[S-IT20] 地球深部科学

[SIT20-04] Slip system transition of bridgmanite

グアン ロンリ²、*山崎 大輔¹、辻野 典秀³ (1.岡山大学惑星物質研究所、2.中国地質大学、3.高輝度光科学研究センター)

Although the Earth’s lower mantle globally appears nearly seismically isotropic for most of its depth, the regions in which oceanic lithosphere is subducting in the lower mantle show significant seismic anisotropy. Seismic anisotropy is often caused by lattice preferred orientation of constituting mineral, which is bridgmanite in the case of the lower mantle, formed mainly during plastic deformation in the dislocation creep regime but not in diffusion creep regime. The evolution of lattice preferred orientation of bridgmanite remains poorly constrained despite its key role to understand deformation mechanism and resultant mantle flow in the lower mantle because of experimental difficulties. In this study, we investigated the effect of temperature, to simulate the conditions of subducting lithosphere and surrounding mantle, on the evolution of lattice preferred orientation of iron-free and iron bearing bridgmanite during deformation by means of high-pressure experiments under the uppermost lower mantle conditions, 25 GPa and 1,700-2,100 K, using the D111-type apparatus. Observation of lattice preferred orientation across the temperature range showed the change of pattern of lattice preferred orientation and suggest that the transition of the dominant slip system from [010](100) below 1,800 K to [100](010) above 1,800 K, negligibly affected by Fe content. Calculated elastic anisotropy for lattice preferred orientation formed at high temperature is weaker, while lattice preferred orientation formed at lower temperature produces stronger, with the assumption of the dominant flow of horizontal shear in the uppermost lower mantle. These results reasonably explain the strong seismic anisotropy observed beneath subduction zones and the global nearly isotropic surroundings with single deformation mechanism of dislocation creep and hence viscosity in the lower mantle strongly depends on stress and is insensitive to grain size, providing critical insights into the viscosity structure and dynamics of the lower mantle.