Rock magnetic properties and their anisotropy from host rock and impact lithologies of drillings at the Chesapeake Bay impact structure, USA

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The ICDP-USGS Eyreville drilling at Chesapeake Bay produced one of the most complete geologic sections of an impact structure. Core samples from the 1766 m deep drilling into the central zone of the 35 Ma old impact structure allows the study of different target and impact lithologies and their rock magnetic properties. The dominant magnetic minerals creating crustal magnetic anomalies are pyrrhotite and magnetite. The main target of this study is to find new clues about the different hypotheses on remagnetization mechanisms during impact-related processes. Our initial investigations have shown that the target rocks contain magnetite and pyrrhotite, which are responsible for the regional magnetic anomaly pattern. This pattern is disrupted by the impact, which is in accordance with an interpretation of displaced basement-derived megablocks embedded in the lithic and suevitic breccia unit. The only modification of magnetic minerals has been observed in this latter unit, while the basement-derived blocks do not show any shock deformation. We found shock-induced remanent magnetization and chemical remanent magnetization to be the only remagnetization mechanisms, and no indication for a TRM can be confirmed, in contrast to most hypothesis in literature. Shocked pyrrhotite (4C modification with abundant mechanical twins and defect structures, a significant metal deficieny and a TC at 360 °C) and secondary magnetite in the suevite matrix (interpreted to be formed from Febearing fluids, which derived mainly from melt alteration) are the magnetic carriers of a stable remanent magnetization (Jr > Ji) in the suevite unit. Because only one NRM direction has been observed in the suevite, probably both minerals have acquired their magnetization close to the timing of the impact event. We plan to conduct low-temperature magnetization experiments with the suevite samples as well as with experimentally shocked pyrrhotite. These data might enhance our understanding of shock-induced acquisition processes in impact rocks and help to understand the relation between microstructures in pyrrhotite and shock pressure, which is not well constrained up to now. Furthermore, the magnetite only bearing samples will also be studied in order to learn if these samples do show any indications of superparamagnetic behaviour as we would suspect from the observed nanocrystals, and to find clues on the large range of Curie temperatures from 500 to 640 °C.