U-Pb dating of the Hummeln impact structure, Sweden

The aim of this project is to provide the direct radiometric age of material produced or transformed during the formation of the Hummeln impact structure (Ø 1.2 km), southern Sweden. An impact origin for this structure has only been confirmed relatively recently (Alwmark et al., 2015), and the crater-forming impact event is thought to be Ordovician in age, from biostratigraphic constraints. However, this structure has not been studied directly through radiometric dating of impact materials. Firstly, through microstructural characterisation (SEM-EBSD) of U-bearing accessory mineral grains (e.g., zircon, monazite) within impact melt, we will identify shock-related features in the grains which formed in response to the enormous pressures and temperatures produced during impacting. We will then conduct U-Pb geochronology (LA-ICPMS/SIMS) on the shocked grains, which will have experience variable radiogenic Pb loss, resulting in an accurate date of the impact event. Furthermore, analysis of PGE’s within the melt will be undertaken, enabling us to fingerprint the impactor. Ultimately, these results will enable us to definitively link the Hummeln impact structure to the L-chondrite break-up event, or not.