Petrographic and Geochemical Study of an anomalous
Melt Rock from the Gilf Kebir Plateau, close to the Libyan Desert Glass
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
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
segregation veinlets in the basalt