Chemical and isotopic characterization of products of small-scale hypervelocity impacts. The Gebel Kamil event

This study presents chemical and oxygen isotope data on a series of materials found in the area of the Gebel Kamil crater, a recent well preserved impact crater in southern Egypt, which was formed by the explosive impact of a metal projectile. Intracrateric target rocks, fallback deposits, impact vitreous ejecta (impactite) and recent or present-day aeolian sands have been analysed. Exposed crater wall rocks essentially consist of quartz arenites with minor siltstone beds chemically and isotopically quite distinct, the latter resulting enriched in lithophile elements (Al, K, Ba, Y, REE, etc.) and O-18. Abundance of siderophile elements (Ni, Co, Cu) indicates that none of the exposed target rocks and recent aeolian sand deposits was contaminated by materials of the metal impactor. On the other hand, a projectile-derived component was found in all samples of fallback deposits and in impact melt ejecta. Lithophile element and isotope chemistry of the fallback deposit results intermediate between that of quartz arenite and silstone target rocks. White- light-coloured glassy ejecta representing a minor (< 10% vol.) component of the impact melted products are chemically and isotopically distinct from dark-coloured vitreous clasts. The latter are enriched in lithophile elements and also 180 with respect to estimated "average" composition of the intracrateric target rocks. The presence of a projectile component in all samples of vitreous ejecta clasts suggests that melting of terrestrial material becomes extensive only when the projectile is directly involved in melting. The extreme variability of siderophile element ratios (e.g. Ni/Co, Ni/Cu) in materials (i.e., fallback deposits and vitreous ejecta) contaminated by projectile components suggests that significant element fractionation occurs during the explosive disruption of an impactor. A distinctive chemical and isotope signature of most of the impact melted products (i.e., dark vitreous ejecta), with respect to "average" composition of the exposed crater target rocks, poses problems concerning the actual terrestrial material source. On the basis of knowledge of surface geological stratigraphy, we hypothesize that the terrestrial source material for most of impact melted products (i.e., the dark impactites), is represented by a 2 m tick weathered unconsolidated cover enriched in lithophile elements originally present on the preimpact surface. The data suggest that in the case of small-scale explosive impacts effective melting of terrestrial material occur only at or nearly the earth's surface with the direct involvement of the projectile.