NORTHWEST AFRICA 4485

Two paired stones weighing 64.3 g (NWA 4472) and 188 g (NWA 4485) were found in the Algerian desert and subsequently purchased by separate collectors (G. Hupé and S. Ralew, respectively). A portion of each was submitted for analysis to the University of Washington in Seattle (S. Kuehner and A. Irving) and Washington University in St. Louis (R. Korotev), and they were classified as unique KREEP-rich, basaltic melt breccias.

This is a heterogeneous breccia composed of clasts from diverse lunar locations including mare basalt, Mg-suite, and ferroan anorthosite (FAN) lithologies. The probable KREEP-bearing assemblages are composed of granophyric textured clasts consisting of intergrowths of silica and K-feldspar, together with the high-temperature mineral zirconium oxide, or baddeleyite, and the Fe(Zr,Y)Ti-silicate known as tranquillityite, a mineral first recognized as a late-stage fractional crystallization product in Apollo 11 and 12 basalts. The bulk composition of this meteorite reflects a high REE abundance with a strong negative Eu anomaly, with overall incompatible element abundances in the range of the only known KREEP-rich lunaite SaU 169. Basaltic clasts sample both low- to very low-Ti source regions and exhibit a range of metamorphic textures. Some of these are fayalite-rich, quenched-textured glass thought to be derived from impact melting of mare basalt lithologies. Also present are a variety of feldspathic impact melt, fragmental, and granulitic breccias, as well as metal clasts, and glass spheres, all consistent with lithification within the lunar regolith.

The composition of NWA 4472/4485 is similar to that of the Th-rich, mafic, LKFM impact melt breccias recognized from the Apollo collection. The four constituents of the LKFM material—KREEP norite, forsteritic dunite, feldspathic upper crust, and FeNi-metal—are thought to be the likely products of a basin-sized impact into the ancient ‘Great Lunar Hot Spot’, creating the Imbrium basin within the Th-rich Procellarum KREEP Terrane (PKT) (R. Korotev, 1999). The impactor is thought to have been an iron meteorite which mixed upper mantle dunite with KREEP-contaminated Mg-rich magma, and incorporated clasts of ferroan anorthositic upper crust. The LKFM composition is unique to the Procellarum KREEP Terrane, and with its high FeO (noritic) composition and incompatible element abundances (>10 ppm Sm), NWA 4472/4485 is likely derived from within this region (Joy et al., 2008).

A thorough chronological history of this lunar rock following the initial basin-forming impact, including time on the lunar surface, Moon-Earth transit, and terrestrial residence, is yet to be resolved. A more thorough treatment of the chemical classification of lunar meteorites can be found on the WUSL—Lunar Meteorites website. The photo of NWA 4485 shown above (courtesy of Chladni's Heirs) is a 0.32 g partial slice. Pictured below is the uncut main mass as found.