A shocked lithic clast with compositionally contrasting melt domains in the impact melt breccia-a first report from the Dhala impact structure, India

Abstract

Bolide impacts are associated with enormous amount of kinetic energy which transforms into humongous shock pressures (∼100 GPa) and temperatures (up to 20,000 °C) on the planetary surfaces. These extreme conditions result in unique shock features that are routinely used as diagnostic evidence to confirm the meteorite impact origin of a structure. Occurrence of diaplectic glass pseudomorphing various minerals is one of the unequivocal evidences of shock metamorphism. Similarly, complete rock melting requires pressure in the excess of 60 GPa; however, the melting processes are not well constrained and remain ambiguous. The present study focuses on shock-induced melting of a quartzo-feldspathic lithic clast within an impact melt breccia sample from the Dhala structure, India. Multiple felsic melt stringers and a silicic melt (similar to lechatelierite) lense were observed across the clast, displaying a linear mixing trend between K-feldspar and silica. The occurrences of planar deformation features in quartz, melt clasts, spherules, and traces of coesite indicate characteristics of shock metamorphism. The presence of coesite (a high-pressure silica polymorph), formed during shock pressure release from the silica melt, was verified through Raman spectroscopy. Compositions of felsic melt stringers seem to mimic eutectic melting similar to a synthetic K₂O-Al₂O₃-SiO₂ system suggesting the dominant role of eutectic type punctuated melting rather than instantaneous melt behaviour. This pattern contrasts with an immediate, widespread melting often reported in impactites worldwide.