No abstract

The Northwest Africa (NWA) 7475 meteorite is one of the several stones of paired regolith breccias from Mars based on petrography, oxygen isotope, mineral compositions, and bulk rock compositions. Its inventory of lithic clasts is dominated by vitrophyre impact melts that were emplaced while they were still molten. Other clast types include crystallized impact melt rocks, evolved plutonic rocks, possible basalts, contact metamorphosed rocks, and siltstones. Impact spherules and vitrophyre shards record airborne transport, and accreted dust rims were sintered on most clasts, presumably during residence in an ejecta plume. The clast assemblage records at least three impact events, one that formed an impact melt sheet on Mars ≤4.4 Ga ago, a second that assembled NWA 7475 from impactites associated with the impact melt sheet at 1.7–1.4 Ga, and a third that launched NWA 7475 from Mars ~5 Ma ago. Mildly shocked pyroxene and plagioclase constrain shock metamorphic conditions during launch to >5 and <15 GPa. The mild postshock‐heating that resulted from these shock pressures would have been insufficient to sterilize this water‐bearing lithology during launch. Magnetite, maghemite, and pyrite are likely products of secondary alteration on Mars. Textural relationships suggest that calcium‐carbonate and goethite are probably of terrestrial origin, yet trace element chemistry indicates relatively low terrestrial alteration. Comparison of Mars Odyssey gamma‐ray spectrometer data with the Fe and Th abundances of NWA 7475 points to a provenance in the ancient southern highlands of Mars. Gratteri crater, with an age of ~5 Ma and an apparent diameter of 6.9 km, marks one possible launch site of NWA 7475.

Integrated electron nanobeam (EBSD, CL, EDS) and isotopic measurements (U–Pb, (U–Th)/He) of zircon from the collar and centre of the 80 km wide central uplift of the 2020 ± 3 Ma Vredefort impact structure reveal new shock features in a microstructural progression related to impact basin formation and degree of U–Pb age resetting: (1) planar fractures in {1K0} and {1K2} orientation during initial shock wave compression; (2) curviplanar fractures in {1K1} orientation, now annealed, which host glassy inclusions of partial melt of the host rock; (3) microtwin lamellae in an orientation of 65° about [110], attributed to shock wave rarefaction; (4) nucleation of impact-age crystallites, possibly on microtwins, during post-shock heating by impact melt; and (5) crystal-plastic deformation linked to crater modification of the core of the central uplift. Planar fracturing and microtwinning ≥20 GPa in “cold shock” zircon in granitoid at a radial distance of 25 km failed to reset zircon age. Single-grain ID–TIMS data extend between pre-impact age of 2077 ± 11 Ma and a secondary Pb-loss event at ca. 1.0 Ga — the latter reflecting Kibaran igneous activity between 1.110 and 1.021 Ga. Age resetting by the impact event operated in an ∼15 km wide “hot shock” zone of impact-elevated temperatures ≥700 °C at the core of the central uplift. Mechanisms include internal recrystallization, defect-accelerated Pb diffusion via shock microstructures and melt films, and late crystal-plastic deformation. Igneous zircons from a 2019 ± 2 Ma foliated norite impact melt yield a mean (U–Th)/He date of 923 ± 61 Ma, indicating exposure of the present surface after this time.