Zircon (U-Th)/He impact crater thermochronometry and the effects of shock microstructures on He diffusion kinetics

Abstract

Accurate age determination of impact cratering events remains difficult and controversial. Less than a quarter of all documented impact craters are regarded as accurately and precisely age dated. Zircon (U-Th)/He (ZHe) dating of impactites has received recent attention as a novel technique to date impact structures. Complicated and localized post-impact hydrothermal systems could also affect the reliability of ZHe impact ages. We quantitatively define the effects of shock-induced microstructures on He diffusion kinetics in zircon with different shock microstructures characterized by Secondary Electron Microscopy (SEM) and electron backscatter diffraction (EBSD). We investigated samples from the Chicxulub and Ries impact craters, to compare diffusion kinetics from structures of different sizes, ages, and hydrothermal activity. Step-heating fractional-release experiments were used to quantify He diffusion kinetics of variably shocked zircon and examine the relationship of the experimentally derived diffusion domain sizes to the EBSD-defined subgrains. The diffusion data show a consistent activation energy independent of shock level, while shock microstructures, such as planar deformation bands (PDBs) and granular textures systemically and dramatically lower the He retentivity due to the microstructural reduction of the effective diffusion domain size. Shocked zircon were dated by ZHe and the less deformed grains yield ages within error of the accepted impact ages as determined by other methods (40Ar/39Ar and U-Pb), whereas the grains with PDBs or granular textures commonly exhibited younger ages. Thus, the characterization of impact-induced zircon microstructures is critical for determining accurate impact ages because shocked grains are less suitable for reliable impact dating, but potentially powerful for exploring temperature–time history of hydrothermal activity.