Tying Shock Features to Impact Conditions: The Significance of Shear Deformation During Impact Cratering.

Impact cratering is associated with extreme physical conditions with temperatures and pressures far exceeding conditions otherwise prevailing at the surfaces of terrestrial planets. As a consequence, shock‐metamorphosed rocks contain unique deformation features such as planar deformation features in quartz, high‐pressure mineral polymorphs and melted rock. While the physical conditions of formation for impact‐induced melting following the highest pressure and temperature conditions is relatively well understood, aspects of the formation of melt‐veins in otherwise seemingly relatively low shock material has been the topic of discussion. In a new study, Hamann et al. (2023, https://doi.org/10.1029/2023JE007742) are able to largely reproduce the current classification of progressive shock metamorphism of felsic rocks using a modern experimental set up that eliminates multiple shock wave reflections at sample containers and excavation and ejection of target material. Importantly, however, they find that shear deformation results in the formation of melt veins at pressures as low as 6 GPa. The authors recover stishovite in melt veins formed at low‐moderate (<18 GPa) shock pressure, lower than most previous studies. These results have bearing on our understanding of the conditions of progressive shock metamorphism at terrestrial impact structures. However, since the results are similar to data obtained from experiments on basaltic rocks, the results also have broader implications for understanding the shock histories of meteorite parent bodies. Hamann et al. show the importance of experimental impact cratering for bridging the gap between observations in shocked rocks from terrestrial impact structures, in meteorites, and in returned samples, and their formational conditions. Plain Language Summary: A new publication refines understanding of the conditions and processes required to melt rock during impact cratering. Impact cratering represents one of the most extreme geological processes there is, in terms of pressures and temperatures reached. The process is ubiquitous throughout the Solar System, today and in the past, and therefore understanding of the history of impacts on Earth and in meteorites and returned samples from other planetary bodies is essential in order to interpret Solar System evolution. Hamann et al. (2023, https://doi.org/10.1029/2023JE007742) show that experimental impact cratering is a critical link between observations of shock features and the pressure‐temperature conditions that formed those features. Specifically, Hamann et al. see that low shock pressures (6 GPa) is required for generation of small amounts of localized melt and they recover shock‐formed stishovite, a mineral with the same chemical composition as quartz, but with a different crystal structure, in their experiment at unusually low shock pressures compared to previous studies. Key Points: Intense shear deformation during impact experiments leads to formation of melt veins in granite at pressures as low as 6 GPaMelt veins formed in quartz grains subjected to >10–12 GPa during the experiments contain stishoviteAn update to the classification system of shock metamorphism of felsic rock is proposed by the authors [ABSTRACT FROM AUTHOR]