Strontium and neodymium isotopic study of Libyan Desert Glass: Inherited Pan-African age signatures and new evidence for target material

Abstract

Libyan Desert Glass (LDG) is an impact-related, natural glass of still unknown target material. We have determined Rb-Sr and Sm-Nd isotopic ratios from seven LDG samples and five associated sandstones from the LDG strewn field in the Great Sand Sea, western Egypt. Planar deformation features were recently detected in quartz from these sandstones. 87Sr/86Sr ratios and e-Nd values for LDG range between 0.71219 and 0.71344, and between -16.6 and -17.8, respectively, and hence are distinct from the less radiogenic 87Sr/86Sr ratios of 0.70910-0.71053 and e-Nd values from -6.9 to -9.6 for the local sandstones from the LDG strewn field. Previously published isotopic ratios from the Libyan BP and Oasis crater sandstones are generally incompatible with our LDG values. LDG formation undoubtedly occurred at 29 Ma, but neither the Rb-Sr nor the Sm-Nd isotopic system were rehomogenised during the impact event, as we can deduce from Pan-African ages of ~540 Ma determined from the regression lines from a total of 14 LDG samples from this work and the literature. Together with similar Sr and Nd isotopic values for LDG and granitoid rocks from northeast Africa west of the Nile, these findings point to a sandy matrix target material for the LDG derived from a Precambrian crystalline basement, ruling out the Cretaceous sandstones of the former "Nubian Group" as possible precursors for LDG.