Northwest Africa 11575 (NWA 11575)

Mali

Purchased: 2016 Sept

Classification: Ungrouped achondrite

History: Found in June 2016 at the border region of Mali and Algeria; purchased in Mauritania in September 2016.

Physical characteristics: Single stone, 80% covered by fusion crust. Saw cut reveals a surface of approximately 12 square cm with a light colored, felsic-appearing, aphanitic texture. A single, ~1 mm wide, dark colored shock melt vein crosscuts this surface. Also present are a few smaller xenoliths which are dark in color, angular, mafic-appearing, and aphanitic; the largest of which is approximately 1 cm across.

Petrography: (C. Agee, UNM) Microprobe and SEM examinations were performed on a polished probe mount and on the saw-face of the deposit sample respectively. An ophitic texture of pyroxene and plagioclase grains, making up approximately 90-95% of the model mineralogy, was observed. Pyroxenes were typically 300-500 μm in size and showed significant igneous zoning. The pyroxene grains had cores of magnesian pigeonite, outwardly transitioning to a zone of augite, and rimmed by a zone of sub-calcic ferroaugite to ferropigeonite. Plagioclase grains were oligoclase, mostly lath-like in shape, and typically 200x50 μm in size. We also observed ubiquitous potassium feldspar and silica, which were commonly found in contact with each other, and together made up ~5% of the modal mineralogy. Opaque phases made up ~1% of the modal mineralogy; these included ilmenite, iron sulfide, iron metal (nickel not detected), chromite, and iron oxide. The mafic xenolith clasts were basaltic in major mineralogy and lacked K-feldspar. The shock melt vein composition within the host felsic lithology plots in the andesite field of the TAS diagram, whereas the composition of the fusion crust of the meteorite, plots in the basaltic andesite field.

Geochemistry: (C. Agee, UNM) Magnesian pigeonite Fs_28.3±3.3Wo_7.3±1.6, Fe/Mn=37±4, n=5; augite Fs_24.4±4.2Wo_30.0±3.8, Fe/Mn=33±4, n=21; sub-calcic-ferroaugite/ferropigeonite Fs_57.3±6.2Wo_14.7±3.7, Fe/Mn=48±3, n=19; plagioclase Ab_81.3±1.9An_17.4±2.0Or_1.3±0.2, n=20; potassium feldspar Ab_4.0±0.9An_0.4±0.3Or_95.6±1.1, n=23; shock melt vein (proxy for felsic lithology bulk composition) SiO₂=59.1±5.7, Al₂O₃=10.1±1.5, MgO=4.5±1.3, FeO=15.6±5.9, CaO=6.5±1.5, Na₂O=3.8±1.2 (all wt%), n=11.; fusion crust (proxy for meteorite bulk composition) SiO₂=55.8±3.7, Al₂O₃=8.2±3.4, MgO=6.7±1.0, FeO=16.2±6.2, CaO=7.2±0.7, Na₂O=2.6±0.5, K₂O=0.5±0.2 (all wt%), n=8. Oxygen isotopes (K. Ziegler, UNM): 3 acid-washed fragments analyzed by laser fluorination gave δ¹⁸O= 4.875, 5.583, 5.349; δ¹⁷O= 3.760, 4.137, 4.006; Δ¹⁷O= 1.186, 1.189, 1.182 (linearized, all per mil, TFL slope=0.528).

Classification: Ungrouped achondrite. Pyroxene compositional core-rim zoning trends are similar to that observed in Martian basalt QUE 94201 (Papike et al. 2009) and Apollo 15 Mare basalts (Kushiro, 1973). Oxygen isotopes plot within the field defined by LL-chondrites, however the 3 measured data points, from different fragments of the meteorite, form a slope of 0.53, parallel to the mass dependent terrestrial fractionation line. The andesite to basaltic andesite compositions of the shock melt vein and fusion crust of this meteorite plot on the TAS diagram in a broadly similar location to the estimated bulk composition of ungrouped achondrites GRA 06128/GRA 06129; and this meteorite and GRA 06128/06129 have similar oligoclase plagioclase. However, GRA 06128/06129 are not likely related to this meteorite since they are olivine-bearing, lack K-feldspar, and have oxygen isotopes that plot below the TFL.

Specimens: 22.1 g on deposit at UNM, Darryl Pitt holds the main mass.