New evidence on the formation conditions of the Libyan Desert Glass (Western Egypt): Clues from a dendritic zircon inclusion

Libyan Desert Glass (LDG) is an~29 million years old, silica-rich glass found in Western Egypt. Whether this glass formed in an impact cratering context associated with the hypervelocity collision of a cometary/asteroidal body or radiative heating during an airburst is debated. Determination of the formation temperatures and pressures of rare mineral components in LDG can provide key petrogenetic constraints on its origin. Here, we report the occurrence of a zircon inclusion, whose textural, chemical, and crystallographic features point to a rapid formation during solidification of the silica-rich LDG melt. The study was conducted combining dual beam microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and three-dimensional electron diffraction. The inclusion is a few tens of micrometer in size and consists of dendritic branches of zircon arranged in a reticulate-cruciform texture. The high-silica glass filling interstices between dendrites have longer chemical bonds compared to matrix glass, as indicated by electron pair distribution function analysis, and is enriched in Al2O3. The lack of incongruent melt products (ZrO2, SiO2) suggests that the inclusion formed during cooling from supraliquidus conditions, by dynamic crystallization from an (immiscible) undercooled liquid droplet. Such droplet would derive from shock-induced melting of a precursor zircon grain, possibly mixed with the SiO2-rich liquid formed by melting of the LDG precursor material. The formation model proposed for this inclusion does not allow us to discriminate between the two genetic processes proposed for LDG, but sets a new minimum to the liquidus temperature of the corresponding chemical system of~2250°C.