Northwest Africa 12689 (NWA 12689)

Morocco

Purchased: 2014

Classification: HED achondrite (Eucrite, brecciated)

History: A single stone of 629 g was purchased from a Moroccan dealer by Dr. R. McKenzie in 2014.

Physical characteristics: Light-brown lithic clasts 0.5 to 1.5 cm across protrude through partly preserved, dark-brown fusion crust. Cut surface shows rounded lithic clasts, some of which contain feldspar laths up to ~1 mm long, surrounded by fine grained, dark-brown to black matrix.

Petrography: (L. Vonopartis and L. Ashwal, Wits) A monomict eucrite breccia with angular to rounded lithic clasts up to 15 mm across, in a glassy matrix that contains mineral fragments and broken lithic clasts 0.1 to 5 mm across. Most of the lithic clasts are composed of subequal proportions of coarse-grained, subhedral plagioclase laths up to 1.2 mm long, surrounded by irregular sub-ophitic grains of low-Ca pyroxene, many of which contain augite exsolution lamellae of variable thickness. The overall texture of these lithic clasts resembles terrestrial dolerites or coarse-grained basalts, but differs from the coarse, equigranular textures typical of unbrecciated cumulate eucrites. A small proportion of lithic clasts consist of fine-grained (0.01 to 0.05 mm) granoblastic aggregates of plagioclase and pyroxene, with angular mineral fragments up to 0.5 mm across. The matrix consists of dark brown, partly devitrified glass with some evidence for flow textures, which is charged with highly comminuted lithic and mineral fragments 0.01 to 0.5 mm across, very likely representing fragmental material derived from the larger lithic clasts. Both feldspars and pyroxenes in all components of the specimen show variable shock features, including kinked and offset twins, undulatory extinction and mosaicism; some plagioclases have been completely maskelynitized. Accessory minerals include chromite, ilmenite and a silica phase.

Geochemistry: Mineral compositions and geochemistry: (L. Vonopartis and L. Ashwal, Wits; D. Van Niekerk, Rhodes University; R. Greenwood, OU) Low-Ca pyroxene is homogeneous (Fs_59.6±1.4Wo_3.4±1.4) and contains augite exsolution lamellae (Fs_25.5±0.7Wo_44.1±1.6). Plagioclase is somewhat more variable (Ab_10.4±1.6An_88.9±1.7Or_0.6±0.2), but individual grains show no compositional zoning. Accessory chromite has Mg/(Mg+Fe) = 3.75 and Cr/(Cr+Al) = 80.29; ilmenite has MgO = 0.75±0.05 and MnO = 1.52±0.05. Dark brown glass in the matrix is basaltic, with SiO₂ = 49.1±0.6, Al₂O₃ = 19.4±2.1 and Mg/(Mg+Fe) = 34.4±2.2. Oxygen isotope analyses on 150 mg of powdered sample yielded: δ¹⁷O = 1.855±0.45 ‰, δ¹⁸O = 3.997±0.10 ‰, Δ¹⁷O = -0.223±0.007 ‰.

Classification: (L. Vonopartis and L. Ashwal, Wits) Achondrite (eucrite). High degree of shock (S5), and low degree of weathering.

Specimens: Main mass 490 g held in private collection of Dr. R. McKenzie. Type specimen 86 g, and 3 chips (8, 10, 18 g) remaining after thin section preparation Wits. A 150 mg subsample from an 18 g chip was powdered and consumed for oxygen isotopic analyses at OU.