NORTHWEST AFRICA 4483


Lunar Feldspathic Breccia
(granulitic breccia)
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Found August 2005
no coordinates recorded

Initially, a fresh, fusion-crusted stone weighing 1,634 g was found in the Mauritania–Algeria desert region. The meteorite exhibited a thin, translucent, greenish fusion crust with clasts visible underneath. The stone was purchased by G. Hupé 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 rare feldspathic granulitic breccia, similar samples of which were recovered at most Apollo lunar highlands sites. This meteorite has the composition of a noritic anorthosite. Subsequent to this classification, 12 additional paired fragments from the strewn field were acquired by S. Ralew, and these submitted samples were given NWA-series designations of NWA 4483 (208 g) and NWA 4881 (606 g). An additional 5 fragments weighing together 1.3 g was designated NWA 6275. It was determined that these four meteorites, which constitute a single stone having a weight of 2,449 g, have compositions that overlap each other, and these NWA-designated stones 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). By comparison, Apollo granulitic breccias have trace and siderophile element contents, Ar–Ar ages, and shock effects that indicate they were derived from a different region from that of the NWA 4483 pairing group (Hudgins et al., 2011). Northwest Africa 4483 is composed of ~70 vol% plagioclase that encloses small grains of pyroxene (pigeonite with minor augite), olivine, and accessory minerals. Maskelynized anorthitic plagioclase is present in shock veins and pockets throughout (S3), but they are thought to antedate the ejection event which launched the meteoroid 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 where temperatures reached ~1070°C.

Lunar granulites cooled rapidly at shallow depths (<200 m) and are associated with small impact craters. Metamorphic textures were developed 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 which 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).

The Rb–Sr chronometer places the separation of the melt from its source reservoir at 4.56 (±1.2) 0.1 b.y. ago. The Ar–Ar-based age was calculated to be ~3.327(±0.029) b.y., which reflects late impact resetting of this isotopic chronometer associated with extensive metamorphism at depth—possibly recording the event that produced the granulitic texture (McLeod et al. 2013). An even younger Ar–Ar age of ~2 b.y. was calculated for the paired NWA 4881. The CRE age was calculated to be 14.5 (±1.2) m.y. A terrestrial residence age of 12,000 years has been measured for NWA 4483 (Fernandes et al., 2009 and references therein).

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 paired stone.

nwa 4483
42.387 g stone with primary fusion crust
click on photo for a magnified view
nwa 4483
42.387 g stone with secondary crust ("green icing")
click on photo for a magnified view
nwa 4483
NWA 4483 Group Photo
click on photo for a magnified view

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Photos courtesy of Chladni's Heirs—Stefan Ralew & Martin Altmann