F. Fröhlich^1,*, G. Poupeau^1,2, A. Badou¹, F. X. Le Bourdonnec², Y. Sacquin³, S. Dubernet², J. M. Bardintzeff^4,5, M. Véran⁶, D. C. Smith⁷, E. Diemer^†

¹Département de Préhistoire, Muséum National d’Histoire Naturelle, UMR CNRS, Paris, France
²Université de Bordeaux 3, CRP2A – UMR CNRS IRAMAT 5060, Pessac, France
³CEA/Saclay, DSM/Irfu, Gif-sur-Yvette Cedex, France
⁴Laboratoire de Pétrographie-Volcanologie/équipe Planétologie, Université Paris-Sud, UMR CNRS IDES 8148, Orsay Cédex, France
⁵Université de Cergy-Pontoise, IUFM, Cergy-Pontoise, France
⁶Département Histoire de la Terre, Muséum National d’Histoire Naturelle, USM 203, Paris, France
⁷Département Histoire de la Terre, Muséum National d’Histoire Naturelle, UMR CNRS 7202, Paris, France
^† Deceased

Results are presented of new geological observations and laboratory analyses on Libyan Desert Glass (LDG), a unique kind of impact glass found in Egypt, probably 28.5–29.4 million years in age. A new LDG occurrence has been discovered some 50 km southward of the main LDG occurrences in the Great Sand Sea. From Fourier transform infrared (FTIR) analysis, the molecular structure of LDG is refined and significant differences are shown between LDG specimens and other pure silica glasses (fulgurite, industrial fused quartz, and amorphous biogenic silica) that are related to differences in their structures. The slight variations observed here for the mean Si-O-Si angle between the different glasses are attributed to their thermal histories. With regard to the other glasses analyzed, the LDG infrared spectral parameters point to a higher ratio of discontinuities and defects in the tetrahedral (SiO₄) network. The quantitative mineralogical constitutions of sandstones and quartzites from the LDG geological setting were analyzed by FTIR. Cretaceous sandstones have a specific composition (about 90 wt% quartz, 10% dickite), clearly different from the Paleozoic ones (about 90 wt% quartz, but ≥7% kaolinite). It is shown that the reddish silts bearing the LDG are constituted mainly of microquartz enriched with dickite, whose particle size distribution is characteristic of fluvio-lacustrine deposits, probably Oligocene to Miocene in age. The target rocks, most probably quartz sand, resulted from the weathering (loss of the cementing microquartz) of the Cretaceous sandstones from the Gilf Khebir Plateau with deposition in a high-energy environment.

Reference
Fröhlich F, Poupeau G, Badou A, Le Bourdonnec FX, Sacquin Y, Dubernet S, Bardintzeff JM, Véran M, Smith DC and Diemer E (in press) Libyan Desert Glass: New field and Fourier transform infrared data. Meteoritics & Planetary Science 
[doi:10.1111/maps.12223]
Published by arrangement with John Wiley & Sons

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Agnese Fazio^1,*, Massimo D’Orazio¹, Luigi Folco¹, Jérôme Gattacceca², Corinne Sonzogni²

¹Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy
²CNRS-IRD Aix-Marseille Université, Centre Européen de Recherche et d’Enseignement de Géosciences de l’Environnement (CEREGE), UM34, Aix-en-Provence, France

Northwest Africa (NWA) 6583 is a silicate-bearing iron meteorite with Ni = 18 wt%. The oxygen isotope composition of the silicates (∆′¹⁷O = −0.439 ‰) indicates a genetic link with the IAB-complex. Other chemical, mineralogical, and textural features of NWA 6583 are consistent with classification as a new member of the IAB-complex. However, some unique features, e.g., the low Au content (1.13 μg g⁻¹) and the extremely reducing conditions of formation (approximately −3.5 ∆IW), distinguish NWA 6583 from the known IAB-complex irons and extend the properties of this group of meteorites. The chemical and textural features of NWA 6583 can be ascribed to a genesis by impact melting on a parent body of chondritic composition. This model is also consistent with one of the most recent models for the genesis of the IAB-complex. Northwest Africa 6583 provides a further example of the wide lithological and mineralogical variety that impact melting could produce on the surface of a single asteroid, especially if characterized by an important compositional heterogeneity in space and time like a regolith.

Reference
Fazio A, D’Orazio M, Folco L, Gattacceca J and Sonzogni C (in press) The extremely reduced silicate-bearing iron meteorite Northwest Africa 6583: Implications on the variety of the impact melt rocks of the IAB-complex parent body. Meteoritics & Planetary Science 
[doi:10.1111/maps.12231]
Published by arrangement with John Wiley & Sons

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Gabriele Giuli^1,*, Maria Rita Cicconi¹, Sigrid Griet Eeckhout², Christian Koeberl³, Billy P. Glass⁴, Giovanni Pratesi⁵, Mariangela Cestelli-Guidi⁶ and Eleonora Paris¹

¹School of Science and Technology, Geology Division, University of Camerino, Via Gentile III da Varano, 62032, Italy
²European Synchrotron Radiation Facility (ESRF), 6 rue Jules Horowitz, 38043 Grenoble, France
³Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria; and Natural History Museum, Burgring 7, A-1010 Vienna, Austria
⁴Department of Geological Sciences, University of Delaware, Newark, Delaware 19716, U.S.A.
⁵Dipartimento di Scienze della Terra, Università di Firenze, Via G. La Pira 4, 50121, Firenze, Italy
⁶Laboratori Nazionali Frascati, Istituto Nazionale Fisica Nucleare, Via Enrico Fermi, Frascati, Italy

Iron oxidation states and coordination numbers have been determined by micro-X-ray absorption near edge spectroscopy (XANES) on the cores of a large group of microtektites from the Australasian, Ivory Coast, and North American (NA) tektite strewn field. The North American microtektites used in this study have been collected from five sites at different distances from the source crater; most have SiO₂ content between 70 and 80 wt%. Accurate analysis of the pre-edge peak energy position and integrated area allowed determination of Fe³⁺/(Fe²⁺+Fe³⁺) ratios on all samples with an estimated error of ±0.05.
Microtektites from the Australasian and Ivory Coast strewn fields show low values of the Fe³⁺/(Fe²⁺+Fe³⁺) ratios, in fair agreement with tektites from the same strewn field. In contrast, microtektites from the North American strewn fields show a wide range of Fe³⁺/(Fe²⁺+Fe³⁺) ratios from 0.02 to ca. 0.61. Comparison of Fe oxidation state data with chemical composition do not show any relation between Fe³⁺/(Fe²⁺+Fe³⁺) ratios and Na, Ca, or K contents, thus suggesting that the high-Fe oxidation states are not the consequence of sea-water alteration.
The difference between the Fe oxidation state of tektites and microtektites from the North American strewn fields suggests that some factors in the formation of the North American microtektites were different than for the North American tektites and for microtektites in the other strewn fields.
Previous Fe oxidation state data on NA tektites strongly suggest that the wide range in Fe oxidation state we found on NA microtektites is not related to lateral heterogeneity of the target rocks. Despite a correlation between microtektite oxidation state and distance from the source crater, we maintain that Fe oxidation state is not related only to the microtektite droplet flight distance. This is in keeping with the fact that no significant variations in the Fe oxidation state have been found in microtektites from the Australasian strewn field, even for Australasian microtektites recovered in Antarctica. The Fe oxidation state in North American microtektites could be explained by interaction of melt droplets with a H₂O-rich vapor plumes generated during the impact. These data point out that some difference must exist between the thermal histories of microtektites and tektites from the NA strewn field. Moreover, microtektites from the NA strewn field show also distinctively higher oxidation states than those from Ivory Coast or the Australasian strewn fields.

Reference
Giuli G, Cicconi MR, Eeckhout SG, Koeberl C, Glass BP, Pratesi G, Cestelli-Guidi M and Paris E (2013) North American microtektites are more oxidized than tektites. American Mineralogist 98:1930-1937.
[doi:10.2138/am.2013.4505]
Copyright: The Mineralogical Society of America

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