Crystallographic-preferred-orientation of (Mg,Fe)SiO3-bridgmanite : implications for the mantle dynamics in the lower mantle

Abstract:

Bridgmanite, (perovskite-structured (Mg,Fe)SiO₃) is the dominant constituent mineral in the Earth’s lower mantle but whose rheological property is largely unknown. Seismic shear wave anisotropy is observed at the Earth’s uppermost lower mantle near Tonga-Kermadec slab (e.g. Wookey and Kendall 2004). The anisotropy caused by deformation-induced crystallographic-preferred-orientation (CPO) of bridgmanite is the most plausible candidates of this feature.

We carried out shear deformation experiments of bridgmanite under the lower mantle conditions (25 GPa and 1600^°C) employing the deformation-DIA (D-DIA) type multi-anvil press with Kawai-type cell assembly (6-8 tpye). Dense bridgmanite aggregates, which were equigranular with typical grain size of ~15 μm and random crystallographic orientation, were used as starting materials. Both of the crystallographic orientations of the starting and the deformed samples were determined by 2D monochromatic X-ray diffraction pattern method.

In the deformation experiment, the total strain and average strain rate were evaluated to be γ ~0.7 and ý ~2 × 10^-4 /s, respectively, from the tilting angle of the strain marker. The obtained fabrics are characterized by the a-axis perpendicular to shear plane and the c-axis parallel to shear direction, indicating that the dominant slip system of bridgmanite is [001](100). The observed shear wave anisotropy near Tonga-Kermadec slab is explained by the CPO of bridgmanite induced by lateral flow of the slab with the stagnation above 1000 km depth. Present study shows that the flow direction in the lower mantle can be determined based on the combination of CPO of bridgmanite and seismic observation.