^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.

¹Tatsuhiro Michikami,²Axel Hagermann,^3,4,5Akira Tsuchiyama,¹Yushi Otsuka,⁶Michihiko Nakamura,⁶Satoshi Okumura,⁷Harumasa Kano,³Junya Matsuno,⁸Sunao Hasegawa
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2024.116068]
¹Faculty of Engineering, Kindai University, Hiroshima Campus, 1 Takaya Umenobe, Higashi-Hiroshima, Hiroshima 739-2116, Japan
²Luleå University of Technology, Space Campus, Kiruna 981 28, Sweden
³Research Organization of Science and Technology, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan
⁴CAS Key Laboratory of Mineralogy and Metallogeny/Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (CAS), 511 Kehua Street, , Tianhe District, Guangzhou 510640, Wushan, China
⁵CAS Center for Excellence in Deep Earth Science, Guangzhou 510640, China
⁶Department of Earth Science, Graduate School of Science, Tohoku University, Aoba 6-3, Aramaki, Aoba-ku, Sendai, Miyagi 980-8578, Japan
⁷The Tohoku University Museum, Tohoku University, Aoba 6-3, Aramaki, Aoba-ku, Sendai, Miyagi 980-8578, Japan
⁸Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa 252-8510, Japan
Copyright Elsevier

The surfaces of sub-kilometer-sized asteroids directly explored by spacecraft, such as Itokawa, Ryugu and Bennu, are covered with blocks and/or regolith particles, whose shapes are considered clues to understanding their formation and evolution on the asteroid’s surface. Ryugu particles returned by the Hayabusa2 mission are likely fragments resulting from impacts because their shapes resemble impact fragments from laboratory experiments. However, there is a lack of laboratory impact experiments examining the shapes of fragments in carbonaceous chondrites, thought to originate from carbonaceous asteroids such as Ryugu and Bennu. The measured sizes of Ryugu particles are in the mm and sub-mm range, similar to the sizes of chondrules. Also, carbonaceous chondrites are generally structurally weaker than the basalts and granites often used in previous laboratory impact experiments. Differences in the strength of the chondrules and matrix might affect the overall strength of the meteorite. In this study, as a first step towards a better understanding of impact fragment shapes in carbonaceous chondrites, we conducted impact experiments on the carbonaceous meteorite Allende (CV3). A spherical alumina projectile with 1.0 mm and a glass projectile with 0.80 mm in diameter were fired into 1–2 cm-sized Allende targets at nominal impact velocities of 2.0 and 4.0 km/s, respectively. To investigate the correlation between the chondrules (typically sub-mm in size) and the shapes of fine fragments, we measured the shape distributions of sub-mm impact fragments using X-ray microtomography. We observed several impact fracture surfaces along the chondrule boundaries. In addition, these fragments tended to be rounder than fragments from previous impact experiments. However, because the total number of these fragments is relatively small, the fragments were found to have the same overall shape distribution as previous laboratory impact fragments, Itokawa particles and Ryugu particles. This may imply that impact fragment shapes are independent of the bulk material strength. These findings will be useful for understanding the formation process of regolith layers on asteroid surfaces, Itokawa particles, Ryugu particles, and Bennu particles.