Northwest Africa 1685 (NWA 1685)

(Northwest Africa)

Purchased: 2002

Classification: Ordinary chondrite (LL4)

History: Purchased by D. Bessey in 2002 from Morocco in two lots. Fragments were sold under the name "BL" to a number of collectors.

Physical characteristics: Total known mass >11.56 kg in multiple pieces. Individuals have a characteristic glossy, desert-polished black fusion crust. Broken and cut surfaces often show a breccia with angular igneous clasts to 2 cm set in a chondritic matrix (igneous clasts ~20% of volume). Weathering is minor, limited to Fe-oxides developed in association with the fusion crust and with some Fe,Ni metal blebs. Bulk physical properties: Grain density 3.42 ± 0.03 g/cm3 (n=3 pieces); Bulk density 3.15 ± 0.03 g/cm3 (n=4); calculated porosity 7.9%. Magnetic susceptibility of four pieces averages log χ = 4.10.

Petrography: (P. McCausland, UWO) Two polished thin sections with an area of 4 cm² show a large (2 cm) and several small (~mm) subangular metal-poor igneous clasts in a matrix of chondrules and chondrule fragments to 2 mm, as well as minor Fe,Ni and troilite grains <1 mm. Igneous clasts have sharp contacts with the chondritic matrix. The clasts consist of 40-50% fresh, subhedral ~0.1 mm olivine grains set in 55-45% symplectic to skeletal low-Ca pyroxene in a feldspathic glass with accessory chromite; also present within clasts are relict mm olivine grains, which retain inclusions of low-Ca pyroxene and troilite and that host ~0.2 mm inclusion-free olivine rims. Clasts have sinuous fractures and irregular patches filled with glass (dark in cross-polarized light). The fractures and associated melt patches sharply offset clast olivine grains and also contain partially resorbed clast olivine and pyroxene-feldspar symplectite; they do not extend beyond the igneous clasts.

Geochemistry: (R. Flemming, P. McCausland UWO; UCalgary) Chondritic matrix: olivine (Fa_28.4±0.6 n=25; FeO/MnO = 57.5±5.0); low-Ca pyroxene (Fs_23.0±0.6 Wo_2.4±0.8, n=22; FeO/MnO = 32.7±3.4); high-Ca pyroxene (Fs_9.1±1.2 Wo_43.9±0.6, n=3). Igneous clast: olivine (Fa_27.1±0.4 n=49; FeO/MnO = 53.5±4.7); relict olivine (Fa_27.8±0.5 n=7); low-Ca pyroxene (Fs_22.2±0.6 Wo_2.4±0.5, n=42; FeO/MnO = 31.4±2.6); melt veins of ~pyroxene composition (Fs_26.4±0.6 Wo_2.7±0.6, n=8; with ~5% total Na, K, Al).

Classification: NWA 1685 is a polymict breccia consisting of an LL chondrite matrix and metal-poor igneous clasts, which have experienced variable to significant shock. The similar olivine and pyroxene mineral compositions between the igneous clasts and the host LL chondrite suggest that the clasts are a melt product derived from the LL chondrite host, with selective loss of Fe, Ni, S. A subtle but sharp drop in olivine Fe content visible in BSE images across contacts between matrix and clast olivine implies that the overall bulk meteorite cannot be equilibrated, despite the apparent uniformity of chondrule olivine compositions. The igneous clasts have a similar appearance to those of LL4 Yamato 74442, and to a lesser extent with the igneous clasts in LL chondrites Krähenberg and Bhola.

Specimens: A type specimen (28.64 g) and two polished thin sections are on deposit at UWO. Main masses: 1640 g,  DGregory; 624 g R. Jirasek.