Microstructures and anisotropy in pyrolite at lower mantle pressures and temperatures

In this work, we investigate the microstructures induced by phase transformation and deformation in a pyrolitic composition at Earth's lower mantle pressures and temperatures. The Earth's lower mantle is thought to hold a composition relatively close to that of pyrolite, with approximately 80% of bridgmanite (Mg,Fe)SiO3, 15% of ferropericlase (Mg,Fe)O, and 5% of davemaoite CaSiO3. Deformation of pyrolite at Earth's mantle pressures and temperatures is hence critical to constrain observations, or non-observations, of seismic anisotropy in the deep mantle. We form a lower-mantle pyrolitic composition at pressures between 20 and 30 GPa and temperatures on the order of 1900 K and further compresse it up to pressures above 100 GPa at simultaneous temperatures on the order of 2000 K, corresponding to depths of approximately 2500 km in the Earth's mantle. We rely on a novel technique, multigrain X-ray diffraction in the laser heated diamond anvil cell, which allows tracking hundreds of individual grains simultaneously during processes including phase transformation, pressure increase, and deformation. Our results show that i) pyrolite is formed with a non-random transformation texture, due to the compressive stress applied during the diamond anvil cell experiments, ii) microstructures further evolve with increasing pressure. The experimental results are then used to i) identify deformation mechanisms in bridgmanite within a pyrolitic composition, ii) evaluate the effect of pressure on those mechanisms, and iii) evaluate the resulting seismic anisotropy, either from the synthesis of pyrolite at 660 km depth discontinuity conditions or from deformation deeper in the Earth's lower mantle.