Some petrographic anomalies with relations to the Silica strewnfield

Some petrographic Anomalies with relations to the Silca strewnfield

Norbert Brügge, Germany
Dipl.-Geol.
Upload: 26.03. 2019

1. Silica glass components in breccias from the Qaret-el-Hanash structure (Kimberlite pipe)

2. Magnetite, Mullite and Tridymite in an Silica melt rock

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

"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 (Al_4.2Fe³⁺_0.2Si_1.6O_9.8), 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."

Note: Mullite is formed in magmatic rocks as a decomposition product of sillimanite (sillimanite → mullite + SiO2). Sillimanite is a nesosilicate from the group of aluminosilicates and is considered as a indicator for the thermodynamic transformations of andalusite and kyanite in rocks of the amphibolite and granulite facies.

Tridymite segregation veinlets in the groundmass

Flickering flames over the Libyan Desert ?
John M. Saul -- International Geology Review (2018); DOI: 10.1080/00206814.2018.1512057

Mullite-magnetite rock
"A group of smoothly wind-sculpted medium-dark grey rocks were found partly buried in the sand approximately 160 km SSE of the LDG strewnfield (GPS coordinates 23�48.0ʹN, 26�11.5ʹE). Two blocks estimated to weigh 80 and 50 kg were found, as well as four blocks in the 10 kg range, plus some smaller fragments. A 14-kg block was found at a distance of about 10 m, but searches yielded no additional similar material nearby. No outcrop was identified. The largest rock was found at the centre of a 15-m ring of white rocks. The discovery was made on 1 February 2007 by Didier Bernon and Martine Linares (e-mail of Ren� Lorillard, 1 April 2017).
The rock is very compact, and although also finegrained and highly magnetic, with wind-sculpted concavities that are much larger and smoother, and an absence of voids. Exposed surfaces were slightly greenish.
In addition to magnetite, Fe3O4, the main constituents are SiO2 and mullite (3Al2O₃�2SiO₂ or 2Al2O₃�SiO₂), an uncommon refractory mineral with a melting point of 1840�C whose synthetic equivalent is used in the manufacture of crucibles and furnace linings. The matrix of the rock is of silica and includes silica glass and relict quartz grains, as well as tridymite, a high-temperature polymorph of SiO2. Nickel-bearing magnetite (trevorite, NiFe₂³⁺O₄) was also detected but not awaruite (Ni₂Fe to Ni₃Fe). Other phases, identified by Wolfgang Bach (e-mail, 15 October 2013), were phosphates of calcium, aluminium and rare earths, ilmenite, and hercynite. Bach described his samples as having a �quench texture�. Rocks with similar compositions have not been reported from elsewhere (see Ferri�re et al. (2009). Platinum group element (PGE) concentrations in the magnetite-mullite 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� (Ferri�re et al. 2009)".

3. Megascopic ferruginous Breccia METEORITE IMPACT EFFECTS IN THE LIBYAN GLASS AREA, SOUTHWESTERN EGYPT By Aly Abd Alla Barakat, -- Cairo 2004
"There is a very strange exposure of megascopic breccia observed during the expedition of November/December 1996, at latitude 25� 22�51″ N and longitude 25� 27� 34″ E. The exposure is of about 2-m high above the surrounding level. The exposure represents remnant of an eroded section. To the north, this rock has been replaced by ferruginous breccia and iron deposit. The petrographical and mineralogical studies carried out on the breccia indicate that it represents impact breccia. In addition to the brecciation of the quartz grains some of these grains show PFs and PDFs. The zircon of this breccia shows evidence of deformation as represented by the fragmented nature of some the studied grains and by non-stoichiometric contents of ZrO2 and SiO2". The detection of glass and graphite is accounted also for the impact origin of this breccia.


4. Magnetite spherules MAGNETIC COSMIC (?) SPHERULES FROM THE GREAT SAND SEA (WESTERN DESERT, EGYPT) A. M. Fioretti et al. -- 61st Annual Meteoritical Society Meeting, 5073.pdf "During a joint Italian-Egyptian expedition in the Western Desert (Egypt) dark "brush-strokes" fading were observed along the flanks of some sand dunes. Optical investigations showed that the dark dust is mainly composed of subrounded to well-rounded opaque grains of high sphericity, mixed with minor amounts of quartz and silica glass grains. Among them, small, absolutely perfect spheres, with a smooth, polished surface and a metallic lustre are clearly manifest. Small amounts of magnetic particles studied by reflected light and SEM; chemical compositions were obtained by WDS-EMPA. The rounded opaque minerals are mostly magnetite, ilmenite and hematite. The oxidised spherules are 100 to 200 microns long, with metal Fe-Ni eccentric cores 40 to 100 microns long. The external shells show typical dendritic structures. Spherules with very different core compositions are found (e.g. Fe80Ni20 - Fe5Ni95) within each of the six samples, whereas the oxides constituting the shells show more uniform, Ni-depleted, compositions." Comment: The spherules are certainly not micrometeorites, but aeolian or fluvial reprocessed material from subvolcanic hotspots with iron and magnetite deposits in the Great Sand Sea.

7. The unusual meteorite GSS-020

5420 grams of a supposed meteorite found in 2000. But only 396 grams of it were examined and classified as Chondrite (H). After studying of the published mineral chemistry, I come to the conclusion that the chondrite GSS-020 is none. The matrix is clearly a remelted mix of material from the Earth' mantle. The Fe-Mg silicate matrix with the mineral relics and metal inclusions are not a criterion to postulate an extra-terrestrial origin. Fe-Mg silicates occur in a mixed series (enstatite and ferrosilicate) and are for example part of peridotite.

GREAT SAND SEA 020: AN UNUSUAL H CHONDRITE MELT ROCK.
F. Brandstätter, D. D. Badyukov and G. Kurat -- 64th Annual Meteoritical Society Meeting (2001), 5199.pdf

"The GSS-020 chondrite is an unusual non-brecciated melt rock. On a <1 cm scale the melt portion is texturally homogenous and extends throughout the whole meteorite.
GSS-020 is a fine-grained rock composed of about 66 vol% melt rock matrix, 16 vol% relictic silicate objects, 9.5 vol% opaques (mainly mm to submm-sized metal/sulfide grains), and 8.5 vol% numerous homogeneously distributed voids (on average < 0.3 mm in size).
Microscopically, the major part of the silicate fraction is matrix material that clearly shows the texture of a quenched melt consisting of about 57 vol% Fe-Mg silicates and 9 vol% glass. Relictic olivine and low-Ca pyroxene (16 vol%) are present as isolated grains in the quenched melt matrix and as unmelted remnants in �ghost� chondrules. The latter have circular shapes and are composed of relictic olivine or/and pyroxene with interstitial melts.
The melt rock fraction consists of short-prismatic olivine with grain sizes <30 �m, <50 im-sized augite and interstitial glass. Large olivine grains in melt rocks are zoned with Fe increasing towards the surface. No shock effects are present in either olivines nor pyroxenes. Glasses in melt rock portions are inclusion-free but locally contain crystallites of Fe-Mg silicates. Relictic olivines and pyroxenes are both <0.3 mm in size and contain tiny troilite and metal blebs. In addition, features like planar fractures, mosaicism and mechanical twining indicate shock alteration of the relictic silicates.
Metal and troilite mainly occur as nodules (average size < 0.5 mm) of irregular to rounded shapes. Some troilite nodules include large droplets of metal consisting of Ni (kamacite and taenite). Micrometer - sized metal and troilite globules and particles are dispersed throughout the melt. Regions enriched in such globules resemble shock veins or melt pockets.
Relictic chromite occurs as fractured single grains with subhedral to anhedral shape, whereas newly formed chromite forms clusters of small euhedral to subhedral crystals."

26�01' N / 25�44' E

6. Magnetic Ferro-Basalt
Detected augite ferrobasalt at northeastern site auf Kemal-el-Din plateau It was found a porphyritic, fine-grained igneous rock dominated by phenocrysts of olivine (30 vol% pheno) and augite in a groundmass of augite, abundant magnetite, minor plagioclase and nepheline. Olivine phenocrysts form colourless, euhedral crystals up to 2 mm in size and are partly altered to red brown iddingsite. Augite phenocrysts are euhedral crystals up to 1 mm in size and often contain magnetite inclusions. The groundmass consists of subhedral brown augite, small laths of plagioclase and euhedral magnetite. Nepheline occurs as areas of nepheline. Specimen is a primitive basalt dominated by olivine and pyroxene with very little plagioclase. Minor amounts of nepheline indicate this is a silica undersaturated alkali basalt. Enough magnetite is present in the sample to make it highly magnetic. Rock Library, Imperial College London, Author: Matt Genge (2017)