(NWA 773 Clan)

Lunar Mingled Breccia
(fragmental breccia with clasts of very low-Ti olivine basalt,
olivine gabbro cumulate, fragmental breccias, and regolith breccias)

Found 2005
no coordinates recorded

A single 233 g black-crusted meteorite was found in Algeria. The stone was acquired by M. Farmer and J. Strope in Morocco in October 2005, and a sample was sent for analysis to Northern Arizona University (T. Bunch) and the University of Washington in Seattle (T. Irving). Northwest Africa 2977 has been classified as a lunar highlands cumulate olivine gabbro, with features identical to the olivine gabbro clasts present in the fragmental breccias NWA 773 and NWA 2700. It is considered to be a probable pairing to these two lunar meteorites (Bunch et al., 2006), as well as to several separately analyzed stones, including NWA 2727, NWA 3160, and NWA 3333 (Zeigler et al., 2006). All of these paired stones exhibit a unique chemical signature in that they have the highest Sm/Eu ratios of any other basaltic lunar meteorite or basalt studied from the Apollo collection (Korotev, WUSL). The entire pairing group has a combined weight of ~1,155 g. A comparison of NWA 773 and NWA 2700 follows (comparitive scale of images is estimated):

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Northwest Africa 2977 contains cumulus olivine (41–51%), pigeonite (23–39%), augite (9–12%), plagioclase (7–14%, partially shocked to maskelynite), minor K- and Ba-rich feldspar, various Al,Ti-rich oxides including chromite, ilmenite, and baddeleyite, the phosphates merrillite and apatite, the sulfides pyrrhotite and pentlandite, and FeNi-metal (Bunch et al., 2006; Zhang et al., 2011). This accumulation of crystals might have occurred within a thick lava flow, or perhaps within a magma column that intruded into the crust. Two inclusions types have been identified in the olivines: 1) a Si,Al-rich glass combined with pyroxene which may represent parental melt, and 2) a mafic mixture of high-Ca pyroxene, merrillite, chromite, and baddeleyite which may represent KREEP-rich rock entrained during magma ascent (Zhang et al., 2011). The crystallized rock was subsequently shocked to moderate to high degree (S3–S6) during an impact event(s), producing undulatory extinction in silicates, thin melt veins, and the high-pressure polymorph ringwoodite (S6) that was locally produced. The presence of maskelynite attests to a rapid cooling rate.

A detailed petrogenetic model for mare basalts has been presented by J. Day and L. Taylor (2007), for which a synopsis can be found on the NWA 032 page. This model, which demonstrates that NWA 032/479 is launch paired with the Antarctic LaPaz pairing group, was then expounded to explore the possibilities that the NWA 773 pairing group may also be derived from the same fractionally differentiated stratigraphic magma unit as the NWA 032/479 and LAP samples. Based on chemical compositions, mineralogies, textures, cooling rates, and crystallization and CRE ages, it was argued that the lunar pairing group of NWA 773 may represent the more rapidly cooled, cumulate-rich base of this magma unit where the olivine basalt component, as independently represented by NWA 3160, derives from the lowermost layer adjacent to the local pre-existing rock. The uniformly slow-cooled LAP samples are proposed to have crystallized in the middle of the flow, while the more rapidly cooled NWA 032 is consistent with crystallization at the upper margin. Non-uniform integration of mafic and plagiophile elements as well as KREEP components has resulted in slight compositional variations among members of the pairing group (Nagaoka et al., 2010).

Age data for NWA 2977 obtained by Nyquist et al. (2009) indicates that it has an Ar–Ar age of 2.77 (±0.04) b.y., which likely represents a secondary thermal event after crystallization. A more reliable crystallization age was obtained utilizing Sm–Nd chronometry, resulting in an age of 3.10 (±0.05) b.y., while Rb–Sr dating resulted in an age of 3.29 (±0.11) b.y. (Nyquist et al., 2009). The data for Nd are considered characteristic of KREEP-rich sources. The Pb–Pb data provide an age consistent with the other chronometers at ~3.123 b.y. (Zhang et al., 2011). The age, trace element abundances, and isotopic systematics of NWA 2977 suggest it originated on the lunar nearside near the Procellarum KREEP Terrane. New Sm–Nd data presented by Touboul et al. (2009), utilizing NWA 2977 and other mare basalts, suggests that the Lunar Magma Ocean differentiated ~218 m.y. after CAIs. Given that the Moon was formed ~50 m.y. after CAIs, the 147Sm–144Nd ratio of the bulk Moon is consistent with a superchondritic composition, as proposed for bulk Earth and Mars. The four photos shown below, courtesy of Michael Farmer, exhibit the very fresh nature of this lunaite.

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complete mass of NWA 2977
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close-up of fusion crust
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cut face showing tan to yellow matrix with abundant maskelynite shock veins
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close-up of interior
Photos courtesy of Michael Farmer—Michael Farmer Meteorites