Shock metamorphic features in minerals in crater fill from the Tvären impact structure, Sweden

Collisions between planetary bodies are one of the most ubiquitous and important geological processes in the Solar System. In recent decades, the study of impact events has gained more interest, leading to the identification of over two hundred impact structures on Earth [1]. The object of this study, Tvären, is a 2 km-in-diameter simple impact structure that was formed in a marine environment in the Middle Ordovician [2]. To date, only limited information validating the impact origin of the structure is available [3] and no in-depth studies of impact metamorphism of the crater fill have been published to date, to our knowledge. Our investigation aims to determine the pressure conditions prevalent during the formation of the crater fill.

For this study, forty thin sections from the [161,60-218 m deep] drill core Tvären-2 [4] were investigated and searched for the presence of shocked quartz and shock metamorphic features in other minerals using a petrographic microscope. Subsequently, Leitz five-axes universal stage microscopy was utilized to measure orientations of the poles of PDFs in quartz that were further indexed with the updated stereographic projection template [5]. Quartz grains from five different depth levels of the drill core were measured to confirm that planar features – PDFs and PFs (planar fractures) – occur in the drill core material between 161,60 and 215,70 m. The investigated lithologies are (from top to bottom) (i) light grey sandstone to mudstone; (ii) sandstone; (iii) fine gravel to coarse sand; (iv) limestone gravel breccia; (v) coarse limestone breccia [4]. The presence of quartz grains with PDFs parallel to {1012} and quartz grains with only PFs in the same depth intervals indicates the investigated lithologies contain material shocked over a wide pressure range – from 5 to 25 GPa (see method for calculating shock pressures in [6] and references therein). Shocked quartz grains, which generally do not exceed 0.5 mm in size, are present in both matrix and bedrock fragments. Preliminary findings suggest that PDFs mostly occur in angular quartz grains, while PFs were found in rounded quartz grains.

We have also studied a subset of thin sections with SEM-EBSD, with the purpose of investigating if shock metamorphic features are present in any other minerals than quartz. In our preliminary investigation we have observed heavily fractured monazites and zircons with interesting textures, that will require further investigation. With known pressures, based on quartz PDF shock barometry we may assign the features found in the other minerals to different levels of shock.

Acknowledgements: Swedish Research Council grants #2022-04255 and #2021-03836.

References: [1] Osinski G et al. Earth Sci Rev. 2022;232:104112; [2] Ormö J. GFF. 1994;116(3):139-144; [3] Gajewska K et al. NGWM 2024; 10-12 Jan 2024; Gothenburg, Sweden; [4] Lindström et al. Geol Mag. 1994;131(1):91-103; [5] Ferrière et al. MAPS. 2009;44(6):925-40; [6] Holm-Alwmark S et al. MAPS. 2018;53(1):110-130.