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

Abstract

Strontium isotopes and selected trace elements (Rb, Sr, REE, Zr, Hf, Th, and U) were measured on samples of Libyan Desert Glass (LDG) and a series of terrestrial materials (rocks, LDG-bearing soils, eolic sand) collected over a large area of southwestern Egypt to identify the LDG terrestrial parent material and the site where impact melting occurred. Samples include Upper Cretaceous hypersilicic sandstones outcropping at or near the LDG strewn field and Lower Cretaceous to Silurian sandstones from the Gilf Kebir Plateau highlands. Strontium isotopes and partially Zr, Hf, Th, and U, possibly reflecting the composition of detrital zircon grains, are effective indicators of the geochemical affinity between terrestrial materials and LDG, unlike Rb, Sr, and REE abundances. The best geochemical affinity with LDG was found in LDG-bearing soils collected at the base of intradunal corridors in the Great Sand Sea. Remarkably, abundances of the Zr group elements of the LDG Zr-bearing phase are distinct from all terrestrial detrital zircons from the area. We suggest a mixture of weathering products from sandstones of different ages, including Devonian and Silurian rocks from the Gilf Kebir highlands, as the most likely source for LDG. A loose sedimentary formation exposed 29 Ma ago at the Earth's surface, superimposed over hard bedrock, might have been the true terrestrial target of the impact, but because of its incoherent nature, it was rapidly destroyed, explaining the complete absence of any evidence of an impact structure.

Origin of β-cristobalite in Libyan Desert Glass: The hottest naturally occurring silica polymorph?

“…Experimental data for ßcristobalite formation in vitreous silica represent reasonable analogues for cristobalite formation in LDG, as both the kinetics (rapid melting) and composition (pure silica) appear applicable to LDG. The model presented is consistent with a high-temperature impact origin of LDG (e.g., Kleinmann, 1969;Koeberl, 1997;Greshake et al, 2018;Koeberl and Ferrière, 2019;Cavosie and Koeberl, 2019;Sighinolfi et al, 2020). Our study is the first to quantify a multi-stage growth history involving ß-cristobalite from any environment.…”

“…Annealing experiments on silica glass powder (5-25 μm spheres) found no reduction in porosity from 1200 to 1350 °C; however, above 1550 C°, the spheres sintered into non-porous glass in 5 to 10 minutes (Breneman and Halloran, 2014). This observation may have significance on minimum temperature and heating duration if the source was porous (e.g., Sighinolfi et al, 2020). The LDG contains ubiquitous layering visible in CL and element distribution maps (Gucsik et al, 2004, Greshake et al, 2010 this study); the layering likely reflects mineralogical heterogeneity of the precursor.…”

Machine learning-based Sr isoscape of southern Sardinia: A tool for bio-geographic studies at the Phoenician-Punic site of Nora

“…The 87 Sr/ 86 Sr ratio of the samples was determined using a Neptune MC-ICPMS (Thermo Fisher Scientific) housed at the Centro Interdipartimentale Grandi Strumenti (CIGS) of the University of Modena & Reggio Emilia, as described in [91][92][93][94]. Sr solutions were diluted to *50 ppb and introduced through an APEX desolvating system.…”