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The seismic anisotropy observed in Earth's lowermost mantle (D″ layer) is thought to originate from the lattice‐preferred orientation (LPO) of mantle materials induced by plastic deformation. Ferropericlase is the second most abundant mineral in the lower mantle, and its LPO induced by deformation may significantly contribute to the seismic anisotropy in the D″ layer. The slip system(s) of ferropericlase controlling the LPO in the D″ layer is still controversial, owing to technical difficulties in conducting high‐pressure deformation experiments. We conducted shear deformation experiments on MgO under pressures of up to 120 GPa and reported the LPO after the deformation experiments. Our results indicate that {100}<011> is the dominant slip system in the D″ layer. The LPO developed during shear deformation would result in shear wave splitting with VSH > VSV, consistent with seismic observations in the regions expected to accommodate large shear strain by subducted slabs in the D″ layer. Plain Language Summary: Heat from Earth's core induces active thermal convection involving plastic flow in the Earth's solid mantle. Owing to plastic flow, minerals with elastic anisotropy in the mantle are thought to be aligned in certain directions (lattice‐preferred orientation, LPO) and generate the seismic anisotropy observed in the lowermost mantle. In particular, the LPO of ferropericlase, the second most abundant mineral in the lower mantle, has garnered attention as a likely candidate for attributing the seismic anisotropy of the lowermost mantle. To quantitatively interpret the dynamics of mantle convection in the lowermost mantle from seismic observations, it is essential to understand the LPO patterns of ferropericlase during deformation under the lowermost mantle conditions (for instance, extremely high‐pressure conditions at 1.2 million atmospheric pressure). However, owing to technical difficulties in conducting high‐pressure deformation experiments, there have been uncertainties in the deformation‐induced LPO of ferropericlase in the lowermost mantle. We conducted shear deformation experiments of MgO under the lowermost mantle pressure for the first time and demonstrated that the LPO of MgO produced by shear deformation is indeed consistent with the seismic anisotropy observed in parts of the lowermost mantle expected to accommodate large shear deformation by subducted slabs. Key Points: Strain rate controlled shear deformation experiments on MgO were conducted under lowermost mantle pressure (120 GPa) for the first timeLarge strain deformation experiments on MgO show that the {100}<011> slip is dominant in the D″ layerA {100}<011> slip of MgO by shear deformation results in VSH > VSV shear splitting, consistent with seismic observations in the D″ layer [ABSTRACT FROM AUTHOR]

Abstract: The phase transition boundary between stishovite and CaCl2-type structure in pure SiO2 was investigated at 45–75GPa and 300–2490K based on the in situ X-ray diffraction measurements in a laser-heated diamond-anvil cell (LHDAC). Results show that the boundary has a positive dP/dT slope of 11.1MPa/K and the transition occurs about 70GPa at 2200K along the typical mantle geotherm. This corresponds to a depth of 1700km, somewhat shallower than the previous estimate. Seismic heterogeneities have been found in a wide depth range from 800 to 1850km in the upper to middle layers of the lower mantle. The post-stishovite phase transition is known to be ferroelastic-type, which causes a considerable reduction in shear modulus, leading to a large slow shear velocity anomaly. The almost pure SiO2 phase included in subducted crustal materials may be therefore responsible for the relatively deep mid-lower mantle seismic heterogeneities. The shallow lower mantle anomalies are possibly caused by the similar phase transition in SiO2 phase containing Al2O3 and H2O or by the phase transition in Al-bearing CaSiO3 perovskite from cubic to tetragonal structure. [ABSTRACT FROM AUTHOR]