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

TL;DR

This study reveals that olivine in Martian meteorite NWA 7721 records evidence of ancient crustal deformation, showing distinct microstructures indicative of pre-impact stress and rapid post-shock cooling, thus providing insights into Mars's geological history.

Contribution

It is the first to identify and interpret microstructural evidence of crustal strain in Martian olivine, linking it to ancient deformation processes on Mars.

Findings

Olivine exhibits two distinct subgrain populations with different strain states.

Microstructures indicate pre-impact crustal stress and rapid post-shock cooling.

Evidence of dynamic crustal deformation preserved in Martian meteorite olivine.

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.