Shear waves in the diamond-anvil cell reveal pressure-induced instability in (Mg,Fe)O

The emerging picture of Earth's deep interior from seismic tomography indicates more complexity than previously thought. The presence of lateral anisotropy and heterogeneity in Earth's mantle highlights the need for fully anisotropic elasticity data from mineral physics. A breakthrough in high-frequency (gigahertz) ultrasound has resulted in transmission of pure-mode elastic shear waves into a high-pressure diamond-anvil cell using a P-to-S elastic-wave conversion. The full elastic tensor (c_ij) of high-pressure minerals or metals can be measured at extreme conditions without optical constraints. Here we report the effects of pressure and composition on shear-wave velocities in the major lower-mantle oxide, magnesiowü stite-(Mg,Fe)O. Magnesiowüstite containing more than ≈50% iron exhibits pressure-induced c₄₄ shear-mode softening, indicating an instability in the rocksalt structure. The oxide closer to expected lower-mantle compositions (≈20% iron) shows increasing shear velocities more similar to MgO, indicating that it also should have a wide pressure-stability field. A complete sign reversal in the c₄₄ pressure derivative points to a change in the topology of the (Mg,Fe)O phase diagram at ≈50-60% iron. The relative stability of Mg-rich (Mg,Fe)O and the strong compositional dependence of shear-wave velocities (and ∂c₄₄/∂P) in (Mg,Fe)O implies that seismic heterogeneity in Earth's lower mantle may result from compositional variations rather than phase changes in (Mg,Fe)O.