Northwest Africa 15468 (NWA 15468)

Mali

Find: 2022

Classification: OC4-melt breccia

History: Found at an undisclosed location in Mali in 2022. Most of the material (~30 kg) was purchased from a dealer in Niger by Taleb Ahmed Boulaouane, who sold ~13 kg to Nicholas Gessler. Other material was purchased in June 2022 by Ziyao Wang (1.8 kg) from a meteorite dealer in China and by Shun-Chung Yang (~5 kg) from a dealer in Erfoud, Morocco.

Physical characteristics: The individual dark brown pieces lack fusion crust and some exhibit significant weathering. Interiors of some of the stones are much fresher; cut and polished surfaces have a very distinctive appearance in being overall deep brown in color, but with irregularly-distributed small, shiny metal grains and numerous thin metal coatings or veins along grain or segment boundaries. The proportion of metal to silicate is variable, but is up to ~45 vol.% in some portions.

Petrography: (A. Irving, UWS, P. Carpenter, WUSL and J. Boesenberg, BrownU; A. Greshake, MNB) Three specimens were studied in detail at separate institutions. The unmelted portions of the specimens are composed of well-formed equilibrated chondrules (apparent diameter 500±260 µm, N = 30) containing forsteritic olivine, magnesian orthopyroxene (predominantly enstatite), pigeonite, subcalcic diopside, diopside and devitrified feldspathic glass set in a relatively coarse grained matrix containing abundant kamacite (altered to varying ° to Fe oxides), taenite, chromite, troilite and chlorapatite, but evidently no plagioclase. Partially melted portions have thin metal veins surrounding chondrule-bearing domains. Clasts have shock stage S2.

Geochemistry: Forsterite (Fa_8.5±0.4, range Fa_8.0-9.2, FeO/MnO = 16-20, N = 20), magnesian orthopyroxene (Fs_8.6±1.1Wo_0.7±0.5, range Fs_7.9-12.3Wo_0.2-2.0, FeO/MnO = 10-18, N = 16), magnesian pigeonite (Fs_9.6±1.8Wo_6.7±0.1, range Fs_8.3-10.9Wo_6.7-6.6, FeO/MnO = 8-12; Fs_6.6Wo_17.3, FeO/MnO = 10; N = 3), subcalcic diopside (Fs_8.6±0.3Wo_25.2±0.2, range Fs_8.4-8.8Wo_25.7-24.8, FeO/MnO = 12-14, N = 2; Fs_5.0±0.2Wo_37.0±0.8, range Fs_4.7-5.1Wo_36.1-37.7, FeO/MnO = 7-9, N = 3), diopside (Fs_4.3±1.0Wo_43.1±2.5, range Fs_3.1-5.7Wo_39.9-47.2, FeO/MnO = 6-10, N = 8). Oxygen isotopes (D. Ibarra, BrownU): analyses of both acid-washed and unwashed subsamples of the freshest interior WZY-124 material by laser fluorination gave, the following closely comparable results: unwashed δ¹⁷O=4.034, δ¹⁸O=6.974, Δ¹⁷O=0.366 per mil; acid-washed δ¹⁷O=3.909, δ¹⁸O=6.608, Δ¹⁷O=0.433 per mil (TFL slope value = 0.526). All data were normalized to San Carlos olivine analyzed concurrently, per the recommendations of Sharp and Wostbrock (2021). Magnetic susceptibility was measured at UWB and MNB on the freshest interior material and gave a mean value of log χ (× 10^-9 m³/kg)=5.60.

Classification: Ordinary chondrite (OC4-melt breccia). This meteorite has relatively high metal content and magnetic susceptibility. Macroscopic observations show considerable heterogeneity in metal distribution, suggesting the meteorite is a melt breccia. The equilibrated mafic silicate compositions are much more magnesian than observed in H chondrites. The oxygen isotopic composition plots above the terrestrial fractionation line, but it is distant from the established trend for H, L, and LL ordinary chondrites.

Specimens: 112 g including two polished thin sections at UWB; 40 g including one polished thin section at MNB; remainder with Mr. N. Gessler (~13 kg), WangZ (1710 g), Mr. S.-C-Yang (~5 kg) and Mr. Taleb Ahmed Boulaouane (~8 kg).