14:15 〜 14:30
[SIT16-08] Role of ferropericlase and akimotoite in rheological weakening of subducting slab across the upper and lower mantle boundary
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、フェロペリクレース、アキモトアイト
Rheological weakening of subductiong slab across the 660km seismic discontinuity have been discussed during the past few decades. From the observation of post-spinel transformed microstructure, it is revealed that this transformation is a eutectoid reaction with alternating fine lamellar structure in the colony texture, and suggested that the superplastic flow weakens the slab (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 initially deforms by dislocation creep, and the superplastic flow dominates after the degeneration of the colony, which is an important process for rheological weakening (e.g., Doi et al., 2014). Recently, deformation experiments of the post-spinel assemblage were conducted at lower-mantle pressures without transformation (Girard et al., 2016), and showed the two-phase flow of stiffer bridgmanite and weaker ferropericlase, suggesting that rheological weakening of the assemblage may occur by interconnection of ferropericlase in larger strains. Thus, rheological behaviors of the deep slab across the upper and lower mantle boundary have not been well understood. In addition, it has been suggested that the two-stage post-spinel transformation through the akimotoite and ferropericlase assemblage is kinetically possible (Kubo et al., JPGU18), and it is necessary to consider its effects on the rheology.
In order to assess these issues, 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 two kinds of starting materials, (Mg,Fe)2SiO4 sintered polycrystalline ringwoodite and Mg2SiO4 forsterite powder. The latter was first transformed to ringwoodite just before the deformation stage. The sample was uniaxially deformed at ~21-28 GPa and ~800-1340°C with increasing temperatures or pressures to cause the post-spinel transformation. 2D-XRD patterns and X-ray radiography images were taken every ~1-5 min to obtain stress-strain and transformation-time curves. The strain rates were 3.4-28 x 10-5 s-1 during the post-spinel transformation.
FE-SEM observations revealed that the post-spinel eutectoid colonies are formed from the grain boundary of parental ringwoodite, and its degeneration occurs during deformation. We observed that rheological behaviors and microstructures changes with dP.
When the transformation occurred at small dP, the post-spinel phases deform under nearly iso-stress condition (i.e., σbrg = σfp). The flow stress of bridgmanite is too small to be interpreted by its diffusion creep, suggesting that weaker ferropericlase dominates the bulk deformation. The texture in recovered sample showed that the granular bridgmanite grains are present in the irregularly-shaped ferropericlase matrix. This may be caused by the preferential spheroidizing of the stiffer bridgmanite in relatively large eutectoid colony deformed by dislocation creep at small dP. It has been suggested that the post-spinel transformation occur at small dP under subduction conditions. Therefore, the rheological weakening due to the formation of the interconnected ferropericlase can be an important process for the large deformation of the slab across the upper and lower mantle boundary.
We observed the two-stage post-spinel transformation through the akimotoite and ferropericlase assemblage when using Mg2SiO4 ringwoodite. It is noteworthy that the colony size of bridgmanite and ferropericlase becomes large significantly even at large dP probably because it formed through akimotoite. This possibly leads to the formation of the interconnected ferropericlase when degeneration although further studies are needed.
In order to assess these issues, 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 two kinds of starting materials, (Mg,Fe)2SiO4 sintered polycrystalline ringwoodite and Mg2SiO4 forsterite powder. The latter was first transformed to ringwoodite just before the deformation stage. The sample was uniaxially deformed at ~21-28 GPa and ~800-1340°C with increasing temperatures or pressures to cause the post-spinel transformation. 2D-XRD patterns and X-ray radiography images were taken every ~1-5 min to obtain stress-strain and transformation-time curves. The strain rates were 3.4-28 x 10-5 s-1 during the post-spinel transformation.
FE-SEM observations revealed that the post-spinel eutectoid colonies are formed from the grain boundary of parental ringwoodite, and its degeneration occurs during deformation. We observed that rheological behaviors and microstructures changes with dP.
When the transformation occurred at small dP, the post-spinel phases deform under nearly iso-stress condition (i.e., σbrg = σfp). The flow stress of bridgmanite is too small to be interpreted by its diffusion creep, suggesting that weaker ferropericlase dominates the bulk deformation. The texture in recovered sample showed that the granular bridgmanite grains are present in the irregularly-shaped ferropericlase matrix. This may be caused by the preferential spheroidizing of the stiffer bridgmanite in relatively large eutectoid colony deformed by dislocation creep at small dP. It has been suggested that the post-spinel transformation occur at small dP under subduction conditions. Therefore, the rheological weakening due to the formation of the interconnected ferropericlase can be an important process for the large deformation of the slab across the upper and lower mantle boundary.
We observed the two-stage post-spinel transformation through the akimotoite and ferropericlase assemblage when using Mg2SiO4 ringwoodite. It is noteworthy that the colony size of bridgmanite and ferropericlase becomes large significantly even at large dP probably because it formed through akimotoite. This possibly leads to the formation of the interconnected ferropericlase when degeneration although further studies are needed.