Remanent crustal strain on Mars in non-poikilitic olivine of NWA 7721

TL;DR

Problem: to determine whether non-poikilitic olivine in NWA 7721 records dynamic crustal deformation on Mars during the Late Amazonian. Approach: a multi-scale, non-destructive microstructural study combining dark-field X-ray microscopy (DFXM), electron backscatter diffraction (EBSD), and in situ 2D X-ray diffraction (µXRD). Key findings: a single olivine crystal hosts two subgrain populations—fine, nearly strain-free Type 1 recrystallites and coarse, highly strained Type 2 relicts with aligned subdomains and slip bands; grain-growth modeling yields a post-shock heating duration of about $2.3~\mathrm{s}$, implying rapid quenching; the Type 2 fabric reflects pre-existing magmatic or crustal strain. Significance: provides the first direct microstructural evidence that non-poikilitic olivine preserves crustal/deformation processes on Mars in the Late Amazonian and demonstrates a powerful multi-scale approach with DFXM for planetary materials.

Abstract

We present a multiscale microstructural analysis of olivine from the non-poikilitic lithology of the poikilitic shergottite NWA 7721, using dark-field X-ray microscopy (DFXM), electron backscatter diffraction (EBSD), and context in situ 2D micro-XRD. A single olivine crystal contains two distinct subgrain populations. Type 1 subgrains are fine (1-5 micrometers), randomly oriented, and nearly strain-free, whereas Type 2 subgrains are coarse (greater than 30 micrometers), aligned, and strongly strained. Layered DFXM data reveal slip-band features in Type 2 that are absent in Type 1. We interpret Type 1 as products of shock-induced recrystallization, whereas Type 2 preserves remnants of a highly deformed parent grain. This bimodal microstructure, not observed in other Martian meteorites including the paired NWA 1950 and ALH A77005, points to a heterogeneous response to impact influenced by pre-existing strain in the olivine grain. We propose that NWA 7721 olivine experienced substantial crustal or magmatic stress before impact. The subsequent shock wave imposed a rapid load-release cycle that mobilized dislocations and produced low-angle boundaries in Type 2, while driving recrystallization of Type 1. Grain-growth constraints limit the post-shock heating duration to approximately 2.3 s, consistent with rapid quenching. These results provide the first evidence that non-poikilitic olivine in NWA 7721 preserves dynamic crustal deformation on Mars in the Late Amazonian.

Figures (8)

• Figure 1: Macroscopic textures of NWA 7721. A: Context petrographic image PPL for investigated non-poikilitic olivine. B: 10X image XPL for investigated non-poikilitic olivine showing fringing C: 2D XRD pattern showing strain-related diffraction streaks and polycrystalline rings along the Debye rings, with targeted in situ X-ray 300 µm diameter beam spot shown in context image of grain at right. D: $\sum(\mathrm{FWHM}_\chi)$ for the integrated peak at lattice plane (021). E: 1D XRD pattern along $2\theta$, matching with forsteritic olivine using Powder Diffraction File (PDF) from the International Center for Diffraction Data.
• Figure 2: EBSD of Type 1 and Type 2 olivine subgrains. A: IPF-colored orientation plot of Type 1 subgrains. B: log scaled low-angle boundary (LAGB) density within each subgrain. C: shape-preferred orientation of Type 1 subgrains. D: misorientation angle distribution plot for Type 1 subgrains compared with a theoretical random distribution (red line). E: IPF-colored orientation plot showing both Type 1 subgrains and Type 2 subgrains. F: log scaled LAGB density for Type 2 subgrains. G: LAGBs (blue lines) in Type 2 subgrains. H: Shape preferred orientation for Type 2 subgrains showing strong alignment. I: misorientation angle distribution plot compared with a theoretical random distribution (red line)
• Figure 3: Dark-field X-ray microscopy for Type 1 subgrains. A: Example of 2D XRD image of the investigated grain. B: Zoom-in view of lab 2D XRD showing the powder ring pattern using near-field camera with 2500 pixels horizontal and 2000 pixels vertical. C: Zoom-in view of selected subgrain imaged by the far-field detector with an objective lens. D: Reconstruction of the projection of the selected subgrain using $100 \times 100 \mu m$ beam size. Dash lines are schematic demonstration of using a line-focus beam (with $500nm$ beam height of $1\mu m$ spacing) to image different layers in the grain. E: Reconstruction of the first layer mosaicity showing small mosaic spread F: First layer Kernal Average Misorientation map highlighting the small misorientation boundaries G: Autocorrelation analysis of the selected area (red box in F). H: Horizontal 1D projection from G. I: Vertical 1D projection from G.
• Figure 4: Dark-Field X-ray Microscopy for Type 2 subgrains. A and B: chi-mu mosaicity plots. C and D: Kernel average misorientation plots calculated from center of mass from mu motor. E and F: autocorrelation orientational analysis
• Figure 5: Dark-field X-ray microscopy for Åheim olivine. A: Mosaicity plot of $\chi-\mu$. B: KAM plot. C: autocorrelation orientational analysis. D: 1D horizontal projection of autocorrelation analysis. E:1D vertical projection of autocorrelation analysis.