Northwest Africa 11042 (NWA 11042)

Mauritania

Purchased: 2016

Classification: Primitive achondrite

History: Purchased by Abdelhadi Aithiba in Morocco, 2016.

Physical characteristics: Single complete stone covered in black fusion crust. Broken surface reveals a phaneritic mix of pale-yellow olivine grains, light-green pyroxene grains, glassy maskelynite patches, and dark-colored shock melt pockets. A single, fine, dark shock melt vein was also observed in hand sample.

Petrography: (C. Agee, UNM) Microprobe examination of two polished mounts shows an ultramafic rock with approximately 46% olivine, 38% pyroxene, 15% maskelynite and 1% chromite. Fe-sulfide is a minor but ubiquitous phase. Trace amounts of Fe-Ni metal and chlorine-rich apatite are present. Dominant igneous texture consists of poikilitic olivine and pyroxene grains most in the size range 500-1500 μm. Plagioclase has been completely transformed to maskelynite. Maskelynite domains appear smooth and glassy, and are associated with radiating fractures into surrounding olivine and pyroxenes. Shock melt pockets are scattered throughout, some up to ~500 μm in diameter, also surrounded by radiating fractures.

Geochemistry: (C. Agee, M. Spilde, Z. Vaci, UNM) olivine Fa_24.6±0.2, Fe/Mn=50±2, n=20; low-Ca pyroxene Fs_20.2±0.4Wo_4.0±0.4, Fe/Mn=30±2, n=29; augite Fs_10.5±0.1Wo_39.5±0.1, Fe/Mn=22±1, n=2; maskelynite Ab_83.4±1.7An_11.9±1.8Or_4.8±0.4, n=10. LA-ICPMS (M. Humayun, FSU) Bulk rock: SiO₂ 46.35, TiO₂ 0.11, Al₂O₃ 2.34, Cr₂O₃ 0.36, FeO 17.93, MnO 0.43, MgO 28.61, CaO 2.39, Na₂O 1.01, K₂O 0.07, P₂O₅ 0.08, S 0.31 (all wt%); La 0.40, Ce 1.27, Pr 0.16, Nd 0.84, Sm 0.28, Eu 0.10, Gd 0.41, Tb 0.08, Dy 0.544, Ho 0.12, Er 0.37, Tm 0.06, Yb 0.36, Lu 0.06, Ni 102, Co 26, Th 0.060, U 0.036 (all ppm). Noble gases (D. Weimer, H. Buseman, ETHZ) results on two sample fragments without fusion crust, 20 and 100 mg, gave ¹²⁹Xe/¹³²Xe= ~1.8, ³⁶Ar/¹³²Xe= ~600, ⁸⁴Kr/¹³²Xe= 3 (100 mg) and 6 (20 mg); U,Th-He age ~0.2 Ga. Oxygen isotopes (K. Ziegler, UNM): 8 acid-washed fragments analyzed by laser fluorination gave, respectively δ¹⁸O= 4.33, 4.45, 4.36, 4.54, 4.64, 4.42, 4.49, 4.46; δ¹⁷O= 3.33, 3.38, 3.34, 3.42, 3.52, 3.35, 3.41, 3.36; Δ¹⁷O= 1.04, 1.04, 1.04, 1.03, 1.07, 1.02, 1.03, 1.00 (linearized, all per mil, TFL slope=0.528); weighted average δ¹⁸O= 4.46; δ¹⁷O= 3.34; Δ¹⁷O= 1.03.

Classification: Primitive achondrite. Δ¹⁷O values coincide with the L-chondrite field. The bulk rock pattern is LREE-depleted as expected for an igneous cumulate rock. Possibly related to NWA 4284, but not paired, as NWA 4284 has no shock melt pockets, no maskelynite, and a higher modal abundance of plagioclase.

Specimens: 18.5 g, including a probe mount, on deposit at UNM, Abdelhadi Aithiba holds the main mass.