The dynamic response of polycrystalline MgO to cyclic loading at seismic frequencies and high pressures

Summary The propagation of seismic waves through the Earth’s mantle is controlled by the dynamic response of the mantle to oscillating stresses and strains. This dynamic response arises from the elastic, anelastic, and plastic properties of the minerals and rocks in the mantle and can vary with the amplitude and frequency of strain oscillations. Previous experiments at pressures of the lower mantle have either determined the elastic properties of minerals and mineral aggregates at much higher frequencies than those of seismic waves or have focussed on their plastic deformation under static stresses. Here, we present cyclic loading experiments that probe the dynamic response of magnesium oxide (MgO) powder at pressures of the lower mantle and at frequencies that extend into the range of seismic frequencies. MgO powder was compressed in a diamond anvil cell and subjected to loading cycles generated by a piezoelectric actuator. During loading cycles with varying strain amplitudes and frequencies between 10 and 100 millihertz, we collected X-ray diffraction patterns in radial diffraction geometry and at a rate of one pattern per second. We analysed the resulting time series of diffraction patterns by extracting lattice spacings from the reflections 200 and 220 of MgO to derive volume strains and differential lattice strains from both reflections. While the oscillating components of volume strains reflect the isotropic compressibility of MgO, the oscillating differential lattice strains reveal a more complex response to cyclic loading that varies with the amplitude of strain oscillations. For larger strain amplitudes, we resolve phase shifts between the oscillating differential lattice strains of both reflections that indicate significant dissipation of strain energy by anisotropic plastic or anelastic deformation. At the smallest amplitudes of strain oscillations, the dynamic response of the textured MgO powder is described by almost uniform oscillating lattice strains consistent with elastic isostrain and minor dissipation. By following our experimental procedure, future cyclic loading experiments may characterise the dynamic response of polycrystalline materials at high pressures and quantify how dissipation of strain energy affects the seismic properties of materials in the Earth’s mantle.