Abstract
The CV3 Allende is one of the most extensively studied meteorites in worldwide collections. It is currently classified as S1—essentially unshocked—using the classification scheme of Stöffler et al. (1991), however recent modelling suggests the low porosity observed in Allende indicates the body should have undergone compaction-related deformation. In this study, we detail previously undetected evidence of impact through use of Electron Backscatter Diffraction mapping to identify deformation microstructures in chondrules, AOAs and matrix grains. Our results demonstrate that forsterite-rich chondrules commonly preserve crystal-plastic microstructures (particularly at their margins); that low-angle boundaries in deformed matrix grains of olivine have a preferred orientation; and that disparities in deformation occur between chondrules, surrounding and non-adjacent matrix grains. We find heterogeneous compaction effects present throughout the matrix, consistent with a highly porous initial material. Given the spatial distribution of these crystal-plastic deformation microstructures, we suggest that this is evidence that Allende has undergone impact-induced compaction from an initially heterogeneous and porous parent body. We suggest that current shock classifications (Stöffler et al., 1991) relying upon data from chondrule interiors do not constrain the complete shock history of a sample.
Highlights
IntroductionPlanetary systems evolve from a molecular cloud comprised of dust (condensed minerals and presolar grains) and gas, to a protoplanetary disk, where most of the mass is concentrated in the ‘mid-plane’ of that disk
Planetary systems evolve from a molecular cloud comprised of dust and gas, to a protoplanetary disk, where most of the mass is concentrated in the ‘mid-plane’ of that disk
Within the 1.28 cm2 sample area of our sample, we observed a large variation in matrix grain size (Supplementary Figs. 1–4)
Summary
Planetary systems evolve from a molecular cloud comprised of dust (condensed minerals and presolar grains) and gas, to a protoplanetary disk, where most of the mass is concentrated in the ‘mid-plane’ of that disk. Interactions with the gas and collisions lead these solids to grow and accumulate into planetesimals measuring 10 s to 100 s of kilometers in diameter As this accumulation must be relatively gentle to ensure sticking and adhesion among the components, primordial planetesimals formed in this way would preserve large pore spaces in between each component, resulting in very high porosities (>65%) (Blum, 2003; Weidenschilling and Cuzzi, 2006). Planetary evolution from these primordial objects to the asteroids that we see today requires that porosity be greatly reduced, as evidenced by the worldwide collection of meteorites that comprises comparatively low-porosity rocks.
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