The first U-Pb age for shocked zircon from the Mien impact structure, Sweden, and implications for metamictization-induced zircon texture formed during impact events

Shocked zircon from impactites from the Mien impact structure, Sweden, has been investigated with the aim to date the impact event and correlate the degree of U-Pb age resetting with shock-related microtextures. In situ U-Pb spot isotope analyses of granular and microporous-granular zircon grains from the impact melt rocks give an age of 120.0 +/- 1.0 Ma. This essentially confirms the previous best estimate age of 122.4 +/- 2.3 Ma, while also increasing precision on the Mien impact age. U-Pb isotope mapping shows that radiation damage likely explains the similar U-Pb age reset associated with different shock-related microtextures. Microporous and some of the granular and microporous-granular domains yield higher U concentrations along with younger 238U/206Pb dates. Lower U contents with older 238U/206Pb dates are predominately observed in pristine domains. Due to the U-decay, the zircon lattice is damaged, a process through which Pb can be lost. This would result in younger 238U/206Pb dates, as observed for the high U domains. As the zircon crystal lattices were locally weakened, metamictization possibly facilitated the development of microporous and granular textures during the impact event. Analyses of unshocked Mien zircon confirm that radiation damage already existed before impact. Lead loss from granular domains occurred through recrystallization and from microporous domains through Pb leaching by hydrothermal fluids. In addition, our study demonstrates the utility of combined U-Pb isotope mapping and spot analysis in unraveling the link between U-Pb resetting and shock-related microtextures, the formation of which was in this case likely promoted by pre-existing radiation damage.