Using Multigrain Crystallography to Explore the Microstructural Evolution of the α-Olivine to γ-Ringwoodite Transformation and ε-Mg2SiO4 at High Pressure and Temperature

The introduction of multigrain crystallography (MGC) applied in a laser-heated diamond anvil cell (LH-DAC) using synchrotron X-rays has provided a new path to investigate the microstructural evolution of materials at extreme conditions, allowing for simultaneous investigations of phase identification, strain state determination, and orientation relations across phase transitions in a single experiment. Here, we applied this method to a sample of San Carlos olivine beginning at ambient conditions and through the alpha-olivine -> gamma-ringwoodite phase transition. At ambient temperatures, by measuring the evolution of individual Bragg reflections, olivine shows profuse angular streaking consistent with the onset of yielding at a measured stress of similar to 1.5 GPa, considerably lower than previously reported, which may have implications for mantle evolution. Furthermore, gamma-ringwoodite phase was found to nucleate as micron to sub-micron grains imbedded with small amounts of a secondary phase at 15 GPa and 1000 degrees C. Using MGC, we were able to extract and refine individual crystallites of the secondary unknown phase where it was found to have a structure consistent with the epsilon-phase previously described in chondritic meteorites.