Shock experiments on anhydrite and new constraints on the impact‐induced SO_x release at the K‐Pg boundary

Abstract

Abstract-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 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 46.5 GPa. At higher pressures, peak broadening diminishes and the X-ray diffraction pattern of the 85 GPa sample resembles essentially that of unshocked, wellcrystallized 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 postshock temperatures caused 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 30-41 GPa.

Dynamic water loss of antigorite by impact process

“…Several experimental reports on shock-recovered samples have been available for representative volatiles such as H 2 O, CO 2 , and SO 3 from serpentine (Boslough et al, 1980;Lange et al, 1985;Tyburczy and Ahrens, 1988;Akai and Sekine, 1994), carbonaceous chondrites with serpentine (Tyburczy et al, 1986;Tomeoka et al, 2003;Tomioka et al, 2007), carbonates (Boslough et al, 1982;Lange and Ahrens, 1986;Martinez et al, 1995;Agrinier et al, 2001;Skála et al, 2002;Ohno et al, 2008Ohno et al, , 2014a, and hydrous and anhydrous sulfates (Zhang and Sekine, 2007;Bell, 2010;Preschuer et al, 2011;Ohno et al, 2014b), in which quenched samples were investigated after impacts to analyze the degree of degassing or impact-induced gasses as a function of shock pressure. In these shock-recovery experiments, degassing conditions were classified to be vented or not to be vented, but the classification will be subjected to the degree or amount of degassing as well (Lange et al, 1985).…”

The reaction of carbonates in contact with laser‐generated, superheated silicate melts: Constraining impact metamorphism of carbonate‐bearing target rocks

“…; Bell ), differences in shock‐induced entropy at a given nominal shock pressure in impact and shock reverberation experiments (Prescher et al. ; Kurosawa et al. ), shock pulse durations, water content and grain size of starting materials (Agrinier et al.…”

Evidence for shock‐induced anhydrite recrystallization and decomposition at the UNAM‐7 drill core from the Chicxulub impact structure

“…() and Prescher et al. () that suggested that (1) anhydrite is stable over a large high‐pressure range; (2) low shock pressures cause fragmentation and undulatory extinction; (3) high post‐shock temperatures induce recrystallization, annealing of deformation features, and lowered birefringence; and (4) voids indicate partial decomposition. At UNAM‐7, the onset of SO x degassing due to decomposition is indicated at fragmented anhydrite by the voids commonly <200 nm in size.…”

“…Transmission electron microscopy studies of experimentally shocked anhydrite (Prescher et al. ) documented shock effects such as formation of twins, dislocations, voids, and polygonization. Prescher et al.…”

“…Prescher et al. () showed that in the pressure range from 46.5 to 85 GPa, high postshock temperatures induced recrystallization and annealing of the deformation features, and concluded that voids up to 180 nm in size in the anhydrite samples shocked at 64 and 85 GPa indicated partial decomposition.…”