Libyan Desert Glass area in western Egypt: Shocked quartz in bedrock points to a possible deeply eroded impact structure in the region

Abstract

Libyan Desert Glass (LDG) is an enigmatic natural glass, about 28.5 million years old, which occurs on the floor of corridors between sand dunes of the southwestern corner of the Great Sand Sea in western Egypt, near the Libyan border. The glass occurs as centimeter‐ to decimeter‐sized, irregularly shaped, and strongly wind‐eroded pieces. The origin of the LDG has been the subject of much debate since its discovery, and a variety of exotic processes were suggested, including a hydrothermal sol‐gel process or a lunar volcanic source. However, evidence of an impact origin of these glasses included the presence of schlieren and partly or completely digested minerals, such as lechatelierite, baddeleyite (a high‐T breakdown product of zircon), and the presence of a meteoritic component in some of the glass samples. The source material of the glass remains an open question. Geochemical data indicate that neither the local sands nor sandstones from various sources in the region are good candidates to be the sole precursors of the LDG. No detailed studies of all local rocks exist, though. There are some chemical and isotopic similarity to rocks from the BP and Oasis impact structures in Libya, but no further evidence for a link between these structures and LDG was found so far. These complications and the lack of a crater structure in the area of the LDG strewn field have rendered an origin by airburst‐induced melting of surface rocks as a much‐discussed alternative. About 20 years ago, a few shocked quartz‐bearing breccias (float samples) were found in the LDG strewn field. To study this question further, several basement rock outcrops in the LDG area were sampled during three expeditions in the area. Here we report on the discovery of shock‐produced planar microdeformation features, namely planar fractures (PFs), planar deformation features (PDFs), and feather features (FFs), in quartz grains from bedrock samples. Our observations show that the investigated samples were shocked to moderate pressure, of at least 16 GPa. We interpret these observations to indicate that there was a physical impact event, not just an airburst, and that the crater has been almost completely eroded since its formation.

Terrestrial target and melting site of Libyan Desert Glass: New evidence from trace elements and Sr isotopes

“…The hypothesis of an asteroidal impact is based on (1) the observation of microdeformation structures (PDF) in quartz crystals from bedrock sandstones attesting high‐energy shock effects (Kleinmann et al. 2001; Koeberl and Ferriere 2019) and (2) the microstructural evidence for former reidite, a high‐pressure polymorph of ZrSiO 4 produced from zircon during shock (Kleinmann 1969; Cavosie and Koeberl 2019a, 2019b). In addition, traces of PDFs have been observed in former quartz, in what is now cristobalite inclusions within the LDG (Schmieder, personal communication).…”

“…2018a, 2018b; Cavosie and Koeberl 2019a, 2019b) suggest that, while high temperature melting and possible ejection of terrestrial material might have occurred at high pressure, the melt solidified quickly at atmospheric pressure. According to Cavosie and Koeberl (2019b) and Koeberl and Ferriere (2019), such a scenario is more compatible with an asteroidal impact than with a cometary impact or with a low‐altitude airburst of a cosmic body. The fact that no remnants of impact structures were found in the silica area does not necessarily play against the asteroidal impact hypothesis.…”

“…The fact that no remnants of impact structures were found in the silica area does not necessarily play against the asteroidal impact hypothesis. Koeberl and Ferriere (2019) argue that primitive impact structures (craters) that exist in the LGD area or nearby might have been demolished by terrestrial surface dynamic (alteration) processes.…”

“…2005; El‐Baz and Ghoneim 2007; Reimold and Koeberl 2014). More recently, Koeberl and Ferriere (2019) have critically discussed all reported hypotheses and relative evidence. In particular, they concluded that these structures are rather distant (hundreds of km) from the LDG area and lack evidence of shock impact effects that could be correlated to the LDG‐forming event.…”

“…Another argument that, in our opinion, reinforces the hypothesis of loose material rather than hard rock as LDG terrestrial target is related to the total absence, confirmed by observations during several field campaigns, of any remnants of potential impact structures in areas proximal to the LDG strewn field and southernmost regions. Recently, Koeberl and Ferriere (2019) suggested that the lack of any remnants of impact structures in the region is simply due to the complete erosion of the primitive structure. In our opinion, however, owing to the relatively young age of the LDG impact event, if impact occurred on hard and compact rocks as sandstones, remnants, or macro evidence of in situ melting among them and/or of an impact structure should have survived.…”