Northwest Africa 12338 (NWA 12338)

(Northwest Africa)

Purchased: 2016 Nov

Classification: Ungrouped achondrite

Physical characteristics: The repository specimen has a brown fusion crust surface and a light-gray, fresh interior.

Petrography: (Z.G. Guo, UWO) This is an unbrecciated basaltic meteorite with fine-grained ophitic texture including ~100 μm-sized clinopyroxene (~55%), and 100-500 μm-sized plagioclase (~35%). Plagioclase is not acicular, as usually found in eucrites, but instead is interstitial between clinopyroxene crystals. Olivine (~5%) is found as ~100 μm individual crystals. It has a low content (<1%) of metal (partially weathered), chromite and troilite. Undulatory (up to mosaic) extinction is observed in plagioclase, with silica veinlets, which indicate a moderate shock stage.

Geochemistry: (EMPA, Z.G. Guo and M. Beauchamp, UWO) Olivine Fa_70.0±0.2, Fe/Mn=44±1 (n=10); pigeonite Fs_40.5±3.3Wo_10.6±4.1, Fe/Mn=30±1 (n=20); plagioclase An_89.1±1.4 (n=10); Bulk chemistry (INAA, R. Korotev, WashU) Na₂O 0.495%, CaO 10.2%, FeO 15.3%, and (all in ppm) Cr 3043, Co 16, Sr 92, Ba 19, La 1.05, Sm 0.74, Hf 0.52, Th 0.10, and U 0.16. (laser fluorination oxygen isotope analyses, A. Alexandre; L. Webb, UWO) acid-washed bulk rock powder Δ¹⁷O=-0.18±0.01, δ¹⁷O=1.980±0.203, δ¹⁸O=4.16±0.37 (n=2).

Classification: Achondrite, ungrouped. This meteorite is an anomalous eucrite-like basaltic meteorite similar to other specimens which have been found to diverge from the HED oxygen mass fractionation line (similarly to PCA 91007 and Pasamonte, and significantly above the HED line). It has geochemical characteristics of eucrites for major element mineral compositions, and of cumulate eucrites for bulk trace element chemistry. But its mineral sssemblage with the presence of igneous olivines, texture, and its oxygen isotopic composition indicate a different parent body from HED, thus an ungrouped achondrite classification.

Specimens: 24.0g at UWO including a probe mount.