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Rocks of the Earth's mantle are composed of minerals with different and anisotropic elastic properties. As a consequence, stress and elastic strain in a polycrystalline rock are partitioned between mineral grains according to their elastic properties and orientations. The way stress and elastic strain are partitioned between grains determines the overall elastic response of the rock which, in turn, controls the propagation of seismic waves through the mantle. Bounds on the elastic moduli of rocks that reflect different assumptions about stress and strain partitioning differ significantly and lead to uncertainties in computed elastic wave speeds of several percents. Such large uncertainties are comparable in magnitude to the observed variations in seismic wave speeds in the mantle and impede their unique inversions to geological structure in terms of composition and temperature.

Here, we present an experimental approach to resolve and quantify the partitioning of elastic strain and stress in polycrystalline materials at high pressures and at frequencies close to those of seismic waves. Using a dynamic diamond anvil cell, we performed cyclic loading experiments on polycrystalline MgO at pressures between 40 and 70 GPa and at frequencies between 10 and 100 mHz. During sequences of loading cycles with varying amplitudes and frequencies, we recorded time-resolved X-ray diffraction patterns in radial diffraction geometry. The results of our experiments and their careful analysis reveal different modes of the elastic behaviour of polycrystalline MgO in terms of stress and strain partitioning. Our experimental approach may therefore resolve the disparate partitioning of stress and elastic strain between the grains of polycrystalline materials and will be used in future studies to search for systematic variations in the elastic response of polycrystalline materials as a function of frequency and temperature.