MINERALOGY, PETROLOGY AND OXYGEN ISOTOPIC COMPOSITION OF NORTHWEST AFRICA (NWA) 12379, A NEW METAL-RICH CHONDRITE WITH AFFINITY TO ORDINARY CHONDRITES.

Abstract

Abstract Northwest Africa (NWA) 12379 is a new metal-rich chondrite with unique characteristics distinguishing it from all previously described meteorites. It contains high Fe,Ni-metal content (∼ 70\u202fvol.%) and completely lacks interchondrule matrix; these characteristics are typical only for metal-rich carbonaceous (CH and CB) and G chondrites. However, chondrule sizes (60 to 1200\u202fμm; mean\u202f=\u202f370\u202fμm), their predominantly porphyritic textures, nearly equilibrated chemical compositions of chondrule olivines (Fa18.1–28.3, average Fa24.9±3.2, PMD\u202f=\u202f12.8; Cr2O3\u202f=\u202f0.03\u202f±\u202f0.02\u202fwt.%; FeO/MnO\u202f=\u202f53.2\u202f±\u202f6.5 (wt.-ratio); n\u202f=\u202f28), less equilibrated compositions of low-Ca pyroxenes (Fs3.2–18.7Wo0.2–4.5; average Fs14.7±3.7Wo1.4±1.3; n\u202f=\u202f20), oxygen-isotope compositions of chondrule olivine phenocrysts (Δ17O ∼ 0.2–1.4‰, average ∼ 0.8‰), and the presence of coarse-grained Ti-bearing chromite, Cl-apatite, and merrillite, all indicate affinity of NWA 12379 to unequilibrated (type 3.8) ordinary chondrites (OCs). Like most OCs, NWA 12379 experienced fluid-assisted thermal metamorphism that resulted in formation of secondary ferroan olivine (Fa27) that replaces low-Ca pyroxene grains in chondrules and in inclusions in Fe,Ni-metal grains. Δ17O of the ferroan olivine (∼ 4‰) is similar to those of aqueously-formed fayalite in type 3 OCs, but its δ18O is significantly higher (15–19‰, average\u202f=\u202f17‰ vs. 3―12‰, average\u202f=\u202f8‰, respectively). We suggest classifying NWA 12379 as the ungrouped metal-rich chondrite with affinities of its non-metal fraction to unequilibrated OCs and speculate that it may have formed by a collision between an OC-like body and a metal-rich body and subsequently experienced fluid-assisted thermal metamorphism. Trace siderophile element abundances and isotopic compositions (e.g., Mo, Ni, Fe) of the NWA 12379 metal could help to constrain its origin.