PETROLOGY, BULK COMPOSITION, AND PROVENANCE OF METEORITE NWA5000

Lunar meteorite Northwest Africa 5000 (NWA5000) is a polymict breccia that contains abundant clasts of leucogabbronorite and has the highest proportion of meteoritic metal in any known lunar sample. The meteorite has only been briefly described in preliminary reports, so this is the first comprehensive study of the meteorite. We examined the petrography and mineral chemistry of a 0.74 gram slab of NWA5000 using a electron microprobe for mineral compositions and EDS element maps and BSE/SEM images to see mineral textures and determine mineral proportions in the clast. From the mineral compositions and proportions, we derived a bulk chemical composition of the clast. The gabbronorite clast in the slab consists of 67% anorthite, 19% augite, 7% pigeonite, 6% olivine, 2% kamacite, and 0.3% merrillite. The clast’s silicates include >200 µm anorthite grains (An₉₆) in a subophitic matrix of less calcic plagioclase (An₉₂) and subhedral olivine (Mg*=68) with interstitial augite and pigeonite (all chemically homogeneous). The gabbronorite clasts sit in a matrix of fragments of fine-grained melt rocks. The NWA5000 gabbronorite clast is interpreted as an impact-melt rock with relict fragments of the protolith (the calcic plagioclase). The sub-ophitic material represents a rapidly solidified impact melt, which then remained at a high temperature for an extended period of time. The metal has chondritic Ni/Co, suggesting that it came from a chondritic asteroid (identity yet unknown) that impacted a mafic area of the lunar highlands. The asteroid traveled slowly enough so that it did not completely vaporize, but instead, it was incorporated into the impact melt (now the gabbronorite). After it cooled, the gabbronorite was shattered in another impact event and mixed with the fine-grained melt rock to form the breccia. NWA5000 then resided in the lunar regolith until it was ejected from the Moon. The meteorite’s relatively high abundance of augite pyroxene gives some hope that its launch site in the lunar highlands may be located.