^1,2S. A. Eckley,²R. A. Ketcham,³Y. Liu,^4,5,6,7J. Gross,⁷F. M. McCubbin
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14165]
¹Jacobs—JETS, Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, Houston, Texas, USA
²Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas, USA
³Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
⁴Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey, USA
⁵Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA
⁶Lunar and Planetary Institute, Houston, Texas, USA
⁷NASA Johnson Space Center, Houston, Texas, USA
Published by arrangement with John Wiley & Sons

Shergottites are mafic to ultramafic igneous rocks that represent the majority of known Martian meteorites. They are subdivided into gabbroic, poikilitic, basaltic, and olivine–phyric categories based on differences in mineralogy and textures. Their geologic contexts are unknown, so analyses of crystal sizes and preferred orientations have commonly been used to infer where shergottites solidified. Such environments range from subsurface cumulates to shallow intrusives to extrusive lava flows, which all have contrasting implications for interactions with crustal material, cooling histories, and potential in situ exposure at the surface. In this study, we present a novel three-dimensional (3-D) approach to better understand the solidification environments of these samples and improve our knowledge of shergottites’ geologic contexts. Shape preferred orientations of most phases and crystal size distributions of late-forming minerals were measured in 3-D using X-ray computed tomography (CT) on eight shergottites representing the gabbroic, poikilitic, basaltic, and olivine–phyric categories. Our analyses show that highly anisotropic, rod-like pyroxene crystals are strongly foliated in the gabbroic samples but have a weaker foliation and a mild lineation in the basaltic sample, indicating a directional flow component in the latter. Star volume distribution analyses revealed that most phases (maskelynite, pyroxene, olivine, and oxides/sulfides) preserve a foliated texture with variable strengths, and that the phases within individual samples are strongly to moderately aligned with respect to one another. In combination with relative cooling rates during the final stages of crystallization determined from interstitial oxide/sulfide crystal size distribution analyses, these results indicate that the olivine–phyric samples were emplaced as shallow intrusives (e.g., dikes/sills) and that the gabbroic, poikilitic, and basaltic samples were emplaced in deeper subsurface environments.