17:15 〜 18:45
[SCG44-P03] Effects of water on kinetic and creep behaviors during the post-spinel transformation
キーワード：ポストスピネル相転移、変形相転移実験、X線その場観察、含水量
It has been suggested that rheological weakening of subductiong slab across the 660km seismic discontinuity is attributed to the post-spinel transformation. We experimentally demonstrated that the weakening occurs during this transformation by the superplastic flow and the interconnection of the weaker phase of periclase (Goto et al., JPGU23). Under subduction zone conditions, water is another possible important factor for rheological weakening, however it has not been examined so far. Here we report results of preliminary experiments on this issue.
In this study, we carried out syn-deformational post-spinel transformation experiments at lower-mantle pressures by in-situ X-ray observation method using D-111 type high-pressure deformation apparatuses at the synchrotron facilities of PF-AR NE-7 and SPring-8 BL04B1 beamlines.We used three kinds of polycrystalline ringwoodite with different water contents as starting materials, those are synthesized at 22 GPa and ~1400°C from San Carlos olivine monocrystal, Mg2SiO4 forsterite monocrystal, and Mg2SiO4 forsterite powder. Their water contents were estimated to be 500-850, 290, and 2500 wt. ppm H2O, respectively, by FTIR measurements.
The sample was uniaxially deformed at ~22-28 GPa and ~500-1350°C (the overpressure of ~0-6.0 GPa) with increasing temperatures (~0.3°C/s) or pressures (~0.6 GPa/h) to cause the post-spinel transformation. 2D-XRD patterns and radiography images were taken every ~1-5 min to obtain stress-strain and transformation-time curves. The strain rates were 4.8-23 x 10-5 s-1 during the post-spinel transformation.
The results of our experiments suggest that kinetics of the post-spinel transformation and creep strengths of ringwoodite and post-spinel phases differ with water contents of the starting ringwoodite. Firstly, water accelerates the post-spinel transformation dramatically. The wet ringwoodite containing 2500 wt. ppm H2O metastably transformed to akimotoite and periclase in ~10 min, and immediately changed to bridgmanite and periclase, at the overpressure (dP) of ~1GPa and 1150°C. In contrast, the post-spinel transformation in the dry ringwoodite with 290 wt. ppm H2O did not start even at dP of 2~ GPa and 1300°C. Because the stability field locates at low temperatures (Kojitani et al., 2022), it has been difficult to observe the stable assemblage of akimotoite and periclase experimentally, which may be possible owing to the fast kinetics under wet conditions.
Secondly, the flow stress of ringwoodite decreases with water contents. In our experiments, the flow stress of the drier ringwoodite with 500-850 and 290 wt. ppm H2O is about ten times larger than that of ringwoodite with 2500 wt. ppm H2O. The transition temperature from Peierls mechanism to dislocation creep are also different between 290 and 500-850 wt. ppm H2O ringwoodite, there was a difference of more than 150°C in the temperature at which flow law switching.
Finally, the strength of post-spinel phases possibly decreases by water. We have already demonstrated the superplastic weakening of bridgmanite + ferropericlase assemblage just after the post-spinel transformation under nominally dry condition(Goto et al., JPGU23). The present study suggests that the flow strength of the post-spinel assemblage from wet ringwoodite with 2500 wt. ppm H2O is lower than that of the fine-grained drier post-spinel assemblage. Thus, water enhances the post-spinel transformation and plastically weakens both reactant and product phases, which is important to understand the deep slab behaviors across the 660 km discontinuity.
In this study, we carried out syn-deformational post-spinel transformation experiments at lower-mantle pressures by in-situ X-ray observation method using D-111 type high-pressure deformation apparatuses at the synchrotron facilities of PF-AR NE-7 and SPring-8 BL04B1 beamlines.We used three kinds of polycrystalline ringwoodite with different water contents as starting materials, those are synthesized at 22 GPa and ~1400°C from San Carlos olivine monocrystal, Mg2SiO4 forsterite monocrystal, and Mg2SiO4 forsterite powder. Their water contents were estimated to be 500-850, 290, and 2500 wt. ppm H2O, respectively, by FTIR measurements.
The sample was uniaxially deformed at ~22-28 GPa and ~500-1350°C (the overpressure of ~0-6.0 GPa) with increasing temperatures (~0.3°C/s) or pressures (~0.6 GPa/h) to cause the post-spinel transformation. 2D-XRD patterns and radiography images were taken every ~1-5 min to obtain stress-strain and transformation-time curves. The strain rates were 4.8-23 x 10-5 s-1 during the post-spinel transformation.
The results of our experiments suggest that kinetics of the post-spinel transformation and creep strengths of ringwoodite and post-spinel phases differ with water contents of the starting ringwoodite. Firstly, water accelerates the post-spinel transformation dramatically. The wet ringwoodite containing 2500 wt. ppm H2O metastably transformed to akimotoite and periclase in ~10 min, and immediately changed to bridgmanite and periclase, at the overpressure (dP) of ~1GPa and 1150°C. In contrast, the post-spinel transformation in the dry ringwoodite with 290 wt. ppm H2O did not start even at dP of 2~ GPa and 1300°C. Because the stability field locates at low temperatures (Kojitani et al., 2022), it has been difficult to observe the stable assemblage of akimotoite and periclase experimentally, which may be possible owing to the fast kinetics under wet conditions.
Secondly, the flow stress of ringwoodite decreases with water contents. In our experiments, the flow stress of the drier ringwoodite with 500-850 and 290 wt. ppm H2O is about ten times larger than that of ringwoodite with 2500 wt. ppm H2O. The transition temperature from Peierls mechanism to dislocation creep are also different between 290 and 500-850 wt. ppm H2O ringwoodite, there was a difference of more than 150°C in the temperature at which flow law switching.
Finally, the strength of post-spinel phases possibly decreases by water. We have already demonstrated the superplastic weakening of bridgmanite + ferropericlase assemblage just after the post-spinel transformation under nominally dry condition(Goto et al., JPGU23). The present study suggests that the flow strength of the post-spinel assemblage from wet ringwoodite with 2500 wt. ppm H2O is lower than that of the fine-grained drier post-spinel assemblage. Thus, water enhances the post-spinel transformation and plastically weakens both reactant and product phases, which is important to understand the deep slab behaviors across the 660 km discontinuity.