Libyan Desert Glass (LDG)

"The Libyan Desert Glass contains many zircon grains. While most reacted to zirconia due to the heat, about 10% preserve evidence of former reidite. But the thing is, reidite is no longer present.
It was examined zircon grains from seven samples and the critical crystal orientation evidence of former reidite was present in each sample.....The reidite fingerprint points to a meteor impact as the only option."

"Multiple zircon grains were found in each sample of LDG analyzed, and a total of 101 grains were documented. The majority of grains (65%) fully dissociated to zirconia (now baddeleyite) and are no longer zircon. The second-largest population of grains (24%) contain ubiquitous zirconia inclusions, and are interpreted to have fully dissociated to zirconia, and then partially or fully back-reacted with silica melt to form a new generation of zircon; the zirconia inclusions are preserved because they were isolated from further reaction with silica melt by newly formed zircon. As the aforementioned grains do not record information about their predissociation history, they are not discussed further; representative BSE images of such grains are shown in the Data Repository. Only 11 of the 101 zircon grains (11%) showed no evidence of dissociation to zirconia, or in a few cases, only the margins of grains had dissociated. The 11 grains are composed of micrometer-sized neoblasts, and are the focus of this study. Orientation analysis by EBSD was done on eight of the 11 granular grains (three were too small or inaccessible). In five of the grains, neoblast orientations are broadly dispersed, with minor variations, and they do not form discrete orientation clusters. In contrast, neoblasts in the other three grains occur in multiple orientation clusters with systematic relations. An example of a grain with systematic orientation clusters is shown in Figure 2. The grain has a granular zircon core consisting of 0.5–1.0 µm neoblasts, and a few zirconia inclusions located along the margin and/or along partings, both adjacent to glass (i.e., former melt) (Fig. 2A). The core is surrounded by a semi-detached rim that consists of ~0.5 µm zirconia grains (baddeleyite) that are enveloped by thin neoblastic overgrowths of zircon (Fig. 2A, inset). Orientation mapping reveals that the entirety of the grain consists of many distinct orientation clusters (Fig. 2B). However, zircon neoblasts comprising the granular core consist of only three mutually orthogonal orientation clusters that are highly systematic; each orientation cluster can be related to another cluster by 90°/<110> (Fig. 2C). High-angle (75°–95°) misorientation axes defined by adjacent neoblasts in the core are also highly systematic (Fig. 2D), and coincide with each of the three <110> clusters (Fig. 2C). In contrast, zircon in the rim consists of >15 orientation clusters, most of which do not preserve a systematic relation to each other or to neoblast clusters in the granular core."

My Conclusion

What an interesting find. No impact or airburst is indicated. It is further evidence of the LDG's volcanic origin. Due to the occurrence of tridymite and cristobalite in the LDG, a last melting temperature of 1700 ° C with already low pressure load is proven. The presence of fingerprints of a former reidite in the LDG is not a puzzling.

It is proposed that the reidite or recrystallized zircon in the LDG are foreign inclusions that come from the Archean Basement of the East-Sahara-Ghost-Craton (ESGC).
 This is also indicated by inclusions of other elements that are enriched as dark streaks in the glass or in dark specimens.

If that is true, the original Reidite are very old (Archean) and they have almost completely re-crystallized to zircon again due to the high temperatures in the LDG.

"Granular zircon from Libyan Desert Glass that preserves evidence of former reidite.
A: Backscattered electron (BSE) image showing granular core (neoblastic zircon 1, nz1) surrounded by partially detached corona (rim) consisting of zirconia (baddeleyite) intergrown with a later generation of neoblastic zircon (nz2).
B: Orientation map shown with inverse pole figure (IPFy) color scheme.
C: Pole figures showing data for granular zircon core (see oval in B). D: Plot showing high-angle (75°-95°) misorientation axes for data in C. Stereonets are equal-area, lowerhemisphere projections in sample x-y-z reference frame."

Source: Cavosie & Koeberl


Overestimation of threat from 100 Mt–class airbursts? High-pressure evidence from zircon in Libyan Desert Glass
A. J .Cavosie & Ch. Koeberl -- The Geological Society of America, Geology 47, p.609-612, (2019) ; DOI: 10.1130/G45974.1

Evidences of former Reidite in granular Zircon from Libyan Desert Glass
A. J. Cavosie & Ch. Koeberl -- 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 1233.pdf