Olivine Microstructure Constraints on Ureilite Parent Body Deformation

Abstract Ureilites are ultramafic achondrites for which the parent body is unknown. Monomict ureilites, consisting primarily of olivine and pyroxene, are thought to represent mantle residues, carrying essential information for their parent body deformation history. All monomict ureilites are found to be shocked variously, complicating the interpretation of their deformation history. In this work, four monomict ureilites, Elephant Moraine 96042, Northwest Africa 2221, Larkman Nunatak 04315, and Alan Hills A81101, are examined using electron backscatter diffraction to study shock‐related and post‐shock microstructural development in the strained olivine. We calculated the unit segment length (USL) to quantify the subdomain development in those olivine grains, and we further applied a modified misorientation index to study the role of shock in subdomain misorientation. A positive trend of increasing USL with increasing shock level is identified, indicating increased microstructural subdivision and decreasing subdomain size with increasing shock deformation. In LAR 04315 and ALH A81101, the development of low‐angle subdomain boundaries defines an apparent foliation, consistent with a non‐instantaneous, high‐temperature deformation overprint following shock. Together, these results demonstrate that EBSD‐derived microstructural metrics provide a robust, quantitative framework for distinguishing shock‐related deformation from post‐shock microstructural modification in ureilitic olivine.