Northwest Africa 869 meteorite, Northwest Africa Meteoritesi

Regional Level Types
Northwest Africa 869 meteorite	Meteorite Fall Location
Northwest Africa Meteorites	Group of Meteorite Fall Locations

Ordinary L chondrite, brecciated (L3-6, br.; S3; W1)
Find, 2000; 2-7 tons

A large number of distinctive meteorite stones ranging in size from >20 kg to < 1g from a large strewn field of undisclosed location have been sold under the name NWA 869. Surfaces with occasional remnant fusion crust possess an unusual and characteristic deep polish produced by millennia of wind erosion. Most samples are individual 'intact' stones, but fragments are also found. The predominant lithology is that of a shocked (S3), but loosely consolidated chondritic regolith breccia. Fine-grained matrix (~75 vol%) and a mixture of unequilibrated and equilibrated clasts (20 vol%) account for most of the meteorite. Shock-darkened clasts [shock levels >3] and a few shock-melted and/or exotic inclusions account for the rest. The matrix olivine (Fa10-49) and Ca-poor pyroxenes (Fs7-24) are unequilibrated. Individual clasts (up to 5.5 mm) may or may not be equilibrated. However, the overall range in composition for clasts is even greater than for the matrix with occasional grains of pure forsterite or even enstatite present. Other silicates are present with both albitic and anorthitic plagioclase present as minor or accessory phases both in matrix and in chondrules. Silica and K-bearing feldspar are present in a few odd clasts and inclusions. Fe-Ni metal and sulfides are found mostly as individual grains (kamacite, taenite, Fe-sulfide) and/or as intergrowths within matrix. A few exotic inclusions (an HED clast, silica-rich material, etc.) have also been identified. The terrestrial age (Be-10,C-14) is ~4-5 ka. Indeed, for a few larger pieces terrestrial ages and cosmic ray exposure (CRE) ages offer additional confirmation of identity for petrologically recognized or suspected NWA 869 fragments.

The cosmic ray exposure age (CRE) is a somewhat inconsistent 5 Ma, suggesting that during the last few million years before impacting the earth, the preterrestrial meteoroid suffered one or more strong impacts during the last epoch of its journey to earth (i.e., during the last 50,000 years). Even more intriguing are a number of very old Ar-Ar ages (~4.4; 2.2 ; 1.8 Ga) which suggest that NWA 869 escaped the 470 Ma (shock!?) event which affected many L-chondrites.

NWA 869 is the most massive unequilibrated ordinary chondrite listed at the Meteoritical Bulletin Database. While its confirmed mass is listed at 2 tons, the total number of unrecorded and unpaired fragments suggest a much greater mass. Its total mass might even exceed that of Jilin, the four ton H5 fall of 1976, currently listed as the most massive ordinary chondrite. NWA 869's unusual classification of L3-6 results from the great range in equilibration of it individual clasts. There are a total of 37 of L3-6 chondrites listed at the Meteoritical Bulletin Database (Aug 2015 update). [Some could be unpaired meteorites.] NWA 2239, a 100 kg mass, is the only other truly large L3-6 chondrite while Dubrovnik is the only witnessed L3-6 fall (1951).

NOTE: it has been suggested that the "Mercantour meteorite", said to be from France, is identical to NWA 869 (Rochette et al., 2017; Ludovic Ferrière, pers. comm. to Uwe Kolitsch).

References