NORTHWEST AFRICA 4483

Initially, a fresh, fusion-crusted stone weighing 1,634 g was found in the Mauritania–Algeria desert region. The meteorite exhibited a thin, transparent, greenish fusion crust with clasts visible underneath. The stone was purchased by a collector and a portion was submitted for analysis to the University of Washington in Seattle (A. Irving and S. Kuehner) and Washington University in St. Louis (R. Korotev). The meteorite, designated NWA 3163, was classified as a feldspathic granulitic breccia, similar samples of which were recovered at most Apollo lunar highlands sites. Subsequent to this classification, 12 additional paired stones were acquired from the area by other collectors, and these submitted samples were given NWA-series designations of NWA 4483 and NWA 4881. It was determined that these three meteorites, which constitute a single stone having an initial weight of 2,448 g, compositionally overlap each other, and they are considered to be paired. An additional 57.2 g unnamed stone that was found in 2008 is possibly source-crater paired (R. Korotev; see WUSL website).

Northwest Africa 3163/4483/4881 (NWA 4483) is a feldspathic, granulitic, metamorphosed breccia or impactite from the lunar highlands (Irving et al., 2006). Its low content of incompatible elements attests to an origin far from the KREEP-rich PKT region (Fernandes et al., 2009). It is composed of ~70 vol% plagioclase that encloses small grains of pyroxene, olivine, and accessory minerals, and it contains a very low abundance of incompatible elements. Maskelynized anorthitic plagioclase is present in shock veins throughout, but they are thought to antedate the ejection event which launched it to Earth. Precursor material to this granulitic breccia was probably a mixture of anorthositic rocks, including components of the ferroan anorthositic suite and the feldspathic Mg-suite, derived in part from olivine gabbroic to diabasic lithologies located within the upper layers of the lunar highlands. While the KREEP-poor feldspathic fragmental breccias are texturally and compositionally similar to the granulitic breccias, only the latter experienced significant thermal metamorphism at depth.

Lunar granulites cooled rapidly at shallow depths (<200 m) and are associated with small impact craters. Metamorphic textures were formed beneath an ejecta blanket or at the base of the crater near impact melts. A division of the granulites into three groups has been proposed by Cushing et al. (1999). Poikilitic types are coarse-grained and were cooled from impact melt sheets. Poikilitic–granoblastic (or poikiloblastic) types, similar to NWA 4483, have smaller grain sizes and generally represent metamorphic textures, while possibly experiencing some minor melting. Granoblastic types are metamorphosed fragmental breccias with very fine-grained, equant, granular, hornfelsic textures and prevalent 120° triple junctions.

Granoblastic types have undergone recrystallization and grain coarsening by Ostwald ripening during annealing, and this process has enabled investigators to estimate the cooling rate to be relatively rapid at 0.5–50°C/year. Cushing et al. (1999) used this cooling rate to provide a rough estimate for the burial depth of the granoblastic granulites, which they determined ranged from 20 m for the fastest rate, to 200 m for the slowest rate. By determining the metamorphism rate that would be associated with these cooling rates and burial depths, they arrived at a minimum crater diameter of 30–90 km. The NWA 4483 granulite is thought to have been buried even deeper within the lunar crust—up to tens of km deep (Irving et al., 2006).

A terrestrial residence age of 12,000 years has been determined for NWA 4483 (Fernandes et al. (2009), references therein). An Ar–Ar-based age reflecting late impact resetting and extensive metamorphism at depth was calculated to be ~1.34 b.y. The photo shown above is a 1.522 g slice of NWA 4483. The top two photos below show both sides of an uncut 42.387 g stone from the NWA 3163/4483/4881 pairing group. The next photo shows many of the individual stones which comprise the pairing group. The bottom photo is a close-up view of the large 606 g NWA 4881 pairing member. A photo of a 27.3 g complete slice sectioned from this 606 g mass can be seen on the ‘Rocks From Space Picture of the Day’ website for July 6, 2007.