Melting of Phyllosilicates and Evolution of Impact Glasses in Simulated Cratering Events

Abstract

Impact events involving phyllosilicates, whether present in targets or impactors, are highly probable on various celestial bodies. While impact melting is considered the most important metamorphic feature in shocked phyllosilicates, lack of understanding of this process represents a substantial impediment to constraining shock conditions from melted phyllosilicates and to inferring surface evolution of celestial bodies. To investigate shock metamorphism of phyllosilicates, cratering experiments were conducted on clinochlore targets using a light-gas gun at impact velocities ranging from 0.8 to 7.0 km·s⁻¹, and the shocked fragments were characterized with electron microscopy, X-ray diffraction (XRD), Raman spectroscopy and near-infrared spectroscopy. Clinochlore underwent melting at a low velocity of 0.8 km·s⁻¹ due to localized energy concentration at the micron-scale projectile-target interface. With increasing velocity up to 7.0 km·s⁻¹, the shock-generated glasses evolved from semi-parallel nanofilaments to complex agglutinate-like layers, within which abundant vesicles were present due to shock-induced dehydroxylation. Submicroscopic metallic particles were pervasive in the agglutinate-like layers, possibly owing to melting and solidification of micro-jetted metallic fragments. In line with the morphological characterization results, XRD patterns, near-infrared reflectance spectra and Raman spectra of the shocked fragments also collectively reflect the presence and evolution of the impact glasses. Beneath the impact glasses, shock metamorphism may be indicated by decreased basal spacings of clinochlore in the unmelted matrices. Additionally, olivine bearing exogenous iron composition from projectiles crystallized from high-temperature melts during secondary impacts. This work may provide important constraints for regolith evolution and impact history of extraterrestrial bodies.