11:00 AM - 1:00 PM
[SCG43-P06] Uniaxial deformation and transformation of high-pressure clinoenstatite under mantle transition zone condition
Keywords:high-pressure clinoenstatite, akimotoite, subducting slab, deformation experiment
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.
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.