Reconstructing Impact Histories from Microscopic Analysis: a Multi-Methodology Approach Applied to the Stac Fada Member Impactite and the Northwest Africa (NWA) 11220 Martian Meteorite

Abstract

This thesis explores how impact cratering processes are preserved in the geological record at the microscopic scale, providing necessary context to interpret the history of carbonaceous material preserved within impactites; specifically melt-rich impact breccias. Two case studies are used: (i) the Stac Fada Member, a terrestrial Mesoproterozoic impactite exposed in Northwest Scotland, and; (ii) the martian regolith breccia Northwest Africa (NWA) 11220, paired with NWA 7034, a family of martian meteorites. A range of predominantly non-destructive analytical techniques was used to discern the provenance and impact history of carbon present within both lithologies. In the Stac Fada Member, disordered carbon phases have been modified by mild heating within a hot ejecta blanket rather than shock pressure. The first evidence for impact diamonds within the ejecta horizon is presented, including in shocked gneiss clasts present in sandstones directly underlying the Stac Fada Member contact; implying the Stac Fada Member must have been deposited after ballistic ejection of Lewisian basement. The similarity between disordered carbon phases in the Stac Fada Member and in the underlying sandstones suggests that the impact breccia reworked and included underlying material from the Stoer Group, a provenance further confirmed by X-Ray Diffraction (XRD) mineralogy data. However, altered impact melt clasts (now predominantly chlorite) are unusually mafic in composition assuming they are derived from the same source. A detailed, in situ geochemical elemental dataset is provided for melt clasts. The positive correlation between platinum group elements --- apparently independent of Ni --- indicates two geochemical sources for enrichment. It is proposed that these represent an Mg-Ni rich chondritic impactor and a mafic/ultramafic layered body within the target lithology, mixing with Lewisian basement during melt generation. By applying multi-scale X-ray computed microtomography to monomineralic clasts within the NWA 11220 martian meteorite, a scale invariance in size and shape for sand-sized clasts and smaller is demonstrated. This characteristic is commonly observed for clast populations generated by fragmentation without further sorting from sedimentary transport (e.g., aeolian or fluvial processes). Statistical analysis of the petrofabric indicates NWA 11220 has experienced a minimum of two hypervelocity impact events and should be considered a lithified impact ejecta lithology. Applying Mossbauer spectroscopy in combination with several in situ mass spectrometry techniques, it is shown that aliphatic carbon compounds within NWA 11220 are preferentially found within porosity where it adjoins the mineral surface of titano-magnetite. This relationship suggests catalytic surfaces have enabled Fischer-Tropsch (FT) synthesis of hydrocarbons, occurring during quenching in a post-impact hydrothermal system. As such, NWA 11220 preserves evidence for an impact-derived, transient hydrothermal system with redox conditions suitable for driving organic synthesis. Using the Stac Fada Member as a terrestrial analogue to understand the derivation of reworked impact material within melt-rich impact breccia, this thesis explores in greater resolution impact processes that modify parent lithologies during impact ejecta emplacement. Importantly, it investigates how observations scale when working with small samples of impactite that lack contextualised spatial location to a host crater. At the microscopic scale clast fabrics, mineral relationships, and carbonaceous content is used to reconstruct impact events. This has broad implications for developing emplacement and transport models, specifically for fluidized ejecta blankets. This thesis adds considerable detail to the emplacement history of the Stac Fada Member and impact gardening recorded within NWA 11220. Given that the NASA martian sample return programme is expected to return lithologies from Jezero Crate

Date of Award	31 Dec 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Russell Garwood (Supervisor) & Romain Tartese (Supervisor)