Ries magnetic mineralogy: Exploring impact and post-impact evolution of crater magnetism

Abstract

Large-scale impact events are some of the most catastrophic and instantaneous geological processes in nature, and leave in their wake conspicuous geological structures with characteristic magnetic anomalies. Despite magnetic anomalies in craters being well-documented, their relationship with the magnetic mineral composition of the target and impactites is not always straightforward. Furthermore, the influence of impact shock and post-impact events in the magnetism of natural craters remains elusive. In the Ries crater, Germany, the negative magnetic anomalies are attributed to a reverse polarity remanent magnetization in the impact-melt bearing lithologies. We report new chemical, rock-, and mineral-magnetic data from the shocked basement and impactites, from surface samples, NR73 and SUBO-18 boreholes, and explore how temperature and hydrothermalism may influence the magnetic mineralogy in the crater. We identified shocked, pure magnetite in the basement, and low-cation substituted magnetite in the impactites as the main magnetic carriers. The shocked basement is demagnetized but remains largely unaltered by post-impact hydrothermalism, while the impactites show weak magnetization and are extensively altered by neutral-to-reducing post-impact hydrothermalism. We suggest that the magnetic mineralogy of the demagnetized uplifted basement may contribute significantly to the magnetic anomaly variation, in line with recent findings from the Chicxulub peak-ring.