^1,2Niccolò Magnani,^2,3Enrico Mugnaioli,²Sofia Lorenzon,⁴Lidia Pittarello,⁵Tatiana E. Gorelik,^2,3Matteo Masotta,^2,3Luigi Folco
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70094]
¹Dipartimento di Scienze dell’Ambiente e della Terra, Universita di Milano-Bicocca, Milan, Italy
²Dipartimento di Scienze della Terra, Universita di Pisa, Pisa, Italy
³Centre for Instrument Sharing of the University of Pisa, Pisa, Italy
⁴Mineralogisch-Petrographische Abteilung, Naturhistorisches Museum, Vienna, Austria
⁵Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Juelich, Juelich, Germany
Published by arrangemetn with John Wiley & Sons

Libyan Desert Glass (LDG) is an ~29 million years old, silica-rich glass found inWestern Egypt. Whether this glass formed in an impact cratering context associated withthe hypervelocity collision of a cometary/asteroidal body or radiative heating during anairburst is debated. Determination of the formation temperatures and pressures of raremineral components in LDG can provide key petrogenetic constraints on its origin. Here,we report the occurrence of a zircon inclusion, whose textural, chemical, andcrystallographic features point to a rapid formation during solidification of the silica-richLDG melt. The study was conducted combining dual beam microscopy, transmissionelectron microscopy, energy-dispersive X-ray spectroscopy, and three-dimensional electrondiffraction. The inclusion is a few tens of micrometer in size and consists of dendriticbranches of zircon arranged in a reticulate-cruciform texture. The high-silica glass fillinginterstices between dendrites have longer chemical bonds compared to matrix glass, asindicated by electron pair distribution function analysis, and is enriched in Al 2 O 3 . The lackof incongruent melt products (ZrO 2 , SiO 2 ) suggests that the inclusion formed during coolingfrom supraliquidus conditions, by dynamic crystallization from an (immiscible) undercooledliquid droplet. Such droplet would derive from shock-induced melting of a precursor zircongrain, possibly mixed with the SiO 2 -rich liquid formed by melting of the LDG precursormaterial. The formation model proposed for this inclusion does not allow us to discriminatebetween the two genetic processes proposed for LDG, but sets a new minimum to theliquidus temperature of the corresponding chemical system of ~2250°C.