RESEARCH

Microstructural Recovery of the Upper Mantle

Olivine grain size and crystallographic orientations are two of the most important microstructural properties of the upper mantle, with significance over the strength and viscosity of the mantle as well over its anisotropic elastic properties. Although the way microstructure evolves with deformation has been well studied, its evolution over stagnant static conditions is poorly understood. Experiments using pre-deformed samples under static annealing conditions of high pressure (1 GPa) and temperatures (1250 C) showed a remarkable shift in the microstructure. Grains with low internal strain energy grow rapidly over grains with high internal strain energy (see image below). This transition affects not only the grain size but also modifies olivine's crystallographic preferred orientation (CPO).

High-resolution electron backscatter diffraction (HR-EBSD) enables to display the high resolution intragrin misorientations. These small angle misorientations in the grain can delineate the intragrain strain due to geometrically necessary dislocation (GND). This image highlights the driving force for the growth of the large grains as the differences in dislocation - low dislocation-density crystals consume the highly deformed small grains.

Electron backscatter diffraction (EBSD) map of grain orientations of experimentally deformed (Hansen et al., 2016) and annealed samples (Boneh et al., 2017). Small grains ('matrix') represent the steady-state microstructure produced during deformation. The large faceted grains ('porphyroblasts') represent static growth during annealing.

Post-deformation Recovery in Mantle Xenoliths from the Wyoming Craton

Mantle xenoliths (rock fragments carried from large depths by erupted magma) can tell us about the deep mantle's composition, microstructure, and rheology. Through a microstructural and chemical analysis, the Wyoming xenoliths can reveal processes responsible for the geological history of the North American plate.

The set of xenoliths sampling the Wyoming craton is characterized by deformation microstructure overprinted by annealing microstructure (tabular-shaped olivine grains). The annealing occur during a short period of month-years - extremely fast compared to most tectonic processes but slower than the inferred time scale of hours for kimberlite eruption.

Analysis of time scales of different processes of the xenoliths' history (Boneh et al., G-cubed, 2021).

3-D Electron Backscatter Diffraction of a mantle sample from Wyoming, USA (LabDCT technology at Xnovo Technology). The sample shows olivine grains with sharp boundaries. These tablet-shaped grains formed during static conditions of high temperatures ('annealing'). Lower part shows pole figures of three oliivne axes.