17:15 〜 18:30
[SCG46-P02] Creep behavior during the post-spinel transformation in (Mg, Fe)2SiO4
キーワード：ポストスピネル相転移、共析反応、変形実験、X線その場観察、スラブ変形、レオロジー的弱化
Seismological studies have shown large deformation and no deep earthquakes in the lower mantle slabs, which may be attributed to the post-spinel transformation (the decomposition reaction of (Mg,Fe)2SiO4 ringwoodite into (Mg,Fe)SiO3 bridgmanite and (Mg,Fe)O ferropericlase). Poirier et al. (1986) reported that the post-spinel transformation is a eutectoid reaction with alternating fine lamellar structure in the colony texture. Based on microstructural observations, Ito and Sato (1991) suggested that aseismicity in the lower mantle is caused by superplastic flow of the fine-grained post-spinel assemblage. The weakening of the lower mantle slab has also been suggested from the degree of the grain-size reduction and diffusion creep viscosity (e.g., Kubo et al., 2009). Syn-deformational reaction experiments using analog materials have shown that eutectoid reaction does not directly weaken the material, but the degeneration of eutectoid colony is an important process for rheological weakening (e.g., Doi et al. 2014; Zhao et al. 2012). However, there have been no direct experimental studies to understand effects of the post-spinel transformation on creep behavior.
In this study, syn-deformational post-spinel transformation experiments in (Mg, Fe)2 SiO4 were conducted by in-situ X-ray observation method using a D-111 type deformation apparatus (MAX-III) at PF-AR NE-7 beamline. The starting material is polycrystalline ringwoodite (grain size ~a few to 10 µm) that was synthesized from single crystalline San Carlos olivine. The ringwoodite sample was first uniaxailly deformed at ~27 GPa and 600°C to ~3% strain, and then heated to around 1000°C at a constant rate of 0.06°C/s to cause the post-spinel transformation. 2D-XRD patterns and radiography images were taken every ~5 min to obtain stress-strain and transformation-time curves. Deformation and transformation microstructures in recovered samples were examined by FE-SEM.
We conducted two runs at the similar P, T, and strain rate conditions. The strain rates changed from 1.3-1.5e-5 s-1 to 4.9-5.6e-5 s-1 at the timing of the initiation of the transformation. The final strains and transformed fraction were ~27-37% and ~50-80%, respectively. Creep behaviors in ringwoodite suggested that it reached steady state at the strain of ~5% and the stress of ~4 GPa, and then exhibited weakening with the increase in temperature. The decrease in the flow stress of ringwoodite became more rapidly at higher than ~800-850°C, which may be linked to the initiation of the transformation. However, the stress of bridgmanite newly transformed is larger than that of ringwoodite. FE-SEM observation revealed that many eutectoid colonies with the size of ~1 µm in diameter are present at grain boundaries of ringwoodite. The lamellar spacing in the colony is ~50 nm. It is noteworthy that the eutectoid colonies themselves are not deformed and degenerated after the deformation with nearly 30% strain. Both the colony size and lamellar spacing are much smaller than those observed in transformation experiments with similar overpressures but no deformation (e.g., Kubo et al., 2011).
Each phase (bridgmanite and ferropericlase) in a eutectoid colony is thought to be an interconnected single crystal. No deformation in the eutectoid colonies implies that the stiffer bridgmanite supports the colony (i.e., isostrain in the two phase of the colony), which means that the transformed post-spinel colony is stiffer than the parental ringwoodite. Thus, it is unlikely that the reaction-induced weakening occurs in our experiments. The weakening of ringwoodite observed at higher than ~800-850°C may be originated by the change of the dominant deformation mechanism from low-temperature plasticity to dislocation creep (e.g., Imamura, 2018). Our study demonstrates that rheological weakening due to the post-spinel transformation is difficult at low temperatures ~1000°C and larger strains are required to degenerate the post-spinel colony.
In this study, syn-deformational post-spinel transformation experiments in (Mg, Fe)2 SiO4 were conducted by in-situ X-ray observation method using a D-111 type deformation apparatus (MAX-III) at PF-AR NE-7 beamline. The starting material is polycrystalline ringwoodite (grain size ~a few to 10 µm) that was synthesized from single crystalline San Carlos olivine. The ringwoodite sample was first uniaxailly deformed at ~27 GPa and 600°C to ~3% strain, and then heated to around 1000°C at a constant rate of 0.06°C/s to cause the post-spinel transformation. 2D-XRD patterns and radiography images were taken every ~5 min to obtain stress-strain and transformation-time curves. Deformation and transformation microstructures in recovered samples were examined by FE-SEM.
We conducted two runs at the similar P, T, and strain rate conditions. The strain rates changed from 1.3-1.5e-5 s-1 to 4.9-5.6e-5 s-1 at the timing of the initiation of the transformation. The final strains and transformed fraction were ~27-37% and ~50-80%, respectively. Creep behaviors in ringwoodite suggested that it reached steady state at the strain of ~5% and the stress of ~4 GPa, and then exhibited weakening with the increase in temperature. The decrease in the flow stress of ringwoodite became more rapidly at higher than ~800-850°C, which may be linked to the initiation of the transformation. However, the stress of bridgmanite newly transformed is larger than that of ringwoodite. FE-SEM observation revealed that many eutectoid colonies with the size of ~1 µm in diameter are present at grain boundaries of ringwoodite. The lamellar spacing in the colony is ~50 nm. It is noteworthy that the eutectoid colonies themselves are not deformed and degenerated after the deformation with nearly 30% strain. Both the colony size and lamellar spacing are much smaller than those observed in transformation experiments with similar overpressures but no deformation (e.g., Kubo et al., 2011).
Each phase (bridgmanite and ferropericlase) in a eutectoid colony is thought to be an interconnected single crystal. No deformation in the eutectoid colonies implies that the stiffer bridgmanite supports the colony (i.e., isostrain in the two phase of the colony), which means that the transformed post-spinel colony is stiffer than the parental ringwoodite. Thus, it is unlikely that the reaction-induced weakening occurs in our experiments. The weakening of ringwoodite observed at higher than ~800-850°C may be originated by the change of the dominant deformation mechanism from low-temperature plasticity to dislocation creep (e.g., Imamura, 2018). Our study demonstrates that rheological weakening due to the post-spinel transformation is difficult at low temperatures ~1000°C and larger strains are required to degenerate the post-spinel colony.