Cathodoluminescence as a tool to discriminate impact melt, shocked and unshocked volcanics: A case study of samples from the El'gygytgyn impact structure

El'gygytgyn (Chukotka, Arctic Russia) is a well preserved impact structure, mostly excavated in siliceous volcanic rocks. For this reason, the El'gygytgyn structure has been recently investigated and drilled in 2009 in the framework of an ICDP (International Continental Scientific Drilling Program) project. The target rocks mostly consist of rhyo-dacitic ignimbrites and tuffs, which make it difficult to distinguish impact melt clasts from fragments of unshocked target rock within the impact breccia. Several chemical and petrologic attempts, other than dating individual clasts, have been considered to distinguish impact melt from target unshocked volcanic rocks, but none has proven reliable. Here we propose to use cathodoluminescence (imaging and spectrometry), whose intensity is inversely correlated with the degree of shock metamorphism experienced by the investigated lithology. Specifically, impact melt rocks display low cathodoluminescence intensity, whereas unshocked volcanic rocks from the area show typical high luminescence. This high luminescence decreases with the degree of shock experienced by the individual clasts in the impact breccia, down to very low when the groundmass is completely molten. This might apply only to El'gygytgyn, because the luminescence in volcanic rocks might be due to devitrification and recrystallization processes of the relatively old target rock (Cretaceous) with respect to the young impact event (3.58 Ma). The alteration that mostly affects samples from the drill core does not affect significantly the cathodoluminescence response. In conclusion, cathodoluminescence imaging and spectra potentially provide a useful tool for in situ characterization of siliceous impactites formed in volcanic target.