The enigma of post-perovskite anisotropy: deformation versus transformation textures
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The D′′ region that lies just above the core mantle boundary exhibits complex anisotropy that this is likely due to preferred orientation (texturing) of the constituent minerals. (Mg,Fe)SiO3 post-perovskite is widely thought to be the major mineral phase of the D′′. Texture development has been studied in various post-perovskite phases (MgSiO3, MgGeO3, and CaIrO3), and different results were obtained. To clarify this controversy, we report on transformation and deformation textures in MgGeO3 post-perovskite synthesized and deformed at room temperature in the diamond anvil cell. Transformed from the enstatite phase, MgGeO3 post-perovskite exhibits a transformation texture characterized by (100) planes at high angles to the direction of compression. Upon subsequent deformation, this texture changes and (001) lattice planes become oriented nearly perpendicular to compression, consistent with dominant (001) slip. When MgGeO3 post-perovskite is synthesized from the perovskite phase, a different transformation texture is observed. This texture has (001) planes at high angle to compression and becomes slightly stronger upon compression. We also find that the yield strength of MgGeO3 post-perovskite is dependent on grain size and texture. Finer-grained samples exhibit higher yield strength and are harder to induce plastic deformation. Strong textures also affect the yield strength and can result in higher differential stresses. The inferred dominant (001) slip for pPv is significant for geophysics, because, when applied to geodynamic convection models, it predicts the observed anisotropies of S-waves as well as an anti-correlation between P- and S-waves.