Northwest Africa 13479 (NWA 13479)

(Northwest Africa)

Purchased: 2020 Aug

Classification: Carbonaceous chondrite (C2, ungrouped)

History: Purchased by Rachid Chaoui in August 2020 from a Mauritanian dealer.

Petrography: (A. Irving, UWS, P. Carpenter, WUSL; L. Garvie, ASU) Small, well-formed chondrules (apparent diameter 250±210 µm, N = 23) and sparse very fine grained, amoeboid CAI are set within a very fine grained matrix (~65 vol.%, opaque in thin section). Phases identified by EPMA include kamacite (as droplets and separated grains), Fe and Fe-Ni sulfides (typically intergrown and altered), an Fe₂O₃ oxide phase, and an apparently hydrous, Fe-rich silicate phase with bladed morphology. Powder X-ray diffraction confirms the presence of phyllosilicates, tochilinite, regularly interstratified tochilinite/cronstedtite and maghemite, plus minor calcite and dolomite.

Geochemistry: Olivine (Fa_24.1±14.4, range Fa_1.0-44.7, N = 5; Cr₂O₃ in ferroan olivine 0.27-0.53 wt.%, mean wt.%, sd wt.%, N = 7), low-Ca pyroxene (Fs_3.2±2.8Wo_2.5±2.3, range Fs_0.8-6.2Wo_0.8-5.6, N = 4). clinopyroxene (Fs_5.1±0.1Wo_35.0±0.4, range Fs_5.0-5.1Wo_34.7-35.2, N = 2). Oxygen isotopes (K. Ziegler, UNM): analyses of acid-washed subsamples by laser fluorination gave, respectively, δ¹⁷O -8.313, -7.758, -7.724; δ¹⁸O -4.447, -2.727, -3.473; Δ¹⁷O -5.966, -6.318, -5.890 per mil (all data linearized).

Classification: Carbonaceous chondrite (C2 ungrouped). The presence of phyllosilicates and tochilinite establishes this specimen as a C2 chondrite, yet its oxygen isotopic composition plots far from the established array for CM2 chondrites (to much more 16O-rich compositions) and even further from values for CR2 chondrites. Although the oxygen isotopic composition is close to those for a few CV chondrites, an affinity to CV or CK chondrites is excluded on other grounds (notably the relatively small mean chondrule size and absence of Cr-magnetite). Chondrule sizes are within the range for CO chondrites, but oxygen isotopes plot well below the array for CO chondrites (to more negative Δ¹⁷O values). The presence of maghemite is an additional anomalous feature.

Specimens: 6.1 g including one polished thin section at UWB; remainder with Mr. R. Chaoui.