What tell us the detection of Silica and Tridymite in basalt-boulders at the Gilf Kebir, Egypt
about the origin of the Libyan Desert Glass (LDG) ?

Norbert Brügge, Germany
Dipl.-Geol.

Petrographic and Geochemical Study of an anomalous Melt Rock from the Gilf Kebir Plateau, close to the Libyan Desert Glass Area, Egypt
L. Ferričre, B. Devouard, S. Goderis, P. Vincent, D. Bernon, R. Lorillard, and J. M. Saul.
72nd Annual Meteoritical Society Meeting (2009) 5384.pdf

"Four of the authors collected unusual samples from a meter-sized boulder found among tabular formations of sandstone in the north of the Gilf Kebir plateau, about 160 km SSE of the Libyan desert glass (LDG) area.
The rock is dark gray in color, very compact (i.e., no vesicles), and strongly magnetic. Whole rock analyses are as follow (in wt%): SiO2 40.5; Al2O3 30.8, Fe2O3 22.7, TiO2 4.4, with remarkably low alkali (Na2O+K2O = 0.1 wt%) and CaO (0.1 wt%) contents. In thin section, the rock displays a magmatic aphanitic-microlitic texture with microlites of mullite (Al4.2Fe3+0.2Si1.6O9.8), an Fe-rich spinel close to magnetite, and an Fe-Ti oxide, in a mesostasis of nearly pure Silica. Tridymite has been identified in the mesostasis, as well as in segregation veinlets. Rounded, relict quartz grains also occur and are surrounded by tridymite. Platinum group element (PGE) concentrations in the melt rock are within the range of the average continental crust with iridium as an exception, slightly more elevated. These PGE patterns are similar to those measured for LDG.
This melt rock has a composition and mineralogy that is, to our knowledge, unreported from any terrestrial magmatic rock. Phase diagrams in the system SiO2-Al2O3-FeO-Fe2O3 suggest that temperature about 1600 °C is needed to melt a rock with such a composition. Considering the location of recovery, and the lack of vesicles, an anthropogenic origin is highly unlikely. The PGE signature of the melt rock is neither diagnostic of an impact origin, nor the contrary, as is also the case for the PGEs in LDG. Although the chemical composition of the rock and the high-temperature involved in its formation eem to be pointing to an impact origin, no high pressure phases ave so far been detected."

        

Basalt

Silica



    

 Tridymite segregation veinlets in the basalt

 

My comment: The actual sensation is not the unusual composition of the ground-mass of the boulders, but the detection of involved amorphous dark silica and high temperature quartz in veinlets of the boulders. The high contents of Al2O3, Fe2O3 and TiO2 as well as the very low content of Na2O + K2O is indeed exceptional for a basalt and can only indicate the exceptional origin from great depths along my proposed main-fault along the Howar-Uweinat uplift structure.
This also provides the definitive proof that the LDG, in the silica area, 130 km further north, is also of endogenous origin. All further discussions about an exaterrestrial event are now finished.

Thanks to John M. Saul for sending the LPI 48 presentation and coordinates of the finds to me: 23°48'N/26°11.5'E