Depth Dependent Deformation and Anisotropy of Pyrolite in the Earth's Lower Mantle

Seismic anisotropy is a powerful tool to map deformation processes in the deep Earth. Below 660 km, however, observations are scarce and conflicting. In addition, the underlying crystal scale mechanisms, leading to microstructures and crystal orientations, remain poorly constrained. Here, we use multigrain X‐ray diffraction in the laser‐heated diamond anvil cell to investigate the orientations of hundreds of grains in pyrolite, a model composition of the Earth's mantle, at in situ pressure and temperature. Bridgmanite in pyrolite exhibits three regimes of microstructures, due to transformation and deformation at low and high pressure. These microstructures result in predictions of 1.5%–2% shear wave splitting between 660 and 2,000 km with reversals in fast S‐wave polarization direction at about 1,300 km depth. Anisotropy can develop in pyrolite at lower mantle conditions, but pressure has a significant impact on the plastic behavior of bridgmanite, and hence seismic observations, which may explain conflicting anisotropy observations. Plain Language Summary Seismologists rely on observable data to construct models that describe the dynamic state of the Earth's lower mantle. These models, however, require constraints such as mantle composition and material behavior at high pressures and temperatures, which can be provided through experimental mineral physics. In this study, we use a high pressure devices and X‐rays to impose deformation and image the state of our sample with increasing pressure and temperature. We are able to extract information of individual mineral grains within our assemblage, such as the number of grains per phase and their orientations. Using this experimental data, we identify three regimes of grain orientations in bridgmanite in the lower mantle, corresponding to transformation from lower pressure phases, and deformation at low and high pressure. With this information, we are able to make predictions about how seismic waves travel and behave based on the deformation state of the lower mantle. Key Points Three microstructure/anisotopy regimes in bridgmanite in the Earth's lower mantle: transformation, low‐P deformation, high‐P deformation Effect of pressure on the active slip systems of bridgmanite with a shift at approximately 50 GPa or 1,300 km depth in the Earth's mantle Up to 2% shear wave splitting predicted at all depth with changes in fast polarization direction for each microstructural regime