Microstructure Across Deformation Regimes: 3D Imaging of Olivine by Dark-Field X-ray Microscopy

Olivine is a fundamental rock-forming mineral for which microstructures are closely tied to deformation conditions. However, visualization of olivine deformation has traditionally been limited to two-dimensional observations, ranging from petrographic microscopy at the millimetre– to micrometre scale to electron-based techniques probing crystallographic distortion and ordering at the micro- to nanometre scale (e.g., EBSD and TEM). Here, we introduce dark-field X-ray microscopy (DFXM) and present its first application to geological materials, conducted at beamline ID03 of the ESRF.

Using a focused line beam produced by compound refractive lenses, DFXM enables non- destructive, in-situ imaging with spatial resolution down to ~35 nm. By selectively illuminating a ~500 nm thick volume with the line beam, DFXM allows “slicing” through depth of the crystal volume. By translating the sample through the X-ray beam, the layers can be stacked and reconstructed into full 3D datasets.

In this work, we reconstruct the 3D microstructures of the mineral olivine across a range of deformation settings, spanning from hydrothermal single crystal olivine, to olivine in Åheim orogenic peridotite which experienced long-term dislocation creep, to olivine in heavily shock-metamorphosed martian basalt with relict crustal strain. We observe individual static dislocations and associated lattice strain field in the hydrothermal olivine single crystal, to arranged low-angle boundaries (LABs) formed by geometrically necessary dislocations (GNDs) in the Åheim peridotite, to chaotic dislocation networks connected by dense, short, and randomized LABs in shocked martian basalts.

By bridging conventional 2D crystallographic observations with volumetric 3D microstructural reconstructions, our work enables robust observations of microstructures developed in distinctive deformation conditions, providing a powerful and advanced 3D imaging technique for geological materials. Our study expands the application of DFXM to Earth and planetary materials and demonstrates the power of multi-scale, three-dimensional imaging for resolving complex deformation histories in geological systems.