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NWA 2727

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 possibility 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).

Investigation of a section of NWA 2727 by North et al. (2013) 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 meteorites 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 respective 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 previously established NWA 773/2700/2977 pairing group. Still, the much higher abundance of mare basalt clasts in NWA 2727 and pairings, along with significant differences in the gabbro components, initially persuaded investigators that these lunaite groupings were not paired. However, considering the overall compositional and textural similarities that exist among the various stones, as well as their uniqueness compared to all other lunaites, it was ultimately established that they do represent a single diverse pairing group (Zeigler et al., 2006). Further reasons to accept the pairing argument include the reasonably similar CRE ages among different stones (~ 73–154 m.y.; Fernandes et al., 2003), 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), and the concordant Pb–Pb ages calculated for the majority of the clan members. A 14C terrestrial age of 17 (±1) t.y. was determined for the paired NWA 773 by Nishiizumi et al. (2004). A possible origin for this lunar pairing group from the nearside Procellarum KREEP Terrane is consistent with petrographic results.

A significant discovery was made by Kayama et al. (2018) of abundant (ave. 77 wt%) moganite-bearing silica micrograins in the gabbroic–basaltic breccia martix component of NWA 2727. Moganite is a thermochemically metastable mineral phase that is considered to have formed through precipitation from alkaline fluids under high-pressure (impact-related) conditions on the Moon. Other high-pressure silica phases are also present in this meteorite and associated with moganite, including coesite, stishovite, and cristobalite, all of which were formed as transition minerals from precursor moganite during peak shock pressure; this peak pressure is calculated to have been in the range of 8–22 GPa. Post-shock temperatures are calculated to have been 673–1073 K in the breccia matrix where moganite occurs, and to have reached >1173 K in shock veins where moganite was converted to the other high-pressure phases. Kayama et al. (2018) presented a multi-stage scenario to account for the formation of moganite as follows (also see diagram below):

  1. Alkaline fluids (pH 7.0–12.0) were delivered by carbonaceous chondrite meteorites to the lunar surface <2.67 b.y. ago, where the water was trapped as subsurface ice in permanently shadowed regions (PSR) within a stability depth range of 0.1 mm to >100 m.
  2. Impacts into existing basaltic and gabbroic lithologies in the Procellarum KREEP Terrane (PKT) and South Pole–Aitken [SPA] basin regions produced a mixed gabbroic–basaltic breccia and incorporated a component of the subsurface meteoritic water ice.
  3. Moganite-bearing silica micrograins precipitated from the aqueous fluid component within the gabbroic–basaltic breccia matrix at temperatures of 363–399 K and a pH of 9.5–10.5.
  4. The NWA 2727 lithology (sunlit) and adjacent lithologies (773 clan) were ejected from the PKT region of the Moon ~1–30 m.y. ago, during which time the shock-induced conversion of some moganite to high-pressure silica phases occurred. Based on silica solubility equations, Kayama et al. (2018) calculated that a lunar bulk water content of at least 0.6–12.3 wt % would be required to precipitate the volume of moganite present in NWA 2727.

Schematic History of Moganite Precipitation on the Moon
standby for moganite formation scenario
click on image for a magnified view

Diagram credit: Kayama et al., Science Advances, vol. 4, #5, eaar4378 (2 May 2018, open access link)
‘Discovery of moganite in a lunar meteorite as a trace of H2O ice in the Moon’s regolith’
(https://doi.org/10.1126/sciadv.aar4378)
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’, which created 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 the timeframe for the solidification of the lunar magma ocean at 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. The specimen 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 shock-melt veins. The photo below shows the outside appearance of the 30.6 g parent stone. standby for nwa 2727 photo
Photo courtesy of N. Classen


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NWA 4898

Lunar Mare Basalt
standby for northwest africa 4898 photo
Found 2005/2006
no coordinates recorded A single fusion-crusted stone weighing 137 g was found in the Sahara Desert. It was sold by the finder to German dealer S. Ralew in 2007, and a sample was submitted to the Museum für Naturkunde (A. Greshake) in Berlin, Germany for analysis and classification under the designation NWA 4898.

This coarse-grained (gabbroic), magmatic rock is composed primarily of phenocrysts of pyroxene (Ti-rich pigeonite and augite, 50 vol%) and Ca-rich plagioclase (32 vol%, all of which is converted to maskelynite), with lesser amounts of Fe-zoned olivine megacrysts, needle-shaped grains of ilmenite, high-silica glass, and chromite, along with minor FeNi-metal and FeS. The matrix texture reveals radiating, densely branched, polycrystalline sprays (called ‘spherulites’ because of their large-scale average spherical shape) of clinopyroxene and plagioclase, attesting to rapid quenching of the magma during eruption to the surface. It has been demonstrated that this spherulitic texture forms when lunar basalts cool at ~20–60°C/hour. Besides the presence of maskelynite, other features that reflect moderate shock levels include the occurrence of planar fractures and mosaisicm in silicates, twinning in ilmenite, partial melt veining, and localized melt pockets with embedded FeS droplets.

Northwest Africa 4898 is a low-Ti, low-Fe, high-Al basalt compositionally similar to Apollo 14 mare basalts. However, NWA 4898 is significantly younger than Apollo 14 samples, having a Rb–Sr-based crystallization age of 3,578 (±40) m.y. compared to the calculated age range of 3,950–4,330 m.y. for Apollo 14 samples. Certain elemental ratios also reflect significant differences between the NWA 4898 and Apollo 14 basalt mantle sources. Moreover, NWA 4898 is derived from a highly evolved mantle source, and has the highest Sm/Nd ratio known among lunar mare basalts, reflecting the highest incompatible element depletion of any lunar mantle source studied thus far.

Spacecraft have found that basalts are not present in all topographic low areas, but instead, reservoirs of basaltic magma are confined at great depth with eruptions being contingent on a combination of factors such as the crustal thickness, the concentration of heat-producing elements (K–U–Th), and the extent of the underlying magma columns. Based on spectral data, it was determined that many different basalt units exist within individual maria, these representing a wide range of crystallization ages, from ~4.35 b.y. (components of the lunar breccia Kalahari 009) to as young as ~1.3 b.y. Crater counting methods indicate that some maria could have formed as recently as 1 b.y. ago (G. T. Taylor, 2007). With a crystallization age (Rb–Sr) of 3.578 b.y., NWA 4898 is somewhat older than the mare basalt NWA 032, the latter crystallizing 2.852 b.y. ago (Rb–Sr) and considered to be one of the youngest mare basalts in our collections. A look at the ages determined for the lunar meteorites reveals that none of them have a bulk rock age older than ~3.85 b.y., upholding the lunar cataclysm hypothesis.

Further information detailing the formation of mare basalts can be found on the NWA 032 page of this website. In-depth information about lunar meteorites in general, and NWA 4898 in particular, can be found on the lunar meteorite website of the Department of Earth Sciences, Washington University. The specimen of NWA 4898 pictured above is a crusted partial slice weighing 0.207 g. The Washington University website presents a high-resolution close-up photo of the pyroxene–plagioclase sprays which were formed in the matrix during quenching of this rock.


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NWA 4485

Lunar Mafic, Th-rich, Impact-Melt Breccia
(KREEP-melt regolith breccia)
standby for northwest africa 4485 photo
Found Summer of 2006
no coordinates recorded 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 the meteorite was classified as a unique KREEP-rich, basaltic melt breccia.

This is a polymict breccia composed of clasts from diverse lunar locations including mare basalt, High-Mg Suite (HMS), High-Alkali Suite (HAS), ferroan anorthosite (FAN), and impact-melt lithologies dispersed in the matrix. The 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. Other mineral fragments identified in NWA 4485 include olivine, pyroxene, plagioclase, ilmenite, chromite, K-feldspar, apatite, merrillite, silica, Fe-metal, and FeS, most reflecting derivation from a KREEP-rich precursor magma (Arai et al., 2009) referred to as urKREEP. The investigators observed that some of these KREEP basalt clasts exhibit chemical zoning and thick exsolution lamellae, attesting to slower cooling conditions at a deeper location compared to the Apollo mare basalts. The matrix also contains a variety of glasses, some containing vesicles and others taking the form of spherules enriched in P and K. In their study of apatite grains in NWA 4472, the pairing to NWA 4485, Tartèse et al. (2014) found that they contain moderate amounts of water, in the range of 2,000–6,000 ppm. Associated isotopic studies on the apatite demonstrated elevated δ37Cl values compared to terrestrial values, which suggests this meteorite has retained its original lunar isotopic signature. Moreover, they recognized that the δD values are consistent with lunar rocks associated with HMS, HAS, and KREEP-rich basalts.

The bulk composition of NWA 4485 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 lunaites—the impact-melt breccias Sayh al Uhaymir 169 and Dhofar 1442. SaU 169 contains ~4.0 b.y. old clasts containing very high-K KREEP which best reflect the composition of primordial urKREEP (Lin et al. 2010). Basaltic clasts in NWA 4472/4485 sample low-Ti 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 spherules, all consistent with lithification within the lunar regolith.

The composition of NWA 4472/4485 is similar to that of the Th-rich, mafic, LKFM (low-K Fra Mauro) impact-melt breccias recognized from the Apollo collection; specifically, group-C melt breccias of Apollo 15, group-1S melt breccias of Apollo 16, and the aphanitic and poikilitic impact-melt breccias of Apollo 17 (Korotev 2000). 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’, which created the Imbrium basin within the Th-rich Procellarum KREEP Terrane (PKT) (Korotev, 1999). The impactor is thought to have been an iron meteorite that 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 PKT, and with its high FeO (noritic) composition and incompatible element abundances (>10 ppm Sm; 5.9–7.9 ppm Th), NWA 4472/4485 is likely derived from this nearside region (Joy et al., 2008). Other possible source locations are northwest of Sinus Iridium within the Jura mountains, northwest of Sinus Roris at Herschel crater, regions of the Mons Alpes formation in western Mare Imbrium, regions of the Apennine mountains near Mons Caucasus and Mons Bradley near Apollo 15, and regions near the craters Ptolemaeus and Lalande, the latter suggested to be the source location of SaU 169.

Calzada-Diaz et al. (2015) compared compositional and age data from a large number of lunar meteorites with elemental remote sensing data obtained by the Lunar Prospector gamma ray spectrometer, primarily for Fe, Ti, and Th, to better constrain the meteorite’s source regions. For the KREEP-rich basaltic melt breccia NWA 4485/4472, plausible ejection sites were identified near the John Herschel crater and in Mons Caucasus, having a composition consistent with ejecta from the Imbrium basin (see image below).
Image credit: A. Calzada-Diaz et al.
MAPS, vol. 50, #2, p. 220 (2015)
‘Constraining the source regions of lunar meteorites using orbital geochemical data’
(http://dx.doi.org/10.1111/maps.12412)
Establishment of a thorough chronological history of this lunar rock following the initial basin-forming impact, and including the time spent on the lunar surface, in Moon–Earth transit, and in terrestrial residence has begun (Joy et al., 2009). Cosmogenic Ar–Ar data are indicative of a ~300 m.y. near-surface residence as part of the ancient lunar regolith. The Pb–Pb and U–Pb ages were calculated from the phosphates fluorapatite and merrillite in matrix and basalt clasts, as well as from zircon grains in the KREEP basalt component (Joy et al., 2011). The ages found within this regolith breccia reflect a diversity of lithic fragments with a wide range of crystallization ages (~4.35–3.94 b.y.); the younger ages in this range are consistent with those of Apollo KREEPy mare basalts possibly dating the formation of Mare Imbrium, while the oldest ages were derived from a matrix apatite grain and might reflect the crystallization of the HAS lithology. The lower Ar–Ar apparent age of 1.7–2.2 b.y. obtained for trapped solar wind Ar is thought to reflect a recent impact-resetting event which could represent the consolidation of the NWA 4472/4485 meteorite components.

A more thorough treatment of the chemical classification of lunar meteorites can be found on the WUSL—Lunar Meteorites website, including information on the other (unpaired) KREEP-rich meteorites SaU 169 and Dhofar 961/960/925/SaU 449, the KREEPy clast bearing meteorites Calcalong Creek and Y-983885, and the KREEP basalt meteorites comprising the NWA 733 pairing group, the LAP pairing group, and Dhofar 287a.

NWA 4472/4485 contains Sr and Ba indicative of terrestrial weathering. Both portions of the meteorite also contain high Br concentrations, suggesting that they were contaminated by seawater. The photo of NWA 4485 shown above is a 0.32 g partial slice, while that pictured below is the uncut mass as found (both photos courtesy of Chladni’s-Heirs.com).

standby for nwa 4485 photo
Photo courtesy of Chladni’s-Heirs.com


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NWA 4483

Lunar Feldspathic Breccia
(granulitic breccia)
standby for northwest africa 4483 photo
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
click on photo for a magnified view
42.387 g stone with primary fusion crust
nwa 4483
click on photo for a magnified view
42.387 g stone with secondary crust (‘green icing’)
nwa 4483
click on photo for a magnified view
NWA 4483 Group Photo
standby for northwest africa 4881 photo
Photos shown above courtesy of Chladni’s Heirs—Stefan Ralew & Martin Altmann


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NWA 773

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 773 photo
standby for northwest africa 773 photo
Found September 2000
~26° 49′ N., ~12° 49′ W. While visiting the Western Sahara, American meteorite collector M. Killgore purchased three meteorite fragments from local nomads. The three fragments, weighing 50 g, 224 g, and 359 g (totaling 633 g), were all recovered in close proximity, and it is apparent that they constitute a single meteorite. The find location was reported to be a desert plain near Dchira, Western Sahara. Subsequent paired finds include Anoual [5.92 g], NWA 2700 [31.7 g], NWA 2727, NWA 2977, NWA 3160 [34 g], NWA 3170 [60 g, photo courtesy of S. Ralew], NWA 3333 [33 g], NWA 6950 [1,649 g], NWA 7007 [91 g, photo credit: Kuehner et al., 2012], NWA 8127 [529 g], NWA 10656 [262.5 g, diabase, photo credit: Valencia et al., 2017, #2483], and NWA 10985 [250 g]. The individual meteorites composing this lunar pairing group represent a wide variation in composition including both extrusive and intrusive lithologies.

Among the twelve currently known members of the NWA 773 clan, Valencia et al. (2017) recognized five distinct lithologies along with a fragmental and/or regolith breccia. Four intrusive lithologies represent a crystallization sequence from more magnesian to more ferroan as follows: olivine gabbro → anorthositic gabbro → gabbro → ferroan gabbro; a fifth extrusive lithology is an olivine-phyric basalt. 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. For more information and photos of this lunar pairing group see Korotev–WUSL.

Northwest Africa 773 is primarily composed of two distinct lithologies: a light-green magnesian olivine gabbro cumulate, and a dark-colored polymict impact breccia containing a regolith component that comprises gabbroic, basaltic, and volcanic elements. The diverse clasts composing the breccia lithology, considered likely derived from a common magma unit, were grouped into four categories by Fagan et al. (2014): olivine gabbro cumulate (OC); pyroxene–gabbro clasts (PG); symplectite (hedenbergite+fayalite+SiO2) clasts (S); and alkali-phase-ferroan clasts (AF). Boundary textures between the two lithologies in NWA 773 indicate that the cumulate lithology was a large clast within the breccia lithology (Fagan et al., 2003). Several independent analyses of a number of samples have determined a range of mineral compositions for the cumulate portion. It has a modal composition of 48–66% olivine, 26–40% clinopyroxene (comprising both low-Ca pigeonite and high-Ca augite), 8–15% interstitial plagioclase, 2% orthopyroxene, and ~1.5% K-feldspar, ilmenite, and RE-merrillite, along with trace amounts of Ba-rich K-feldspar (hyalophane), Cr-spinel, and troilite–FeNi-metal assemblages. Both the olivine gabbro cumulate and the breccia lithologies are reported to be enriched in incompatible trace elements. A breccia component enriched in incompatible trace elements has also been described in the paired stones NWA 3160 and NWA 3170.

The polymict breccia portion of NWA 773 has a fragmental texture that contains dispersed fragments of both magnesian and ferroan olivine gabbro, Ti-poor (VLT) olivine-phyric mare basalt, and Fe-rich lithic clasts comprising fayalite gabbros and fayalite granites, with both of the latter representing late-stage differentiates. Other evolved clasts present in the breccia include FeO-rich symplectites (fayalite+hedenbergite+silica) that can be formed by two different processes: 1) breakdown of pyroxferroite (a pyroxenoid formed by rapid cooling of a Mg-depleted melt), and 2) quenching of a silicate liquid. The second formation pathway of direct quenching of a melt is favored for NWA 773 due to the presence of feldspathic components in the silica (Fagan et al., 2003). Other unusual clast types have been identified, one of which contains fayalite, hyalophane, silica, and plagioclase, while another containsg silica, K-feldspar, plagioclase, troilite, baddeleyite, and RE-merrillite. Mineral fragments present include fayalite, silica glass, agglutinate glass, and hedenbergitic pyroxene. The partial conversion of feldspar to maskelynite in NWA 773 reflects a low shock stage of S2, and the presence of some minor calcite filling fractures indicates only minor alteration consistent with a weathering grade of W1.

Although variable mineral proportions observed in studied samples initially indicated that the cumulate portion was a norite or gabbronorite, the dominance of clinopyroxene over orthopyroxene in the plagioclase–orthopyroxene–clinopyroxene ternary diagram (Stöffler et al., 1980) suggests that this lithology is actually a gabbro. In a similar way, when the high proportion of olivine in this cumulate lithology is entered on the plagioclase–pyroxene–olivine ternary diagram (Stöffler et al., 1980), it indicates that this is an olivine gabbro. The plagioclase content of 14.2 vol% differentiates this rock from a peridotite (requiring <10 vol% plagioclase).

The olivine and pyroxene clasts within the breccia component exhibit a wide range of Fe contents constituting a magmatic sequence progressing from the magnesian olivine gabbro cumulate to an extreme enrichment in Fe# in the ferroan symplectite and silica-bearing FeO-alkali clasts; these ferroan clasts have been compared by some researchers to terrestrial plutonic tholeiitic lithologies (Fagan, et al., 2002, 2013). The more ferroan olivine gabbro clasts contain no anorthosite (i.e., no highlands component) and have an LREE-enriched pattern with a strong Eu depletion—an unusual composition for VLT basalts, but one which exhibits some similarities to Apollo 14 basalts.

Although the olivine gabbro lithology lacks solar noble gases, the breccia lithology exhibits an enhancement in the solar gas content indicating that only the breccia resided at the lunar surface for any significant time prior to ejection. The CRE ages of these two lithologies support this finding: 5.2 (±0.8) m.y. for the olivine gabbro and 68 m.y. for the breccia lithology. This age for the breccia is equivalent to a surface residence of 136 m.y. Lorenzetti et al. (2005) argue that after the breccia lithology was exposed near the surface, it was mixed with the cumulate lithology during an impact event and was subsequently buried at a depth of a few meters for the relatively short span of 100 (±20) m.y. The Moon–Earth transit time is considered to have been <30 t.y., and according to Nishiizumi and Caffee (2010) such a short transit time corresponds to a launch event from a more shallow depth of <1–4.7 m.

A detailed petrogenetic model for mare basalts was presented by J. Day and L. Taylor (2007), a synopsis for which can be found on the NWA 032 page. This model, which demonstrates that NWA 032/479 could be launch paired with the Antarctic LaPaz (LAP) 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 and LAP samples (Hallis et al., 2007). 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, whereas the olivine basalt component, well represented in NWA 3160, 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. The NWA 6950 pairing member has a crystallization age that is ~100 m.y. older than NWA 773, inferring a possible faster-cooling position in the cumulate pile for this rock (Shaulis et al., 2012). In-depth studies of the NWA 032/NWA 4734/LAP pairing group mare basalts conducted by Elardo et al. (2014) led them to conclude that these meteorites were formed in a non-KREEPy source reservoir as opposed to the KREEPy source of the NWA 773 pairing group, ruling out any close relationship between them. However, it remains a possibility that the KREEP-rich component present in members of the NWA 773 clan is a late addition incorporating ejecta from a more distant impact.

The K–Ar chronometer associated with NWA 773 and other pairings reflects an age of 2.75 (±0.3) b.y. (2.865 ±0.031 b.y. calculated from Sm–Nd isochron), possibly indicating a late crystallization age (Burgess et al., 2007). The weighted average Pb–Pb age derived from baddeleyite grains from NWA 773 and other pairings, reflecting primary crystallization of the various pairings of this clan, was determined to be 3.1156 (±0.0068) b.y. (Shaulis et al., 2012, 2013, 2017). The matching Pb–Pb ages for both magnesian and ferroan olivine gabbro cumulate components indicate that they formed during the same time period, while the comparative ages of the olivine gabbro lithologies and the polymict breccia attest to the fact that the constituents of the latter were derived from the former. Furthermore, the chronological dataset on the whole is consistent with all members of the NWA 773 clan being magmatically related (Shaulis et al., 2013, 2017). The NWA 773 clan lunaites have one of the youngest ages measured for a lunar rock with only a few exceptions, such as the low-Ti mare basalt LAP 02205 dated at 2.991 (±0.014) b.y.; Rankenburg et al., 2007).

A possible scenario for the formation of the olivine gabbro begins with the formation of a lunar magma ocean (LMO) to a depth of 400–500 km, and possibly as deep as 1,000 km (Ranen and Jacobsen, 2004). As cooling proceeded, differences in density began to show their effect. After 75% crystallization of the magma ocean, buoyant plagioclase-rich rock began to crystallize and float to the surface to form the original anorthite-rich plagioclase crust constituting the uppermost ~5–30 km of the magma ocean. At the same time, an increasingly more dense (i.e., a gradual decreasing ratio of Mg to Fe) and mafic, compositionally-zoned lower crust was accumulated at depths of ~25–55 km. Thereafter, an unstable configuration resulted as rock of a higher density lay above rock of a lower density, which resulted in convective overturn. This event in turn initiated the pressure-release remelting of early magma ocean olivine- and orthopyroxene-rich cumulates.

An anomalous KREEP-rich region of limited extent (16% of the surface), known as the Procellarum KREEP Terrane (PKT), was formed during the final phase of LMO solidification through extreme (>99.5%) fractional crystallization which occurred 4.492 (±0.061) b.y. ago. (Korotev 2005). This region is also thought to be the source of an ultramafic upper mantle melt that was the parent magma of the Mg-suite rocks. Through studies of Sm–Nd data from lunar samples, it was determined that solidification of the LMO was complete in 60–200 m.y. after its onset, at least by 4.417 (±0.006) b.y. ago (Boyet and Carlson, 2007; Grange et al., 2009). Based on Pb–Pb dating of zircon crystals, this LMO crystallization interval has been refined to 100 m.y. (Nemchin et al., 2009). Nemchin et al. (2009) determined that the 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. Based on zircon crystallization studies, it was determined by Grange et al. (2009) that all magmatic activity in at least some locations (e.g., Serenitatis) had completely ended by 4.2 b.y. ago. According to Crow et al. (2011), the Pb–Pb ages of Apollo zircons show a peak at ~4.33 b.y. A somewhat younger Pb–Pb age of 3.953 (±0.018) b.y. was found for a large zircon grain located in the breccia lithology of NWA 773; this attests to the incorporation of some older material located in close proximity to the brecciation event (Shaulis et al., 2017).

It is possible that the parent magma assimilated material containing a high-K KREEP composition and a high LREE/HREE ratio, or alternatively, material with a high RE-merrillite composition. This assimilation produced the high incompatible element abundances present in the later-formed rocks. In contrast to this assimilation scenario, it was suggested by Shearer et al. (2005) that the observed compositional diversity of KREEP-rich magmas is more consistent with the addition of the KREEP component to the basaltic magma during the melting phase, prior to olivine crystallization. In support of the source mixing model, Borg et al. (2005) concluded that the 87Rb–86Sr ratio of NWA 773 would require 22% KREEP assimilation by the parental magma compared to only a 2% KREEP addition to the source magma. The low Fe content and high Mg# determined for NWA 773 are more consistent with a lower proportion of KREEP as predicted by the source mixing model. Nevertheless, it is now being considered by some that the KREEP component was incorporated into those specific lunar samples through a late impact into the Procellarum KREEP Terrane.

The parent magma underwent differentiation by fractional crystallization, and a Ti-containing cumulus ilmenite was gravitationally extracted. The magma eventually formed plutons of Mg-suite material (petrogenetically distinct from the Mg-suite material from the PKT region), which then intruded the lower crust in some regions of the Feldspathic Highlands Terrane. This Fe-enriched magma may have been part of a shallow layered intrusive, or possibly a thick differentiated lava flow. Crystallization occurred as the melt cooled from ~1200°C to 1050°C. The upper mantle, composed of ultramafic, olivine-rich dunite or harzburgite, may have contributed melt material to the crystallization process of the Mg-suite rocks. The crystallization sequence from the base of the crust upwards was inferred as follows: dunite (>90 vol% olivine) → troctolite (magnesian olivine + 10–60 vol% plagioclase) → norite (low-Ca orthopyroxene + 10–60 vol% plagioclase) → gabbro (high-Ca clinopyroxene + 10–60 vol% plagioclase) → anorthosite (>90 vol% plagioclase). This lunar crystallization sequence is unlike that of any terrestrial oceanic or continental basalt; in the terrestrial case, gabbroic high-Ca pyroxene crystallizes before noritic low-Ca pyroxene. <!– As a result of the proportional variability inherent in the plagioclase–orthopyroxene–clinopyroxene ternary system, a broader terminology may be utilized:

1.anorthosite
2.gabbroic anorthosite
3.noritic anorthosite
4.troctolitic anorthosite
5.anorthositic gabbro
6.anorthositic gabbro-norite
7.anorthositic norite
8.anorthositic troctolite
9.gabbro
10.noritic gabbro
11.gabbroic norite
12.norite
13.troctolite
14.pyroxenite
15.peridotite
16.dunite
–> The International Union of Geological Sciences—Subcommission on the Systematics of Igneous Rocks, having established a Working Party on the classification of lunar rocks, has adopted a Classification System for Lunar Rocks.

Northwest Africa 773 is probably derived from an upper mantle or lower-crustal parent melt that was mixed with a KREEP-rich melt component, possibly within a deep pluton. This mixture later intruded into upper crustal rock where crystallization under relatively rapid cooling conditions occurred within a shallow dike or sill (a hypabyssal rocktype), or possibly within a thick lava pool. Elemental distributions within the cumulate lithology provide evidence of cooling rates consistent with this scenario. Ultimately, differentiation of the melt produced the various components that are incorporated into the NWA 773 breccia lithology.

Jolliff et al. (2003) have found that close compositional similarities exist between NWA 773 and Apollo 14 Green Glass B, Type 1, and they suggest that NWA 773 may have originated from a parent melt in proximity to the source region of these picritic green volcanic glasses—located within the Procellarum KREEP Terrane (PKT). It was proposed that the high KREEP concentration was incorporated as the melt transited from the mantle to the surface. As the melt cooled near the surface, crystallization of olivine and Ca-rich pyroxene was initiated and a trapped melt component of 15–25 vol% was incorporated. Various surface volcanic basaltic lithologies were mixed at this stage to produce the NWA 773 impact-breccia lithology. The high concentration of the heat-producing elements Th, U and K present in the PKT region could have permitted an extended period of melting and mixing that is consistent with the young age of NWA 773; however, VLT material has not been identified in significant amounts in this region.

In the PSRD article ‘Damp Moon Rising’ by G. Jeffrey Taylor (July 2010), it was described how studies at the Carnegie Institute of Washington (McCubbin et al., 2010) and Okayama University in Japan (Yamashita et al., 2010) employed a technique called ‘hydrogen manometry’ to construct an OH calibration curve from apatite standards. This curve was then employed to determine the amount of water present in specific lunar meteorites such as NWA 773. Fluorapatite in NWA 773 and pairings contains 0.4 to 0.7 wt% (4,000–7,000 ppm) water, significantly more than previously supposed. This converts to a minimum of 0.7 to 1.7 wt% (7,000–17,000 ppm) water in the KREEP-bearing, late-stage magma from which NWA 773 and pairings were derived. In their study of apatite grains in NWA 773, Tartèse et al. (2014) found significantly higher water contents, and determined that the apatites could be resolved into two distinct groups: one characterized by moderate amounts of H2O in the range of 700–2,500 ppm, and another having high to extremely-high amounts of H2O in the range of 5,400–16,700 ppm. The extraordinarily high water content in some apatite grains of the brecciated lithology of NWA 773 has been conjectured to be related to silicate liquid immiscibility, possibly as a result of depletion of F and Cl in the Si-rich melt fraction.

For their calculations, Yamashita et al. (2010) presumed that apatite would have formed from such an evolved magma only after 90–95% crystallization, so they argue that the original basaltic magma would have proportionally contained 360–850 ppm water. Moreover, given the reasonable scenario in which 10% partial melting of the lunar interior occurred, the unmelted interior source region which hosted NWA 773 and pairings would have contained 7–17 ppm water. Although no longer thought to be dry, the Moon certainly contains much less water than the 500–1,000 ppm incorporated in the Earth.

Through remote sensing technology aboard the Clementine spacecraft, utilizing multispectral reflectance imaging, measurements of the mafic minerals olivine, pyroxene, and plagioclase feldspar, and indirectly, anorthosite, have been performed across nearly the entire lunar surface. Notwithstanding the proposal by Jolliff et al. suggesting that the PKT region is a possible source location for NWA 773, a different gabbro-containing site located on the far side of the Moon, in the South Pole–Aitken (SPA) basin, has been identified by Clementine. This 2,500 km-diameter impact structure has had its upper crust completely removed, and a homogeneous melt sheet was formed. This large basin is thought to preserve ancient crustal rock that is mostly uncontaminated by subsequent basin ejecta transported over the Moon’s surface (Petro and Pieters (2008). Subsequent impact events onto the SPA basin, such as those which formed the 64-km-diameter Bhabha crater and the 505-km-diameter Apollo basin, have excavated lower-crustal material from depths greater than 20 km. Although SPA is predominantly noritic in composition, a small rise known as ‘Olivine Hill’ is interpreted to be an olivine gabbro lithology. Volcanic activity associated with small mare ponds that occurred after basin formation is consistent with the presence of the VLT basaltic component identified in the NWA 773 breccia.

Calzada-Diaz et al. (2015) compared compositional and age data from a large number of lunar meteorites with elemental remote sensing data obtained by the Lunar Prospector gamma ray spectrometer, primarily for Fe, Ti, and Th, to better constrain the meteorite’s source regions. For the basaltic breccia NWA 773, plausible ejection sites were identified in Mare Serenitatis, Mare Crisium, and the western boundary regions of Oceanus Procellarum, while Mare Fecunditatis was found to be inconsistent with the age data (see image below).
Image credit: A. Calzada-Diaz et al.
MAPS, vol. 50, #2, p. 219 (2015)
‘Constraining the source regions of lunar meteorites using orbital geochemical data’
(http://dx.doi.org/10.1111/maps.12412)
The lunar meteorites Y-793274, QUE 94281, and EET 87521/96008 share many compositional characteristics with NWA 773 and may have experienced a similar petrogenesis. With the recovery of NWA 773, representing lower-crustal olivine gabbro, our ability to understand the Moon’s early history has been greatly enhanced. The top photo above shows both the cumulate lithology and the breccia which constitute the NWA 773 meteorite. The specimen on the left is a 0.094 g cut fragment of the dark-colored polymict breccia component, pervaded by fragments of the cumulate lithology and other diverse mineral and lithic clasts, while the specimen on the right is a 0.085 g cut fragment of the olivine gabbro cumulate component consisting of green to tan olivine crystals within pyroxene, interspersed with black chromite grains and transected by a small shock-melt vein. An enlarged photo of this cumulate specimen is shown as well.