Leave no trace: A non-destructive correlative approach providing new insights into impactites and meteorites

Impact cratering is today recognized as a fundamental geological process on all rocky bodies in the solar system. On Earth, however, processes such as plate tectonics and erosion have eradicated most craters from the geological record, or they may be buried under sediments, oceans, and vegetation. The formation of a hypervelocity impact crater involves extreme pressures and temperatures that induce permanent changes in the target rocks, so-called shock-metamorphic effects, which can be used to identify and confirm impact structures.

The research in this thesis focuses on the impact cratering process, both during the formation, and post-impact. A number of terrestrial impactites and meteorites were analyzed using a multi-modal approach, including correlative non-destructive neutron and X-ray imaging, and detailed 2D analysis using scanning electron microscopy and electron backscatter diffraction. The material encompasses (1) impactites from the Mien impact structure, (2) a sample of the Martian Miller Range (MIL) 03346 meteorite, (3) a Chicxulub drill core sample, (4) a sample of Libyan Desert Glass, and (5) a sample of impact melt rock from the Luizi impact structure.

The first study investigated shock deformation in zircon grains from the Mien impact structure in Sweden, using electron backscatter diffraction (EBSD). The results show that several of these grains contain evidence of the former presence of a high-pressure phase that is only known from impact structures. These grains would be suitable candidates for refining the age of the impact event. In paper II, combined NCT and XCT were employed to investigate the three-dimensional distribution of hydrogen-rich material in MIL 03346, by utilizing the neutrons’ sensitivity to hydrogen. The results revealed that the hydrogen-rich material occurs in localized clusters, with limited interconnectivity between clusters. This suggests that the fluid source could be small patches of sub-surface ice and that the alteration event likely was short-lived, meaning that the source terrain of this sample was likely not habitable. In Paper III we combined XCT and NCT to test if these methods can be used to locate projectile material in impactites. After careful investigations of the 3D images, an iron-nickel silicide spherule could be pin-pointed in the Libyan Desert glass. The sample was then polished for detailed analysis using scanning electron microscopy. Overall, the non-destructive nature of XCT and NCT makes these methods highly relevant for studying rare samples, such as meteorites and returned samples.