IMPACT GLASSES AND IMPACT MELT BRECCIA FROM THE EL'GYGYTGYN STRUCTURE, CHUKOTKA, RUSSIA

The El’gygytgyn impact structure, 18 km in diameter and 3.6 Ma old, centered at 67°30′N and 172°34′E, is unusual because it is the only known meteorite impact crater excavated in felsic volcanic rocks on Earth. The target is dominated by Late Cretaceous lava, tuffs, and ignimbrites of rhyolitic composition, with local occurrence of dacite, andesite, basalt, and small plutonic bodies. The structure is filled by a lake, which has been drilled in 2009 in the framework of an ICDP project, providing a 517 m –long drill core through sediments, impactites, and the bedrock. Although the ejecta blanket around the crater seems to have been completely eroded, bomb-shaped impact glasses, locally including fragments of impact melt breccia, were deposited after the impact event and currently crop out in the lacustrine terraces within the depression.

Here we present petrographic and geochemical characterizations of 24 new samples of such glasses, collected in 2009. Our studies are complementary to preliminary descriptions provided in the literature prior to the drilling project. In particular, we focus on the different types of glasses, on the relationship between such glasses and impact melt breccia fragments, on the shock effects, and on some features that allow to somewhat constrain the cooling conditions of the melt. For petrographic studies, optical and electron microscopes were used, whereas a detailed geochemical characterization has been done by electron microprobe analysis and neutron activation analysis, the latter one to determine the trace elements abundances.

The silicate glass includes microlites, variously organized in aggregates, mainly consisting of plagioclase or pyroxene, with a flow fabric determined by schlieren and by layers of different crystallization stage. Rare quartz fragments are preserved and display a variety of shock features. Lenses of impact melt breccia are locally embedded into the glasses and consist of variously shocked rock, glass, and mineral fragments chaotically distributed in a glassy matrix. Such breccia portions likely resulted from inclusion into the glasses during ejection and deposition. After deposition the glassy portions continued to deform, whereas the impact melt breccia inclusions, probably already cooled down, behaved as rigid bodies in the flow.