Large Meteorite Impacts VI 2019 (LPI Contrib. No. 2136) 5105.pdf

WHAT DO WE KNOW ABOUT THE FORMATION OF LIBYAN DESERT ? Christian Koeberl1,2, 1Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria (chris- [email protected]), 2Natural History Museum, Burgring 7, A-1010 Vienna, Austria.

Introduction: Libyan Desert Glass (LDG) is an indications for the presence of a meteoritic component enigmatic silica-rich natural glass, which occurs be- in dark streaks or layers of the desert glass from the tween dunes of the southwestern corner of the presence of platinum-group element anomalies. This Great Sand Sea in western Egypt. The glass shows a was confirmed in an osmium isotopic study; un- limited variation in major and trace element abun- published Cr isotope data point in the same direction. dances. Some cristobalite inclusions occur, but other- Crustal strontium and neodymium isotopic values ex- wise LDG is perfectly glassy. Although the origin of clude a significant mantle component; thus, the osmi- LDG is still debated by some workers, an origin by um abundances and isotopic values confirm the pres- impact seems most likely. There are, however, some ence of a meteoritic component in LDG. The iron oxi- differences to “classical” impact , which occur dation state, measured by iron K-edge high resolution in most cases directly at or within an impact crater. X-ray absorption near edge structure (XANES) spec- Evidence for an impact origin includes the presence of troscopy indicates that in the layers with higher iron schlieren and partly digested mineral phases, such as content, iron occurs in a more reduced state, which (a high-temperature mineral melt of suggests that some or most of the iron in these layers ), and baddeleyite, a high temperature break- may be directly derived from the meteoritic projectile down product of zircon, as well as traces of meteoritic and that it is not of terrestrial origin. material. References for all the statements made here The source material of the glass remains a mys- can be found in two recent publications on this topic tery. The rare earth element (REE) abundance pat- [1, 2]. terns are indicative of a sedimentary precursor rock, Location: Libyan Desert Glass is found in an area and the trace element abundances and ratios are in of about 2500 to 6000 km2 (the exact numbers vary agreement with an upper crustal source. Specifically, between different publications, and the higher num- Zr/U and REE data suggest that none of the or bers seem to include pieces transported by, e.g., wind). sandstones from various sources in the region are The glass occurs as centimeter to decimeter-sized ir- good candidates to be the sole precursors of LDG. regular and strongly wind-eroded pieces. The total Compositional data for surface sands show significant preserved quantity of the glass has been estimated at differences from the average LDG composition. The 1.4x109 grams, but it is quite likely that the original high-temperature involved was recently confirmed by mass was much larger. Attempts to determine the age the detection of mullite in LDG samples. Also, LDG of the LDG were made using the K-Ar and fission- has high δ7Li at ≥24.7‰, which m ay represent the track methods. Due to the low K content of the glass, previous fluvial history of parental material that was errors on K-Ar ages are too high to be meaningful. perhaps deposited under lacustrine conditions or in The only precise ages of the LDG come from fission coastal seawater. The δ18O values of bulk rock and track determinations, which gave ages ranging from quartz from intrusives of Pan-African age and the 28.5 ± 2.3 Ma to 29.4 ± 0.5 Ma (plateau age), and results obtained were compatible with their values 28.5 ± 0.8 Ma. obtained for LDG samples (9.0-11.9‰). Thus, geo- Information on Origin: The origin of LDG has chemical measurements have been essential to deter- been the subject of much debate since its discovery, mine the origin of the LDG as some type of impact and a variety of exotic processes were suggested (e.g., glass, and have also given valuable indications regard- a hydrothermal sol-gel process, or a lunar volcanic ing the source material. source). Recently, there has been much interest in the Several workers (see references in [1, 2]) have possibility that LDG could have formed by airburst. considered the possibility that the LDG might be re- However, there is abundant evidence of an impact lated to one or both of the Libyan impact structures, origin of these glasses, including the presence of based on the proximity of these structures to the area schlieren and partly or completely digested minerals, of the Libyan Desert Glass (LDG), which is found in a such as lechatelierite, or baddeleyite, and evidence for limited strewn field about 150 km to the east, and the the former presence of reidite, a high-pressure poly- lack of disturbed Lower Cretaceous sandstone strata in morph of ZrSiO4, which requires shock pressures on the area of the occurrence of the glass. There are indi- the order of 30 GPa [1]. Also, there are some strong cations that the BP and Oasis structures could be older Large Meteorite Impacts VI 2019 (LPI Contrib. No. 2136) 5105.pdf

than the LDG. There is some chemical and isotopic similarity to rocks from the BP and Oasis impact structures in Libya, but a Rb-Sr and Sm-Nd isotopic study of LDG suggests that “Nubian” rocks are not likely precursors of LDG. The study of rocks from these structures did not yield any convincing evidence of a connection to LDG. To complicate things even further, earlier work in the 1990s noted some shocked quartz-bearing breccias in the LDG strewn field, but – so far – no evidence for an actual crater has been found in this area. Most recently, Koeberl and Ferrière [2] reported on the presence of shocked quartz from bedrock sam- ples within the LDG strewn field, providing evidence for the possible existence of the erosional remnant of a former impact structure in the area. Conclusions: LDG remains an enigmatic glass. The question of the mode of formation, airburst or impact, or a combination of the two, is still not re- solved. This author prefers an explanation by impact, or at least predominantly by impact, with a deeply eroded impact structure in the area, due to the discov- ery of shocked bedrock and clear indications of physi- cal interaction of meteoritic material with the target rocks, as shown by the admixture of meteoritic mate- rial in the glass. The absence of a crater, or at least the fact that none has been discovered yet, is problematic but not impossible to explain, given the erosion rate in such areas. To explain LDG just by airburst brings up the question of why there is evidence of high-pressure, high-temperature, and meteoritic admixture. Also, if LDG formed by impact, one wonders why, as airburst would be much more common than impact, there are not more similar deposits on Earth. This all indicates that LDG is something rather unusual, no matter what the formation mechanism was.

Acknowledgements: I thank many colleagues over the years for discussions on the origin of LDG. The search continues.

References: [1] Cavosie A. and Koeberl C. (2019) Geology, 47, 609-612. [2] Koeberl C. and Ferrière L. (2019) MAPS, doi.org/10.1111/maps.13250.