(NWA 773 Clan)

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

standby for northwest africa 2727 photo
click on photo for a magnified view

Purchased November 2004
no coordinates reported

A stone weighing 26 g was purchased from a dealer in Morocco by N. Oakes. A portion was submitted for classification to Northern Arizona University (T. Bunch and J. Wittke). All together, four stones (11.6, 30.6, 64, and 85 g) having a combined weight of 191.2 g were classified under the NWA 2727 designation. Numerous additional stones (or parts thereof) were classified under different NWA-series designations by different labs (e.g., NWA 3160 and NWA 3333; see following photos). All of these similar stones are considered to be a pairing group, and all are also thought to belong to the previously recognized three-member pairing group composed of NWA 773, NWA 2700, and NWA 2977. Consistent with this finding, cosmogenic nuclide studies conducted on NWA 3160 indicate that it is likely paired with NWA 773 (Nishiizumi and Caffee, 2006). Additional paired stones have been recovered and more information and photos of this lunar pairing group can be found on the website of Randy L. Korotev–WUSL.

standby for northwest africa 3160/3333 photo

A detailed petrogenetic model for mare basalts was 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 upon to explore the possibilities that the NWA 773 pairing group might also be derived from the same 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 could represent the more rapidly cooled cumulate-rich base of this magma unit, while the olivine-phyric basalt component (constituting NWA 3160 in its entirity) derives from the lowermost layer adjacent to 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.

Northwest Africa 2727 comprises three of the five compositionally diverse components identified in different members of the NWA 773 clan. The olivine-phyric basalt is a fragmental breccia of a VLT basalt with a geochemical relationship to Apollo 14 Green Glass B1 and KREEP. Although major-element concentrations are consistent with a parental melt composition similar to Apollo 14 Green Glass, the parental melt of NWA 2727 would have had a lower Ni concentration more consistent with that of Apollo 15 Green Glass (Gibson et al., 2010). Large olivine-phyric mare basalt lithic clasts and glass porphyrys are embedded within a fine- to coarse-grained brecciated matrix (Bunch et al., 2006; Jolliff et al., 2007). Zoned olivine phenocrysts are present as spinifex- to dendritic-textured or hopper crystals, along with skeletal pyroxene and plagioclase, and the olivine is less evolved than the olivine in the cumulate olivine gabbro lithology.

Another component of NWA 2727 is a ferroan olivine gabbro cumulate, which along with the olivine-phyric basalt lithology represent the main constituents of this meteorite. A fragmental or regolith breccia lithology is present in lesser abundance and is composed of ferroan olivine gabbro cumulate intrusive material (26 vol%) mixed with surface porphyritic olivine basalt (60 vol%). The gabbro and basalt were derived from a similar parental melt, but the basalt source was less evolved (following 20% olivine crystallization) than that of the gabbro. In addition, a magnesian olivine gabbro lithology likely related to the ferroan olivine gabbro has been identified, and an incompatible-element-rich basaltic lithology derived from trapped intercumulus melt is present in some of the other paired stones (Shaulis et al., 2013).

Studies of a section of NWA 2727 by North et al. (2013) have revealed the presence of a pyroxene-rich clast ~3 mm in size associated with the ferroan gabbro. This clast contains pigeonite and augite along with plagioclase and high-Ba K-feldspar with accessory silica, ilmenite, and sulfide. The pyroxene is similar to pyroxenes found in the paired lunaite NWA 7007, including the occurrence in both of Fe-rich pyroxferroite with its breakdown product symplectite. The presence of these mineral species are consistent with rapid cooling and crystallization near the surface. Furthermore, North-Valencia et al. (2014) recently described a leucogabbro component in NWA 2727 composed primarily of plagioclase (61.8%) and pyroxene (38.2%); this lithology is likely related to that found in the paired NWA 3170 which Shaulis et al. (2017) termed anorthositic gabbro.

Studies of the paired stone NWA 3160 revealed that light-REE abundances and incompatible trace element concentrations, especially the highly incompatible element Th, are higher compared to most other basaltic lunar samples, while plagiophile element concentrations (Na, Sr, and Eu) are lower; these characteristics demonstrate the uniqueness of this lunar meteorite and help establish a probable pairing group (Zeigler et al., 2006). The REE concentrations in the cumulate olivine gabbro lithology show significant variation among the different members of this lunar meteorite clan, with NWA 2727 and NWA 773 showing higher abundances compared to NWA 2977. This variation in REE abundance among the different samples could reflect the crystallization stage (Nagaoka et al., 2015).

Components of NWA 2727 and its pairings are compositionally similar to the olivine-phyric basalt and cumulate olivine gabbro components in the NWA 773/2700/2977 pairing group, but the differences that do exist, e.g., a much higher abundance of mare basalt clasts in NWA 2727 and pairings, and significant differences in the gabbro components, initially persuaded investigators that these lunaites were not paired. However, because of the overall compositional and textural similarities of these various stones and their uniqueness compared to all other lunaites, they are now considered to be paired (Zeigler et al., 2006). Further reasons to accept the pairing argument are the reasonably similar CRE ages among different stones and the identical young Ar–Ar ages (2.7–2.8 b.y.) of like components from separate stones (Zeigler et al, 2007; Burgess et al., 2007); moreover, Pb–Pb ages calculated for the majority of the clan members are concordant. A possible origin for this pairing group within the Procellarum KREEP Terrane is consistent with petrographic results.

It is generally accepted that the Moon was formed from the debris that resulted from a collision between Earth and a smaller body named 'Theia', creating an all-encompassing magma ocean. It was calculated from isotopic data that the earliest time this event could have occurred is 4.517 b.y. ago (Nemchin et al., 2009), or 30–110 m.y. after the beginning of the solar system (Yin et al., 2002; Kleine et al., 2005). Based on Pb–Pb dating of zircon crystals, which is a late crystallization product derived from the last dregs of the lunar magma ocean, Nemchin et al. (2009) determined that crystallization of the lunar magma ocean was complete by 4.417 (±0.006) b.y. ago, thus establishing a timeframe for the solidification of the lunar magma ocean of 100 m.y. They also reasoned that formation of an anorthosite crust could not begin until 80–85% of the magma ocean had crystallized, which would allow relatively rapid cooling over a time interval of ~50 m.y. The final 25% of crystallization would have taken place under an insulating anorthosite crust over a similar time interval of ~50 m.y.

A transmitted light view of a petrographic thin section of NWA 2727 can be seen on John Kashuba's page. The photo of NWA 2727 shown above is a 2.0 g slice sectioned from the original 30.6 g stone. It consists of porphyritic olivine basalt clasts of varying grain size, clasts of ferroan olivine gabbro cumulate, and a regolith breccia component, each sintered into a composite rock by melt veins. The photo below shows the outside appearance of the 30.6 g parent stone.

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Photo courtesy of N. Classen