日本地球惑星科学連合2022年大会

講演情報

[E] ポスター発表

セッション記号 S (固体地球科学) » S-CG 固体地球科学複合領域・一般

[S-CG43] スラブ内地震とその発生メカニズム

2022年6月2日(木) 11:00 〜 13:00 オンラインポスターZoom会場 (28) (Ch.28)

コンビーナ:北 佐枝子(建築研究所)、コンビーナ:大内 智博(愛媛大学地球深部ダイナミクス研究センター)、Manea Marina(Computational Geodynamics Laboratory, Geosciences Center, National Autonomous University of Mexico)、コンビーナ:大久保 蔵馬(防災科学技術研究所)、座長:北 佐枝子(建築研究所)、大内 智博(愛媛大学地球深部ダイナミクス研究センター)、大久保 蔵馬(防災科学技術研究所)、Marina Manea(Computational Geodynamics Laboratory, Geosciences Center, National Autonomous University of Mexico)

11:00 〜 13:00

[SCG43-P06] Uniaxial deformation and transformation of high-pressure clinoenstatite under mantle transition zone condition

*坪川 祐美子1久保 友明1肥後 祐司2丹下 慶範2西原 遊3本田 陸人1 (1.九州大学 大学院 理学研究院、2.高輝度光科学研究センター、3.愛媛大学 地球深部ダイナミクス研究センター)

キーワード:高圧型単斜エンスタタイト、アキモトアイト、沈み込むスラブ、変形実験

In the subducting slab, phase transformation in mantle minerals is kinetically inhibited by slow chemical diffusion (Rubie and Ross, 1994). Enstatite (low-Ca Mg-pyroxene, MgSiO3), the second most abundant mineral in the upper mantle, is likely to survive as a metastable phase and its high-pressure transformations may control the rheology of deep slab (Nishi et al., 2008). Especially at low temperature in the cold slab, enstatite-akimotoite direct transition (with a change in coordination number of Si ion from 4 to 6) accompanied by a large decrease in volume (~12 vol.%) may contribute to the origin of a deep-focus earthquake (Hogrefe et al., 1994). To understand creep behavior and phase transformation of enstatite in the subducting slab, we conducted in-situ deformation experiments of enstatite under PT conditions of the mantle transition zone.
High-pressure and high-temperature deformation experiments were conducted using a deformation-111 type high-pressure apparatus combined with synchrotron X-ray at BL04B1 of SPring-8 (SPEED Mk-Ⅱ) and at NE7A of KEK (MAX-Ⅲ). Orthoenstatite aggregate was used as a starting material. This was compressed to 12-15 GPa, and heated at 1473 K for 1 h to cause the transformation into high-pressure clinoenstatite (HP-Cen). Then, the polycrystalline HP-Cen was uniaxially deformed at 16.4-20.8 GPa and 873-1423 K. Stress-strain curves and transformation rates were measured by X-ray radiography and two-dimensional X-ray diffraction.
The final axial strain reached 13-35 % with almost constant strain rates of 7.6×10-5-1.0×10-4 s-1. At lower temperatures (873-1273 K), temperature dependence of stress (2200-5200 MPa) is small, and operation of exponential flow low of HP-Cen is expected. On the other hand, relatively strong temperature dependence of stress (~1200 MPa) is observed at higher temperatures (1273-1423 K). During the deformation with increasing temperature, akimotoite appeared at around 1273 K and its stress (240-660 MPa) is apparently lower than that of HP-Cen. The change in the temperature dependence of HP-Cen stress at ~1273 K was likely caused by (1) the change in the deformation mechanism of HP-Cen to dislocation creep, or/and (2) the effect of the secondary weak phase (akimotoite). Based on SEM observations, preferential nucleation of akimotoite occurred at HP-Cen grain boundaries perpendicular to the compression axis. On the other hand, lamellar intergrowth of akimotoite within HP-Cen reported in the Tenham meteorite (Tomioka, 2007) was not identified in this study. Our study suggests that the direct transformation to akimotoite may cause rheological weakening. The process of the enstatite transformation is affected by uniaxial stress, however we have not obtained any evidences for shear localization and shear instability induced by this reaction so far. We will continue to search for the conditions for triggering shear instability in enstatite system as well as introducing AE measurements.