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

Abstract

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 α olivine → γ-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 ~1.5 GPa, considerably lower than previously reported, which may have implications for mantle evolution. Furthermore, γ-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 °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 ε-phase previously described in chondritic meteorites.

Deformation Mechanisms, Microstructures, and Seismic Anisotropy of Wadsleyite in the Earth's Transition Zone

“…The detailed procedure has been described in Rosa et al. (2015) and used in several publications (Chandler, Bernier, et al., 2021; Chandler, Devoe, et al., 2021; Gay et al., 2023; Krug et al., 2022; Langrand et al., 2017; Rosa et al., 2016).…”

“…The data are then processed to (a) separate the individual diffraction spots of the grains of the sample large enough to produce a single-crystallike signal, from the rest of the signals, which includes powder-like diffraction signal of the smaller sample grains, signal from the pressure medium, diamond Compton scattering, etc., (b) identify these extracted diffraction spots, and (c) determine the grain orientation matrices. The detailed procedure has been described in Rosa et al (2015) and used in several publications (Chandler, Bernier, et al, 2021;Chandler, Devoe, et al, 2021;Gay et al, 2023;Krug et al, 2022;Langrand et al, 2017;Rosa et al, 2016).…”