Northwest Africa 16493 (NWA 16493)

(Northwest Africa)

Purchased: 2023 Jun

Classification: Ungrouped achondrite

History: Purchased by Fabien Kuntz in June 2023 from a dealer in Laayoune, Morocco.

Petrography: (A. Irving, UWS, P. Carpenter, WUSL, and J. Boesenberg, BrownU) This meteorite has a triple grain junction metamorphic texture and is completely devoid of chondrule structures and fine grained matrix. Most of the specimen has a mean grainsize of ~200 µm but there are some larger grains (up to 1.2 mm). Composed predominantly of olivine with subordinate augite and intermediate plagioclase plus accessory low-Ca pyroxene, merrillite, nepheline, Ti-chromite, Ti-Cr-magnetite, magnetite, pentlandite and troilite. Metal is absent. Sulfides are partly altered to Fe and FeNi oxides (red-brown in thin section) and minor secondary calcite is present.

Geochemistry: Olivine (Fa_40.1±0.4, range Fa_39.5-40.6, FeO/MnO = 89-110, N = 6), augite (Fs_11.4±0.7Wo_48.6±0.7, range Fs_10.4-12.0Wo_47.6-49.1, FeO/MnO = 59-129, N = 6), plagioclase (Ab_58.5±0.8An_40.9±0.9Or_0.4±0.2, range Ab_57.5An_41.8Or_0.7-Ab_59.3An_40.4Or_0.3, N = 3), low-Ca pyroxene (Fs_38.9Wo_3.6, FeO/MnO = 107). Oxygen isotopes (K. Ziegler, UNM): analyses of acid-washed subsamples by laser fluorination gave, respectively δ¹⁷O 1.113, 1.347, 1.448; δ¹⁸O 7.619, 8.500, 8.601; Δ¹⁷O -2.909, -3.141, -3.093 per mil (all data linearized, TFL slope = 0.528). Magnetic susceptibility log χ (× 10^-9 m³/kg) = 3.48.

Classification: Achondrite (ungrouped, metamorphic, with oxygen isotope affinities to ungrouped carbonaceous chondrites). Although the olivine compositions in this meteorite could be consistent with an affinity to equilibrated R chondrites, the magnetic susceptibility and oxygen isotopic composition rule out such an affinity. Magnetic susceptibility is, however, within the range for CM chondrites, including specimens such as Mighei, Murchison and Murray (see Macke et al., 2011). The mean olivine composition is not inconsistent with the median olivine composition in unequilibrated CM2 and ungrouped C2 chondrites coupled with a contribution from regenerated olivine produced by dehydration of original cronstedtite in such thermally metamorphosed specimens (as described in PCA 91018, PCA 02010 and PCA 02012 by King et al. (2021). The prevalence of Ni-bearing sulfides and paucity of metal are also consistent with an affinity to CM or similar carbonaceous chondrites. However, the oxygen isotopic composition plots significantly below the CCAM trend and the field for CM chondrites to more negative Δ¹⁷O values, and lies on the same trend as values for ungrouped carbonaceous chondrites such as Telakoast 001 and similar specimens Irving et al. (2022). The prevalence of Fe oxides, augite and intermediate plagioclase would be consistent with CK chondrites; however, the oxygen isotopes do not support an affinity with those meteorites.

Specimens: 26.1 g including one polished thin section at UWB; remainder with Mr. F. Kuntz.