Microstructural deformation of zircon during impact metamorphism

Impact craters are among the most pervasive geological features on solid bodies throughout the inner Solar System. The heavily cratered surface of the Moon offers a stark visual record of the Solar System’s early history of intense bombardment, a record largely absents from Earth’s dynamic surface. Despite having experienced a comparable flux of impact events over deep time, Earth's geological processes—plate tectonics, erosion, sedimentation, and biological overprint—have significantly obscured or obliterated much of this record. Today, fewer than 200 confirmed impact structures are known on our planet. However, Impact cratering is now firmly established as a fundamental geological process with far-reaching implications for planetary evolution. On Earth, the consequences of large-scale impacts extend beyond crustal modification. One Impact have been directly linked to profound shifts in the geobiosphere, the Chicxulub event at ~66 million years ago, which coincides with the Cretaceous–Paleogene mass extinction. Such events highlight the critical role that impact cratering has played in shaping not only planetary surfaces, but also the trajectory of life itself.In addition to global consequences, impact events produce a distinct suite of mineralogical deformation features within the target rocks. The brief but intense pressures (>10–100 GPa) and temperatures (>1,000°C) gradients generated during hypervelocity impact are sufficiently high to induce permanent structural deformations in both rocks and minerals—a phenomenon known as impact metamorphism. These shock features are unique to impact cratering processes and are critical to confirming the impact origin of a structure but can also be used to reconstructing the conditions of crater formation. Studying shock metamorphic effects at the micro- and nanoscale allows for quantitative reconstruction of the impact process, including pressure-temperature calibration, and to understand the underlying deformation mechanisms. Importantly, the mineral zircon does not only record these extreme conditions, but also preserve geochronological information that can be utilized to constrain dates of impact events (e.g. through the U-Pb isotopic system). Understanding how zircon respond to various pressure-temperature conditions is therefore crucial.The research presented in this thesis investigates how zircon responds to the extreme pressure-temperature conditions generated during hypervelocity impact events. Using a combination of high-resolution analytical techniques—scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), Raman spectroscopy, and transmission electron microscopy (TEM)—zircon grains from the Rochechouart impact structure in France were examined in order to understand various deformation mechanisms, high-pressure phase transformations, and shock-induced recrystallization.Paper I focuses on the preservation and diversity of reidite, a high-pressure polymorph of zircon. Five distinct reidite morphologies were here characterized, including three newly documented habits. These habits exhibit systematic crosscutting relationships, indicative that reidite can form during both compression and decompression during impact. The study also documents the first evidence of localized reversion of reidite to zircon, forming distinct microstructural textures, and providing new constraints on post-shock thermal conditions and insights to zircon → reidite → zircon phase transformation.In Paper II, Raman spectroscopy was used to characterize spectral signatures of shocked zircon and reidite. The study identifies diagnostic spectral shifts associated with increasing shock intensity, revealing how strain is partitioned during both peak-pressure conditions and during the post-shock temperature excursion. Most importantly, this study highlights that potential misidentifications of reidite can occur. Erroneous interpretations arise from spectral interference by rare earth elements, which tend to overlap spectral signatures of reidite. A refined methodological framework is here proposed for distinguishing between various zircon polymorphs, improving the reliability of Raman spectroscopy in impact metamorphic studies.Paper III explores the role of shear localization in driving solid-state recrystallization of zircon. Through EBSD and TEM analyses, the study outlines a sequence of microstructural transitions and evolution, from dislocation accumulation to the formation of granular neoblastic shear bands. The results show that dynamic recrystallization in zircon is facilitated by localized heating and intense strain, offering new insights into how zircon both deforms and recrystallizes under extreme pressure-temperature conditions.Together, the three studies of this thesis provide a comprehensive view of how zircon records impact metamorphic conditions associated with the extreme pressures and temperatures associated with impact cratering. The findings thus offer new tools for interpreting pressure-temperature histories in impact structures and contribute to a broader understanding of the complex physical processes that operate in zircon during the impact cratering process.