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Dar al Gani 896 meteorite (DaG 896), Dar al Gani, Al Jufrah District, Libya

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Latitude & Longitude (WGS84): 27° 45' 0'' North , 16° 52' 60'' East
Latitude & Longitude (decimal): 27.75, 16.8833333333
 
Ordinary Chondrite, impact melt (H-imp melt; S3; W2)
Found,2000; 22.6 g, single stone

A single dark and moderately weathered meteoritic fragment was recovered by an anonymous finder during a meteorite recovery mission. A cut specimen immediately reveals dark shock veins within an otherwise nearly uniform grey-green lithology. The stone's microporphyritic textures reveal abundant small euhedral olivine crystals within a feldspathic glass and pigeonite groundmass. Fe-Ni metal and sulfides are quite sparse. An assortae of small relic grains are also present, but the overall petrography of the meteorite appears to be that of a silicate-rich achondrite dominated by olivine, especially, accompanied by pyroxene and glass of plagioclase composition. An indeed, there is very little iron — much less than we would expect for the low and very low iron L and LL ordinary chondrites — let alone that of an H (high iron chondrite). However, appearances can be deceiving. The olivine and pyroxene compositions as well as the critical oxygen isotope and Fe/Mn ratios are those of H chondrites. The textures and very diversely populated relic grains help us to fill in some of the details. H chondritic material was shock melted and then subsequently lost most of its metal, sulfides, and other volatile components. The remaining material cooled so quickly, however, that extensive differentiation did not occur. In the language of meteoritics, the child of an H chondrite is still an H chondrite — even if it is no longer relatively 'high' in overall iron content. Evidence of shock is extensive (glass, blackening, mosaicism in olivine, planar fracture, melt veins, relic grains). However, the actual shock level (S3) is only moderately high for the meteorite as a whole. Presumably, the decisive shock-melting event had been followed by equilibrating and annealing under a hot debris blanket. Subsequent shocks evident in the reconstituted mass would be less dramatic. Weathering (level W2) is usually moderate — most obvious in the usual limonite staining and in some carbonate veins. However, portions of the meteorite in immediate contact with the soil appear to have suffered additional alterations.

The minor components of the ground mass (augite, chromite, sulfides, opaques, phosphates) retain a (partially decipherable) record of events before and after the decisive shock-melting event. This reconstruction of events is further supported by a 3.7 Ga degassing event [Ar-Ar date] following the ancient (~4.5 Ga) original original planet body aggregation. Another ~2 Ga degassing event [He loss] is also reported. In time, a final fragmentation event ~20 Ma ago (i.e., the cosmic ray exposure age) ejected the DaG 896 meteoroid into its eventual earth-crossing orbit and fall.

Mineral List



16 entries listed. 7 valid minerals.

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References

Russell, S.S. et al. (2003). The Meteoritical Bulletin, No. 87, July 2003. Meteoritics & Planetary Science 38: (7, Suppl.): A189-A248. (July 2003).

Patzer, A., Schultz, L. & Franke, L. (2003) New noble gas data of primitive and differentiated achondrites including Northwest Africa 011 and Tafassasset: Meteoritics & Planetary Science 38(10): 1485-1497. (Oct 2003).

Folco, L. et al. (2004) Extensive impact melting on the H-chondrite parent asteroid during the cataclysmic bombardment of the early solar system: Evidence from the achondritic meteorite Dar al Gani 896. Geochimica et Cosmochimica Acta 68 (10): 2379-2397. (May 2004).

Folco, L., Burbine, T. H. & D'Orazio, M. (2005) Surface mineralogy changes induced by impact melting on ordinary chondritic parent asteroids: clues from the DaG 896 meteorite. Memorie della Società Astronomica Italiana Supplement, vol 6:116-121. (2005).

Grady, M.M., Pratesi, G. & Moggi-Cecchi, V. (2015) Atlas of Meteorites. Cambridge University Press: Cambridge, United Kingdom. 373 pp.

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