Petrography of impact glasses and melt breccias from the El'gygytgyn impact structure, Russia

Abstract

Abstract-The El'gygytgyn impact structure, 18 km in diameter and 3.6 Ma old, in Arctic Siberia, Russia, is the only impact structure on Earth mostly excavated in acidic volcanic rocks. The Late Cretaceous volcanic target includes lavas, tuffs, and ignimbrites of rhyolitic, dacitic, and andesitic composition, and local occurrence of basalt. Although the ejecta blanket around the crater is nearly completely eroded, bomb-shaped impact glasses, redeposited after the impact event, occur in lacustrine terraces within the crater. Here we present detailed petrographic descriptions of newly collected impact glass-bearing samples. The observed features contribute to constrain the formation of the melt and its cooling history within the framework of the impact process. The collected samples can be grouped into two types, characterized by specific features: (1) "pure" glasses, containing very few clasts or new crystals and which were likely formed during the early stages of cratering and (2) a second type, which represents composite samples with impact melt breccia lenses embedded in silicate glass. These mixed samples probably resulted from inclusion of unmelted impact debris during ejection and deposition. After deposition the glassy portions continued to deform, whereas the impact melt breccia inclusions that probably had already cooled down behaved as rigid bodies in the flow.

Strontium and neodymium isotope systematics of target rocks and impactites from the El'gygytgyn impact structure: Linking impactites and target rocks

“…b) Simplified geological map of the El'gygytgyn impact structure, from Raschke et al (). Sample locations (red points for target rocks and black squares for impact glasses) are from Pittarello and Koeberl () and Pittarello et al ().…”

“…). Occurrences of larger centimeter‐sized cobbles and meter‐sized blocks are restricted to recent terraces (Gurov and Koeberl ; Pittarello and Koeberl ; Raschke et al. ).…”

“…A detailed petrographic investigation of El'gygytgyn impact glasses and melt breccias was done by Pittarello and Koeberl () on samples collected in the north‐western part of the crater. They defined two types of impact glasses and melt breccias.…”

“…All impact glasses and melt breccias show characteristic shock deformation, such as planar deformation features (PDFs) and lechatelierite bands, which classify them as impact glasses. A detailed description (meso‐ and microstructures, as well as the shock features) of the different types of impact glass and melt breccias is given in Pittarello and Koeberl (). In this study, we present geochemical and isotopic data of selected samples (mainly Type 1 samples) from the Pittarello and Koeberl () study, which we call “impact glasses” in the following text.…”

“…Based on the work of, and the samples described in, Pittarello and Koeberl () and Pittarello et al. (), we present new Sr and Nd isotope data for selected samples from drill core D1c, a representative selection of target rocks and some impact glasses.…”