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 (XCT) to monomineralic clasts, we identify three phases that can be automatically segmented by thresholding X-ray attenuation greyscales 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 identify 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.
|Journal of Geophysical Research: Planets
|Accepted/In press - 9 Oct 2023