Phase Change of Pyrolitic Material: In-situ Transformation and Induced Microstructures at 660 km Depth

Abstract : It has long been known that phase transitions have contributed to seismic reflections in the mantle. These transitions are caused by pressure and temperature variations. More specifically, at the 660 km discontinuity, phase transitions in pyrolitic composition samples, where ringwoodite and majoritic garnet decompose to form bridgmanite and ferropericlase, have been observed. Transformations at the 660 km discontinuity are not martensitic due to chemical variations within the material. Questions remain on the microstructures obtained after the phase transition. Are there preferred orientation within individual phases after the synthesis? Do grain sizes change during this transition? Better understanding the microstructures induced by phase transformations will give better insight to seismic anisotropy shortly after the discontinuity, the observed seismic reflection and its overall effect on mantle dynamics. Determining transformation microstructures at 660 km conditions will hence lead to a better understanding of the Earth’s mantle. By implementing synchroton multigrain x-ray diffraction in a laser heated diamond anvil cell, we are able to track individual grains, their crystal structure, and orientations, while being compressed in-situ at pressures ranging from 18-55 GPa and temperatures of ~1800K. Using this experimental approach, we can then use obtained transformation textures to refine current seismic models to better understand observable anisotropies and reflections at conditions relevant to the 660 km discontinuity.