the talc capsule used in Run hrwdf11 decomposed into Phase D + stishovite + ringwoodite, and the antigorite capsule used in Run hrwdf12 decomposed into Phase D + superhydrous phase B (ShB) + H2O when increasing temperature to 1073K at ~18-20 GPa. The latter is a dehydration reaction, and the sample part is expected to be under hydrous condition. The flow stress of ringwoodite in this Run (hrwdf12) is 1/3 of that in Run hrwdf11, implying the water weakening in ringwoodite. In-situ X-ray and SEM observations indicate that grain growth and dynamic recrystallization occurred in Run hrwdf12. These processes may also be enhanced by water. However, since there were no significant differences in water contents of recovered ringwoodite samples between these runs (~560 wt. ppm H2O), water might be escaped from the sample during the deformation stage. The weakening in ringwoodite in Run hrwdf12 may be derived from the differences not only in hydrous conditions but also in temperatures and grain sizes.