Microstructure Decoding the Deformation history of the highly shocked Martian Shergottite NWA 7721

Martian meteorites offer insights into Martian magmatic processes and impact history, critical for understanding terrestrial planet evolution. Among over 100 identified Martian meteorites (4.4 Ga-165 Ma; Nyquist et al., 2001; Moser et al., 2013), shergottites are the most common, resembling terrestrial basalts (McSween, 2015; Kizovski et al., 2019) but showing strong shock metamorphism. Key shock features include plagioclase-to-maskelynite transitions, olivine and pyroxene mosaicism, and planar fractures in olivine (Stöffler et al., 1986; Walton & Herd, 2006; Jones, 2014). However, deformation history interpretations using shock and post-shock features remain ambiguous due to limited quantitative constraints and direct observation at a mesoscale. This study analyzes olivine microstructures in poikilitic shergottite NWA 7721 using electron backscatter diffraction (EBSD) and dark-field X-ray microscopy (DFXM). We discovered a bimodal morphological subgrain distribution in the large olivine grain: (1) almost strain-free recrystallized crystallites (<5 µm) forming rims and filling fractures and (2) irregular subgrain fragments (>15 µm) with strong alignment and low-angle boundaries (< 15º). With DFXM, it further revealed two dislocation distributions in the 3D grain volume that 1) “dislocation network” formed by very-low-angle misorientation boundaries (<0.1º) and 2) incipient subdomain walls formed by low-angle misorientation boundaries (> 0.3º). These textures suggest a complex deformation-recovery process for the emplacement of shergottite on Mars. The small crystallites formed via shock-induced heterogeneous nucleation at olivine grain edges and fractures (Walton & Herd, 2006), facilitated by eutectic melting followed by recrystallization during brief post-shock heating that is less than 0.2 hours of 1600-2000K (Takenouchi et al., 2017; Speciale et al., 2020). The irregular subgrain fragments are preserved olivine relics, isolated by very-low-angle boundary networks developed during compressive shock wave passage, migrating to form low-angle boundaries during rapid quenching. This records the final deformation episode before meteorite delivery to the Earth, shedding light on shock metamorphism and recovery processes in Martian rocks.