A 3D Petrofabric Examination of Martian Breccia NWA 11220 via X‐Ray Computed Microtomography: Evidence for an Impact Lithology

• Published in: Journal of geophysical research. Planets 2023-11, Vol.128 (11), p.n/a
• Main Authors: Goodwin, A., Tartèse, R., Garwood, R. J., Almeida, N. V.
• Format: Article
• Language: English
• Subjects: Apatite; Asteroids; Breccia; Computation; Ejecta; Electron microscopy; Ellipsoids; Feldspars; Fragmentation; Fragments; grain size distribution; Hypervelocity impact; Iron; Lithology; Mars; Mars surface; Mathematical functions; Meteorites; Meteors & meteorites; Microtomography; Minerals; NWA 11220; petrofabric; Preferred orientation; Pyroxenes; Raman spectroscopy; Regolith; Rocks; Scale invariance; Scanning electron microscopy; SNC meteorites; Spectroscopy; Spectrum analysis; Sulfides; X‐ray computed tomography
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1029/2023JE007916

The Martian regolith breccia Northwest Africa (NWA) 11220 and paired stones represent the only known meteorites that sample a clastic sub‐surface lithology from Mars. By applying X‐ray computed microtomography to monomineralic clasts, we identify three phases that can be automatically segmented by thresholding X‐ray attenuation greyscale values: (A) feldspars, (B) pyroxene and apatite, and (C) iron‐rich oxides and sulfides, confirmed via scanning electron microscopy and Raman spectroscopy. For these three phases, we demonstrate scale invariance in size and shape for sand‐sized clasts and smaller, a characteristic commonly observed for clast populations generated by fragmentation without further sorting from sedimentary transport (e.g., Aeolian or fluvial processes). Additionally, by assessing the preferred orientation of fitted ellipses and ellipsoids to manually segmented proto‐breccia clasts in two and three dimensions, we identified a weak planar fabric that likely resulted from compaction rather than impact transport. Combining clast size distribution with evidence for nested textures inside proto‐breccia clasts, we propose that NWA 11220 has experienced a minimum of two hypervelocity impact events and should be considered a lithified impact ejecta lithology with little to no reworking via surface regolith processes. Plain Language Summary Northwest Africa (NWA) 11220 is a rocky meteorite from Mars that we studied using X‐ray computed microtomography, a technique that uses X‐rays to image the interior of a sample. From these images, we can model the particles (clasts) of the rock in 3D. We identified three types of particles which correspond to different minerals: (A) feldspars, (B) pyroxene and apatite, and (C) iron‐rich oxides and sulfides. We confirmed this via scanning electron microscopy and Raman spectroscopy, a technique that reflects laser light off a substance, generating a spectrum that acts as a fingerprint for different minerals. By organizing clasts by size, we show their distribution fits mathematical functions used to describe particles generated by fragmentation. Combining this with evidence for clasts inside other clasts, we propose that NWA 11220 has experienced a minimum of two hypervelocity impact events—when an asteroid or similar collided explosively with the surface of Mars. Because the transport of loose fragments either by wind or water on the surface of Mars will preferentially remove finer and less dense particles, the absenc