Uranium and Zirconium Enrichments in Libyan Desert Glass: Zircon Baddeleyite, and High Temperature History of the Glass

INTRODUCTION. Libyan Desert Glass (LDG) is an enigmatic natural glass found in an area of at least 3500 km2 between sand dunes of the southwestern comer of the Great Sand Sea in western Egypt, near the border to Libya. The glass shows irregular shapes with signs of sand abrasion and other erosion features. The age of the glass, as determined by the fission track method, is around 29 Ma [I]. Chemically, LDG is remarkably homogeneous With about 96.599 W% SiOT Major and trace element abundances fall into a narrow range (see, e.g., Fudali 121, Barnes and Underwood 131, Weeks et al. [4], and Koeberl [S]), although some trace elements, such as the rare earth elements, occur at levels typical for upper crustal rocks 14-61. The origin of LDG is the subject of a controversy which is still not settled. The majonty of workers favor an origin by impact, although there are some differences to 'classical' impact glasses, and no impact crater has been found to be associated with the glass occurrence. A relatively high water content of about 0.15-0.2 wt% was reported by Frischat et al. (e.g., m), which is not typical for impact glasses. Furthermore, Jux 181 claims to have found neogene microfossils in the glass (! ) and argues vividly for some kind of a sol-gel origin of LDG. Our study aims at providing further evidence for an impact origin of LDG. SAMPLES AND METHODS. During the determination,of fission track ages in several LDG samples, high local concentrations of uranium were noted in some sectionsfrom the study of the distribution of fission tracks after etching. These samples were then studied by electron microprobe to determine the major element chemistry of the high-U zones. RESULTS AND DISCUSSION. Fig. 1 shows the results of the study of the uranium-rich zones. There is a good positiie correlation between the concentrations of U and Zr. Two main groups of Zr/U rich zones were found. One type (very high in Zr and U) coincides with baddeleyite which is also visible by optical microscopy. Baddeleyite (ZrO,) is a breakdown product of zircon and was described earlier by Kleinrnann [9], who also found some remnant zircon crystals that were only partly transformed into baddeleyite. The other group was not evident from optical microscopy, but was only found from the uranium enrichments in the glass. In these cases, Zr was also found at the same locations that showed U enrichments. In both cases, the distribution of U (as determined from the fission tracks) and Zr follows flow structures and schlieren-iike structures, clearly indicating glass flow. In the case of the second group of Zr/U rich zones, the baddeleyite was already diluted in the flowing glass, and the structures of that flow are preserved. The mere fad of baddeleyite occurrence (which requires temperatures of 1500-2000°C) and the unambigous evidence for glass flow are in clear contradiction to any low temperz2ure origin as suggested in [8]. Fig. 1 shows also that the Zr/U conelation from the baddeleyite is not identical with the Zr/U correlation in the bulk LDG samples. The U content of the bulk LDGs is about 10 times higher than what would be expected if all U would be supplied by zircon. Fig. 2 shows the REE patterns of LDG (from 11 samples; Koeberl, [q, and in prep.) in comparison with local sand and sandstone (from Weeks et al. [4]), and the pattern of zircon [lo]. The zircon pattern has been recalculated to show the contribution to the REE pattern if all Zr in the LDG is supplied by zircon. Obviously, the contribution of zircon to the REE pattern is less than 520% even for the heaviest REE (which seems not to be the case for the Gilf el Kebir (GeK) sandstone (data from [4]), where the HREEs are dominated by zircon). This is in dear agreement with the uranium correlation. The same sands and the sandstone (from (41) are also plotted in Fig. 1 ( d i d triangles), showing that neither of them has U contents high enough to contribute U to the bulk LDG (for the. two sands that have Zr contents comparable to LDG, the U seems to be dominated by zircon). The REE pattern of the Camp F sand is incomplete, but the LREE pattern fits the LDG well; however, this sand cannot be the sole precursor, as evident from the Zr/U correlation. This is in line with Murali et al.3 [6] conclusion, who suggested desert sand (e.g., similar to Ain Dalla sand) and surficial Fecemented ailwial or some Nubia sandstone as precursors. From our Zr/U and REE data we conclude that none of the sands or sandstones analyzed so far, or mixlures thereof, are the sole precursors of LDG. Some admixture of monazite and/or apatite (e.g., from some type of sediment) seems likely to explain both the excess U and the REE patterns.