Northwest Africa 5717

    Western Sahara

    Find: 2008

    Chondrite (ungrouped, 3.05 subtype)

History and physical characteristics: In late 2006, just NW of Mahbas, Western Sahara, a single fresh stone was recovered that weighed 7.310 kg.  Blanketed in fusion crust and showing only the slight spots of oxidation, there is one small fractured face where fusion crust is absent and the internal structure of a possible primitive sub-type is visible. The specimen was acquired in its entirely by a Moroccan meteorite and fossil dealer in Erfoud in late 2008, from whom it was purchased by Darryl Pitt in December of the same year.

Petrography: (T. Bunch and J. Wittke, NAU; A. Irving UWS): an assemblage of two unique chondrule lithologies: Lithology A, the apparent host, consists of dark, tightly packed chondrules that are surrounded by a sparse, cataclastic matrix composed of abraded chondrule rim material (matrix = 8 vol. %). Many of the chondrules have a thin rim of metal, sulfide, and fine-grained silicates. Chondrules are typically sub-round to sub-angular and range in size from 0.04 to 4.1 mm, mean = 0.455 mm. Matrix = 8 vol. % and metal, sulfides and metal oxides = 4 vol. %.  Lithology B occurs as lumpy to swirling arrays of white to gray Mg-rich chondrules that range in size from 0.05 to 3.6 mm together with a prominent subset of microchondrules (5 vol. %) 0.012 to 0.050 mm in diameter. A few large (4 to 8.4 mm), irregular-shaped chondrule-like objects are present and consist mostly of enstatite. Strong compositional zoning is common in chondrule olivine and pyroxenes, which are set in a mesostasis of clear, optically isotropic glasses and quench crystals. Metal and sulfides occur as sub-rounded aggregates in the matrix (6 vol.%).

Geochemistry: Lithology A: Olivine cores, Fa_{9.7 – 20.8} (FeO/MnO mean = 40); rims, Fa_{16.2 – 35.6} (FeO/MnO mean = 36), ferroan olivine Cr₂O₃ = 0.23 - 0. 91 wt %, mean = 0.53.  Lithology B: Olivine cores, Fa _{0.03 – 3.4}; Cr₂O₃ in olivine, 0.24 – 0.84 wt %, mean = 61. Olivine rims, Fa _{17.2 – 27.5}; Cr₂O₃ mean = 0.28 wt %.  Ferroan olivine (Fa >8) Cr₂O₃ = 0.28– 0.78, mean = 0.47 wt %. Mg-rich cores in both lithologies show higher Cr₂O₃ contents relative to FeO rich rims. Chondrule enstatite is Fa _{0.04 – 5.8}Wo _0.7. Average composition of mesostasis glass is (in wt %, N = 6): Na₂O = 4.65; MgO = 1.45; Al₂O₃ = 12.8; SiO₂, 71.3; P₂O5 = 0.45; K₂O = 1.23; CaO = 1.38; TiO₂ = 0.47; Cr₂O₃ = 0.04 and FeO = 5.7. Oxygen isotopes (D. Rumble, CIW): Analyses of two acid-washed whole rock samples by laser fluorination gave, respectively:  lithology A, δ¹⁸O  = 4.17 and 4.47; δ¹⁷O =2.73 and 2.90; Δ¹⁷O = 0.539 and +0.554 (all ‰); lithology B, δ¹⁸O = 3.15 and 3.68; δ¹⁷O = 1.72 and 2.01; Δ¹⁷O = +0.061 and +0.080.

Classification:.Chondrite (ungrouped, 3.05 subtype). Although the oxygen isotopic compositions for lithology A are similar to those of H chondrites, the very low metal content together with other characteristics are inconsistent with an H classification. Lithology B oxygen isotopic compositions are dissimilar from those of all other meteorites.  The Δ¹⁷O values of lithology B are close to those of most enstatite-rich meteorites, but the mafic minerals are significantly more ferroan.  Olivine Fa range and Cr₂O₃ contents of both lithologies are consistent with a subtype 3.05.The weathering grade is W1/2 and the shock level is S2.