Northwest Africa 10416 (NWA 10416)

Mali

Purchased: April 2015

Classification: Martian meteorite

History: Reportedly found in Mali, purchased by Darryl Pitt.

Physical characteristics: Irregular, desert-varnished exterior without fusion crust. A saw cuts reveal green-orange olivine phenocrysts set in a groundmass of white plagioclase and light green pyroxene. Fine, dark, shock-melt veins are present.

Petrography: (C. Agee, UNM) This is an olivine basalt with ~10% olivine phenocrysts (~1 mm in diameter), and intergranular clinopyroxene (~50%) and labradorite plagioclase (~30%). The olivines and clinopyroxenes show igneous zoning. The clinopyroxenes have Mg-rich pigeonite cores and Fe, Ca-rich rims, some of the rims are augitic. Approximately ~10% of the plagioclase has been transformed to maskelynite. Maskelynite occurs primarily in contact with, or near shock melt veins. There is a significant amount of secondary alteration present in the olivines and plagioclase. The olivine alteration occurs primarily in the cores of grains, giving low microprobe totals, and is orange in color (olivine rims are unaltered, give normal totals, are transparent and green in color). The plagioclase alteration occurs in at least 50% of the grains observed, shows low microprobe totals, and depletion of Na and K relative to unaltered labradorite domains. Accessory ilmenite, Ti-Cr-Al-Fe spinel, chromite, and Fe-sulfide were observed throughout the sample.

Geochemistry: (C. Agee and N. Muttik, UNM) Olivine Fa_50.7±7.5, Fe/Mn=57±2, n=19; clinopyroxene Fs_27.6±7.0Wo_9.7±8.0, Fe/Mn=32±2, n=128; plagioclase An_63.5±4.3.1Ab_36.0±4.2Or_0.5±0.1, n=9. Shock melt vein (20 μm electron microprobe beam size) SiO₂ 47.3±2.3, TiO₂ 0.53±0.25, Cr₂O₃ 0.42±0.17, Al₂O₃ 15.9±2.6, MgO 10.2±2.0, FeO 14.1±3.2, MnO 0.33±0.06, NiO 0.01±0.01, CaO 8.8±1.3, Na₂O 1.56±0.60, K₂O 0.07±0.3, n=26. Bulk composition by quad ICPMS (L. Borg, LLNL) Mg 98729±6562, Al 31142±1894, Ca 46482±3540, Ti 3540±180, Mn 3687±212, Fe 132424±8398, La 0.22±0.01, Ce 0.59±0.04, Pr 0.11±0.01, Nd 0.79±0.02, Sm 0.61±0.03, Eu 0.35±0.01, Gd 1.16±0.06, Tb 0.30±0.01, Dy 2.15±0.05, Ho 0.46±0.02, Er 1.36±0.04, Tm 0.19±0.01, Yb 1.25±0.06, Lu 0.19±0.01 (all ppm, 2-sd). Oxygen isotopes (K. Ziegler, UNM) were performed on two types of material: 1) relatively unaltered material and 2) altered material with many orange olivine fragments. All samples were acid-washed and analyzed by laser fluorination. 1) "Unaltered" material: δ¹⁸O=4.539, 4.445, 4.392, 4.740, 4.627, 4.310, 4.500; δ¹⁷O=2.668, 2.674, 2.573, 2.760, 2.716, 2.500, 2.630; Δ¹⁷O=0.271, 0.327, 0.254, 0.257, 0.273, 0.224, 0.254; 2) "altered olivine" material: δ¹⁸O= 4.592, 7.323, 6.592, 8.214, 7.012, 6.091; δ¹⁷O =2.609, 4.041, 3.635, 4.471, 3.850, 3.353; Δ¹⁷O= 0.184, 0.174, 0.154, 0.134, 0.148, 0.137 (linearized, all per mil).

Classification: Martian. This is a martian meteorite based on oxygen isotopes, Fe/Mn of clinopyroxene and olivine, and An-content of plagioclase. This martian meteorite is unlike shergottite basalts in that only about 10% of the plagioclase has been converted to maskelynite. Shock pressures appear to have been lower than for shergottites and in this regard it most resembles NWA 8159. Like NWA 8159, this meteorite also shows a significant LREE-depletion. The altered olivine cores in this meteorite are inconsistent with terrestrial weathering which would normally show chemical attack on the outer surfaces or along internal fractures. The unaltered and altered domains in this sample have different oxygen isotope values, unaltered material gives values that fall within the range of SNC meteorites, whereas altered olivine material shows a much wider range of δ¹⁸O values (up to 8.214 permil).

Specimens: 31.0 g including several probe mounts and polished thin sections on deposit at UNM, DPitt holds the main mass.