Shock experiments on anhydrite and new constraints on the impact-induced SOx release at the K-Pg boundary

Abstract

We have performed six shock experiments at nominal peak-shock pressures of 12.5, 20, 33, 46.5, 64, and 85GPa using polycrystalline anhydrite discs embedded in ARMCO-Fe sample containers and the shock reverberation technique. The recovered samples were analyzed using X-ray powder diffraction and transmission electron microscopy (TEM). The X-ray diffraction patterns recorded on all samples are compatible with the anhydrite structure; extra-peaks have not been observed. Peak intensities decrease and peak broadening increases progressively in the pressure range from 0 to 46.5GPa. At higher pressures, peak broadening diminishes and the X-ray diffraction pattern of the 85GPa sample resembles essentially that of unshocked, well-crystallized anhydrite. Related structural changes at the nanoscale include in the pressure regime up to 20GPa "cold" deformation phenomena such as cracks and deformation twins. Dislocation density increases up to 33GPa and the strain increases up to 46.5GPa. In the pressure range from 46.5 to 85GPa, high postshock temperatures caused annealing of the deformation features. Increasing density and size of voids in the anhydrite samples shocked at 64 and 85GPa indicate partial decomposition of anhydrite. Recalculation of the peak-shock pressure in the experiments to a more realistic natural loading path indicates the onset of degassing of anhydrite in the pressure range of 30-41GPa. © The Meteoritical Society, 2011.