SHOCK EXPERIMENTS ON ANHYDRITE AND NEW CONSTRAINTS ON THE IMPACT-INDUCED SOx RELEASE AT THE K-PG BOUNDARY

In context of the ~190-km-seized Chicxulub impact crater, the “smoking gun of the K-Pg mass extinction”, large amounts of CO2 and SOX were presumably released from the more than 3-km-thick platform sediments that formed the upper part of the target. The instantane-ous input of these gaseous species likely changed composition and radiative balance of at-mosphere and climate at the K-Pg boundary. Published values for the actual re-leased amount of these gases vary by several orders of magnitude (see compilation in Agrinier et al. 2001, Ivanov and Deutsch 2002), mainly due to problems in constructing equations of state (EOS) and phase diagrams relevant for shock compression and unloading of the phases of interest as well as due to limitations in experiments.

In this context, we have performed six shock experiments at nominal peak shock pressures of 12.5, 20, 33, 46.5, 64, and 85 GPa using polycrystalline anhydrite discs embedded in ARMCO-Fe sample containers and the shock reverberation technique. The recovered samples were analysed by means of 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 46.5 GPa. At higher pressures, peak broadening diminishes and the X-ray diffraction pattern of the 85 GPa sample resembles essentially that of unshocked, well-crystallized anhydrite. Related structural changes at the nanoscale include in the pressure regime up to 20 GPa “cold” deformation phenomena such as cracks and deformation twins. Dislocation density increases up to 33 GPa and the strain increases up to 46.5 GPa. In the pressure range from 46.5 to 85 GPa high post-shock temperatures yielded annealing of the deformation features. Increasing density and size of voids in the anhydrite samples shocked at 64 and 85 GPa 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 to 41 GPa.