Experimental Evidence for Shear-Induced Melting and Generation of Stishovite in Granite at Low (<18 GPa) Shock Pressure

Knowledge of the shock behavior of planetary materials is essential to interpret shock metamorphism documented in rocks at hypervelocity impact structures on Earth, in meteorites, and in samples retrieved in space missions. Although our understanding of shock metamorphism has improved considerably within the last decades, the effects of friction and plastic deformation on shock metamorphism of complex, polycrystalline, non-porous rocks are poorly constrained. Here, we report on shock-recovery experiments in which natural granite was dynamically compressed to 0.5-18 GPa by singular, hemispherically decaying shock fronts. We then combine petrographic observations of shocked samples that retained their pre-impact stratigraphy with distributions of peak pressures, temperatures, and volumetric strain rates obtained from numerical modeling to systematically investigate progressive shock metamorphism of granite. We find that the progressive shock metamorphism of granite observed here is mainly consistent with current classification schemes. However, we also find that intense shear deformation during shock compression and release causes the formation of highly localized melt veins at peak pressures as low as 6 GPa, which is an order of magnitude lower than currently thought. We also find that melt veins formed in quartz grains compressed to >10-12 GPa contain the high-pressure silica polymorph stishovite. Our results illustrate the significance of shear and plastic deformation during hypervelocity impact and bear on our understanding of how melt veins containing high-pressure polymorphs form in moderately shocked terrestrial impactites or meteorites.