Geochronology of impact structures - constraining syn- and post-impact processes using the 40Ar/39Ar and U-Pb techniques

Sammanfattning: The discovery, at the beginning of the 20th century, that elements can transform into other elements, due to the spontaneous decay of an instable to a stable atomic nuclei, gave rise to a powerful source for age information in many fields of earth science. For impact structures, it is crucial to establish well-defined and precise ages in order to understand how impact events affect the Earth’s geo and biosphere and also with regard to possible future events that can have devastating effects on our civilization.During the impact cratering process immense amount of energy is released in relatively short time, resulting in extreme temperature and pressure conditions that can even melt and vaporize rocks. The thermal impact is so high that in consequence, the composition between instable and stable nuclei in a melted rock or mineral is changed due to the loss of stable nuclei through diffusion. Thus, the atomic or isotopic clock of a rock or mineral is reset and the accumulation of new stable nuclei starts again, preserving the imprint of the impact event. Traditionally, impact structures are dated by 40Ar/39Ar on impact melts. Such melts quench soon after their formation and thus, inhibit the diffusional loss of newly-formed stable nuclei. Therefore, these melts can be used to date an impact event. Further, individual minerals can yield impact-related ages, too, such as zircon. The U-Pb decay in zircon is the most widely used dating method. Due to the recrystallisation or new growth of zircon within impact melts, Pb is lost, indicating an impact relation. However, even though impact events have a devastating effect, in some cases minerals preserved in the target rock show no sign of shock, but instead, be affected by post-impact processes, such as by hydrothermal activity. Thus, impact crater ages are not always straight forward and should be interpreted with great care.The U-Pb analyses of zircon grains from the target rock of the Siljan impact structure in Sweden can be explained by different residency time of zircon at shallow, cool crustal levels rather than by the impact event. Prior to the uplift by the impact event about 380 Ma ago, zircon grains near the crater centre resided at greater depth, where radiation-damaged lattice is able to anneal due to higher temperatures, leading to much less Pb loss. Whilst zircon grains distal to the crater centre was preserved near the erosional surface at temperatures that inhibit any annealing of radiation damage since >1260 Ma and thus, prone to lose more Pb.The 40Ar/39Ar age data of biotite and amphibole from the Siljan target rock exclude an impact age, as well, and instead, indicate an imprint by hydrothermal fluids driven by the impact. Whilst the 40Ar/39Ar dating of whole rock impactites from the Hummeln and Mien impact structures in Sweden and the Puchezh-Katunki impact structure in Russia suggests a correlation with an impact event, even though Hummeln shows only a partial reset of the 40Ar/39Ar system and Puchezh-Katunki yield an age range between 192 and 196 Ma.Further challenges combine dating with element mapping and microtextural analyses. Highly shocked zircon, extracted from impact melts of the Mien crater, with certain textural features can yield U-Pb ages that are impact-related. It shows that the formation of shock textures in zircon is promoted when the lattice is metamict, i.e., damaged by radiation due to the U-Pb decay.