Preliminary data on the geochemical characterisation and Cr isotope composition of Libyan Desert Glasses and ordinary chondrites

Libyan Desert Glasses (LDG) are an enigmatic type of natural silicate glasses, which occur in well-defined geographic area in the western part of the Great Sand Sea (between Egypt and Libya, North Africa). These materials have gained the attention of many researchers through the last century (early 1930s) but, despite the large number of published studies (> 80) committed to this argument, their origin (terrestrial vs. extra- terrestrial) is still debated. The hypothesis that links their origin to meteoritic impact seems the more likely due to the lack of correlation with the geochemical compositions of the local sand or sandstone analysed from various locations in the common region of recovery. On the other hand, there are some differences with respect to classical impact glasses (i.e., the occurrence of peculiar high-T mineral phases or the presence of an impact crater). According to several authors the strongest indication for the presence of a meteoritic component could be retained in the dark streaks of layers presented by some glass samples. Chromium isotopes systematic can be used to discriminate between terrestrial vs. extra-terrestrial material and to investigate meteorite characteristics. Cr isotope isotopic compositions (53Cr/52Cr and 54Cr/52Cr) can discriminate different meteorite type by the presence of the heterogeneously distributed 54Cr nucleosynthetic anomalies and similar 53Cr excess. Terrestrial rocks on the other hand are not expected to show any variation in Cr isotope composition. This systematic has thus the big advantage, compared to other geochemical systematics such as Os isotopes and PGE abundance, of being selective regarding the terrestrial vs. extraterrestrial origin of the Cr source, and also regarding the meteorite type. Indeed, this method is able to provide at the same time evidence for the presence of extraterrestrial component in impactites, and information about the type of projectiles. We present the preliminary compositional data of Cr isotopes and high-precision trace elements on six ordinary chondrites, which are classified but not yet fully characterised for their geochemical features, used to test the reproducibility of the analytical methods. Two samples of LDG were analysed along with the meteorite samples with the aim of shed light on the origin of these exotic materials. Would be this the solution to the enigma?