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

Abstract

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.

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

“…The study by Hamann et al. (2023) highlights that experimental impact cratering provides a critical link between observations of impact processes and physical conditions, for example, in their observations of the significance of shear heating and melting in production of melt veins at lower shock pressures, and that these factors mediate the transition from quartz to stishovite. These studies enhance our ability to constrain impact environments on Earth, meteorite parent bodies, and other planets, as well as increase understanding of the process itself and its consequences, ultimately allowing interpretations of Solar System dynamics and evolution.…”

“…Continued exploration of the Solar System and retrieval of samples from bodies not as geologically active as Earth, where impact overprinting is frequent if not always present, adds demands on researchers to increase understanding of the full range of shock metamorphism in all rock types and the processes forming observed features. The study by Hamann et al (2023) highlights that experimental impact cratering provides a critical link between observations of impact processes and physical conditions, for example, in their observations of the significance of shear heating and melting in production of melt veins at lower shock pressures, and that these factors mediate the transition from quartz to stishovite. These studies enhance our ability to constrain impact environments on Earth, meteorite parent bodies, and other planets, as well as increase understanding of the process itself and its consequences, ultimately allowing interpretations of Solar System dynamics and evolution.…”

“…Localized melt veins have been generated previously at similarly low pressures in experiments using similar analog materials (Kenkmann et al., 2000; Langenhorst et al., 2002), and for other types of materials (e.g., Ono et al., 2023) but Hamann et al. (2023) can provide further, more detailed, understanding of how shear‐induced melting operates on the scale of individual grains in felsic polymineralic rocks because of the updated experimental setup. It is interesting to consider how these melt veins might compare, in terms of formation, to various melt veins at terrestrial impact structures, including also the larger‐scale melt veins, or pseudotachylitic breccia, in terms of mode of formation.…”

“…No data was used in this paper. The data from Table 1 is from Stöffler et al (2018) andHamann et al (2023).…”

“…A recent publication by Hamann et al. (2023) utilizes this new setup to impact granitic targets and combine experimental data with numerical modeling to estimate the spatial and temporal distributions of peak pressure and temperature as well as volumetric strain rates in the targets. Hamann et al.…”