Brownish inclusions and dark streaks in Libyan Desert Glass: Evidence for high-temperature melting of the target rock

Abstract

Abstract-Dark streaks and different types of inclusions in Libyan Desert Glass (LDG) collected from the LDG strewn field in Egypt were investigated. Rare transparent spherules enclosed in the glassy matrix are characterized by concentric cracks, irregular internal cracks, intense twinning, and considerable amounts of Ti and Al. Raman spectra show that the spherules are a-cristobalite. Their occurrence together with lechatelierite indicates quick heating of the source rock to at least 1550°C, followed by rapid quenching leading to crystallization of b-cristobalite, which upon cooling inverted into a-cristobalite. Brownish inclusions are irregularly shaped, elongated objects with smooth contacts to the surrounding glass. They contain small roundish to elliptical droplets, and a few larger angular grains, which compositionally and according to their Raman spectra most closely resemble low-Ca, Al-rich orthopyroxene. Composition and texture of the orthopyroxene suggest that the brownish inclusions formed by incomplete melting of an Al-rich orthopyroxene bearing precursor, e.g., mafic phases present in desert surface sands or also of orthopyroxenebearing granulite dykes in the LDG target. Experimental data on Ca-poor enstatite also support that the inclusions were heated to about 1550°C. Analyses of dark streaks in LDG reveal high abundances of Al, Ti, Mn, Cr, Fe, and Ni and a pronounced correlation between the abundances of Cr, Mn, Fe, and Ni. As the Fe ⁄ Ni, Mn ⁄ Ni, and Cr ⁄ Ni ratios are all clearly nonchondritic, the source of this material is most likely terrestrial and the dark streaks studied here represent a different type of schlieren compared to those which contain a meteoritic component. These findings suggest LDG formation during a short high-temperature event. Melting of Al-rich orthopyroxene bearing target material seems to suggest an asteroid impact rather than a near-surface airburst.

Libyan Desert Glass: New field and Fourier transform infrared data

“…Most of these observations correspond with the first descriptions by Clayton and Spencer (), Spencer (), and Greshake et al. (); the last paper identified some trace elements in the colored zones (“schlieren”) deduced to be of terrestrial origin. According to Rocchia et al.…”

“…, ; Koeberl ; Bölitz and Langenhorst ; Greshake et al. ). The high content of meteoritic components (Rocchia et al.…”

“…, ; Greshake et al. ) and the presence of the high‐temperature low‐pressure silica phase cristobalite in the glass (e.g., Clayton and Spencer ; Spencer ; Barnes and Underwood ) argue strongly in favor of an impact origin. Moreover, the occurrence in LDG of baddeleyite (ZrO 2 ) (Kleinmann ; Storzer and Koeberl ), a product of the breakdown of zircon (ZrSiO 4 ) that occurs at a temperature above 1680 °C, clearly demonstrates that very high temperature conditions were attained.…”

Libyan Desert Glass area in western Egypt: Shocked quartz in bedrock points to a possible deeply eroded impact structure in the region

“…Greshake et al. () studied a variety of dark streaks that represent a different type of schlieren compared to those which contain a meteoritic component (on the hand specimen scale, there is a slight difference in color). Their findings also suggest that LDG formed during a short high‐temperature event, as shown by melting of aluminum‐rich orthopyroxene‐bearing target material, which then formed the dark schlieren.…”

The Geochemistry and Cosmochemistry of Impacts

“…Although the origin of LDG is still debated by some workers, an origin by impact seems most likely. Greshake et al (2010) studied a variety of dark streaks that represent a different type of schlieren compared to those which contain a meteoritic component (on the hand-specimen scale, there is a slight difference in color). Evidence for an impact origin includes the presence of schlieren and partly digested mineral phases, lechatelierite (a high-temperature mineral melt of quartz), and baddeleyite, a high-temperature breakdown product of zircon.…”