11:45 〜 12:00
[SCG49-17] Rheological weakening of the lower-mantle slab by superplastic flow of the post-spinel assemblage
キーワード：ポストスピネル相転移、変形実験、X線その場観察、レオロジー的弱化、超塑性流動、下部マントルスラブ
The post-spinel transformation, that is a decomposition reaction of (Mg,Fe)2SiO4 ringwoodite into (Mg,Fe)SiO3 bridgmanite and (Mg,Fe)O ferropericlase, occurs at the upper and lower mantle boundary. Seismological studies have shown large deformation and no deep earthquakes in the lower mantle slabs, which may be attributed to rheological weakening by the post-spinel transformation. This is a eutectoid reaction with alternating fine lamellar structure in the colony texture, and superplastic flow of the fine-grained post-spinel assemblage after the degeneration of the colony may be responsible for aseismicity in the lower mantle (Poirier et al., 1986, Ito and Sato, 1991). Experimental studies using analog materials at several GPa have shown that the single-crystal like eutectoid colony deforms by dislocation creep, and the degeneration is an important process for rheological weakening (e.g., Zhao et al. 2012; Doi et al. 2014). Deformation experiments of the post-spinel assemblage without transformation conducted at lower mantle pressures have shown that a large strength contrast exists between bridgmanite and ferropericlase, in which the superplastic flow did not occur, but the stiffer bridgmanite was deformed by dislocation creep (Girard et al., 2016). Thus, although the superplasticity of the post-spinel assemblage is quite important for the rheology of the lower-mantle slab, it has not been experimentally demonstrated so far. To understand superplastic flow of the lower-mantle slab across the 660km discontinuity, it is essential to examine the rheological evolution during the post-spinel transformation.
In this study, syn-deformational post-spinel transformation experiments in (Mg,Fe)2SiO4 were conducted 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. The starting material is a synthesized polycrystalline ringwoodite (grain size ~a few to 10 µm). The sample was uniaxially deformed at ~22-28 GPa and ~800-1350°C (the overpressure of ~0-6.0 GPa) with increasing temperatures (~0.08°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. Deformation and transformation microstructures in recovered samples were examined by FE-SEM and TEM.
The strain rates were 4.8-23 x 10-5 s-1 after the initiation of the transformation. The final strains and transformed fraction were ~30-60% and ~50-100%, respectively. The ringwoodite deformation reached steady state at the strain of ~5% and the stress of ~4 GPa, and then exhibited slightly weakening with temperatures, which can be interpreted by Peierls mechanism. The flow stresses of newly appeared bridgmanite and ferropericlase are generally smaller than that of ringwoodite, and measured to be ~0.5-1.1 GPa and ~0-0.7 GPa, respectively. The weakening of ringwoodite became more significant at higher than ~1000-1200°C, which may result from the phase transformation and/or the transition in flow mechanism to dislocation creep. FE-SEM observation of the recovered samples revealed the overpressure dependencies of the colony size (~1-3 µm), lamellar spacing (~0.05-0.3 µm), and degenerated grain size (~0.2-3 µm). The degenerated weaker ferropericlase grains do not connect each other in the bridgmanite matrix, which supports the mechanical data (i.e., σbrg > σfp).
The flow stress of bridgmanite is much smaller than that expected in dislocation creep (Tsujino et al., 2021), and can be reasonably interpreted by diffusion creep considering not colony size but degenerated grain size and the Si diffusivity (Yamazaki et al., 2000). Our study demonstrated that the degeneration of the post-spinel eutectoid colony readily occurs during the syn-deformational transformation, leading to rheological weakening by superplastic flow of the post-spinel assemblage.
In this study, syn-deformational post-spinel transformation experiments in (Mg,Fe)2SiO4 were conducted 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. The starting material is a synthesized polycrystalline ringwoodite (grain size ~a few to 10 µm). The sample was uniaxially deformed at ~22-28 GPa and ~800-1350°C (the overpressure of ~0-6.0 GPa) with increasing temperatures (~0.08°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. Deformation and transformation microstructures in recovered samples were examined by FE-SEM and TEM.
The strain rates were 4.8-23 x 10-5 s-1 after the initiation of the transformation. The final strains and transformed fraction were ~30-60% and ~50-100%, respectively. The ringwoodite deformation reached steady state at the strain of ~5% and the stress of ~4 GPa, and then exhibited slightly weakening with temperatures, which can be interpreted by Peierls mechanism. The flow stresses of newly appeared bridgmanite and ferropericlase are generally smaller than that of ringwoodite, and measured to be ~0.5-1.1 GPa and ~0-0.7 GPa, respectively. The weakening of ringwoodite became more significant at higher than ~1000-1200°C, which may result from the phase transformation and/or the transition in flow mechanism to dislocation creep. FE-SEM observation of the recovered samples revealed the overpressure dependencies of the colony size (~1-3 µm), lamellar spacing (~0.05-0.3 µm), and degenerated grain size (~0.2-3 µm). The degenerated weaker ferropericlase grains do not connect each other in the bridgmanite matrix, which supports the mechanical data (i.e., σbrg > σfp).
The flow stress of bridgmanite is much smaller than that expected in dislocation creep (Tsujino et al., 2021), and can be reasonably interpreted by diffusion creep considering not colony size but degenerated grain size and the Si diffusivity (Yamazaki et al., 2000). Our study demonstrated that the degeneration of the post-spinel eutectoid colony readily occurs during the syn-deformational transformation, leading to rheological weakening by superplastic flow of the post-spinel assemblage.