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Shear deformation of bridgmanite and magnesiowüstite aggregates at lower mantle conditions

Mantle minerals won't share the strain The deformation of a mixed block of material depends on the strength of the components of which it is made. Weak materials will deform more than the strong ones in a mixture that is squished or stretched. Girard et al. find a large difference in strength between the two primary minerals making up Earth's lower mantle (see the Perspective by Chen). Deformation in the convecting mantle may occur only near boundary layers as a result, leaving large regions potentially unaffected. This could explain long-lived chemical reservoirs in Earth's interior and the lack of seismic anisotropy in the lower mantle. Science, this issue p. 144; see also p. 122 Shearing experiments at lower mantle conditions show that the less abundant of the two major mantle minerals is easily deformed. [Also see Perspective by Chen] Rheological properties of the lower mantle have strong influence on the dynamics and evolution of Earth. By using the improved methods of quantitative deformation experiments at high pressures and temperatures, we deformed a mixture of bridgmanite and magnesiowüstite under the shallow lower mantle conditions. We conducted experiments up to about 100% strain at a strain rate of about 3 × 10−5 second−1. We found that bridgmanite is substantially stronger than magnesiowüstite and that magnesiowüstite largely accommodates the strain. Our results suggest that strain weakening and resultant shear localization likely occur in the lower mantle. This would explain the preservation of long-lived geochemical reservoirs and the lack of seismic anisotropy in the majority of the lower mantle except the boundary layers.