SHOCK RECOVERY EXPERIMENTS AT LOW SHOCK PRESSURE WITH DRY SEEBERGER SANDSTONE

Within the Multidisciplinary Experimental and Modelling Impact research Network (MEMIN) this project investigates shock effects in quartz in the low shock pressure range from < 5 to 15 GPa, and the influence of porosity on progressive shock metamorphism. Shock recovery experiments at 5, 7.5, 10 and 12.5 GPa were carried out with dry Seeberger sandstone (grain size: 0.17 - 0.01 mm, porosity: ~18 vol.%) at the Ernst-Mach-Institute using a high-explosive device. For shock pressure determination Hugoniot data for Coconino sandstone were applied, as at present only incomplete Hugoniot data for Seeberger sandstone are available that indicate an error of ~1-2 GPa in shock pressure determination.

At the microscopic scale the shocked Seeberger sandstone shows a near-complete closure of pore space. Locally, pores are filled with Al-Fe-rich, foamy melts after phyllosilicates (melt abundance increases with shock pressure). Some irregular intergranular fractures have been induced. Quartz grains of the unshocked sample show sharp and undulatory extinction under crossed polarizers, whereas the shocked samples display quartz grains with mainly undulatory extinction at 5 GPa and weak mosaicism at 7.5, 10, 12.5 GPa. All shocked samples show intense intragranular fracturing (irregular and subplanar), which significantly increases from 5 to 7.5 GPa. At even higher pressures to 12.5 GPa, fracturing remains at a more or less constant level. At 5 GPa quartz grains usually display only one set of roughly planar fractures, whereas at 7.5, 10 and 12.5 GPa two or more sets could be observed. The samples shocked at 10 and 12.5 GPa display locally isotropic areas in the optical microscope, which comprise diaplectic quartz glass in the center and deformed quartz in the rim based on Raman and SEM analysis. Our shock experiments have produced shock features as known from naturally shocked porous sandstone.

Additionally, the sandstone cylinders shocked at 10 and 12.5 GPa display curved shear zones starting at the contacts of the sample cylinder with the surrounding ARMCO iron trap. Cataclastic microbreccias occur within broader shear zones, whereas thin shear zones are filled with SiO₂ melts. The shear zones are associated locally with quartz grains displaying subplanar microfeatures with strong similarity to planar deformation features (PDF).