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NWA 3329

Diogenite
Orthopyroxenite
(≥90 vol% orthopyroxene)

standby for northwest africa 3329 photo
Purchased Spring 2005
no coordinates recorded A fragmented stone weighing 252 g was collected in Algeria from a similar location as NWA 2968. The fragments were subsequently purchased in Er-Rachidia, Morocco by collector F. Kuntz. A sample of these fragments was analyzed at the University of Washington at Seattle (A. Irving and S. Kuehner) and NWA 3329 was determined to be a diogenite composed primarily of coarse-grained, dark brown orthopyroxene, together with interstitial plagioclase, silica, phosphate, FeNi-metal, and FeS.

The NWA 3329 fragments were subsequently studied by Barrat et al. (2010). Interestingly, some fragments from the batch were found to be identical to the dunitic diogenite NWA 2968, and petrographic evidence indicates that both lithologies were collected from the same location, with both showing similar degrees of weathering. Importantly, other recovered fragments consist of both lithologies together—diogenite and dunite—which are each identical to their respective type samples. Furthermore, trace element studies conducted on both lithologies were found to be consistent with pairing, and their Δ17O values are indistinguishable as well (Greenwood et al., 2015). These investigators, in accord with an earlier suggestion by Barrat et al. (2010), interpret the evidence to indicate that both the orthopyroxenite and dunite lithologies are fragments from a common fall, likely as components of a mesosiderite. It was also noted by Greenwood et al. (2015) that the REE pattern previously determined for the diogenite NWA 5613 (Barrat et al., 2010) is virtually identical to that for NWA 3329. Further evidence for a mesosiderite–HED genetic relationship is revealed by the identical Δ17O values among the diogenites, such as NWA 3329 and NWA 2968, and the olivine-rich (dunite) clasts that have been identified in most all mesosiderites (Greenwood et al., 2015, 2017). It is considered likely that the dunitic clasts in mesosiderites were initially formed as upper-crustal plutonic cumulates, which were subsequently disrupted through impacts and incorporated with other HED lithologies prior to the formation of the mesosiderites. standby for greenwood diagram
Diagram credit: Greenwood et al., 2015
For an explanation of the diagram components see the open access article in GCA, vol. 169, p. 130 (2015)
Geochemistry and oxygen isotope composition of main-group pallasites and olivine-rich clasts in mesosiderites:
Implications for the “Great Dunite Shortage ” and HED-mesosiderite connection’
(https://doi.org/10.1016/j.gca.2015.07.023)

standby for o-isotopic diagram
Diagram credit: Greenwood et al., 2017
For an explanation of the diagram components see the open access article in Chemie der Erde – Geochemistry, vol. 77, p. 25 (2017)
‘Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies’
(http://dx.doi.org/10.1016/j.chemer.2016.09.005)
A comparison of reflectance spectra of seven near-Earth asteroids to those of HED-group meteorites revealed that all of the pyroxene mineralogies were consistent with eucrites and howardites, but not to diogenites. Therefore, they suggest that there are no km-sized or larger objects composed strictly of diogenite material, but instead, diogenites might exist as a single component within a mixture of lithologies on the HED asteroid. Beck et al. (2012) identified the first olivine-rich melt material present in the howardites of the PCA 02009 pairing group. This olivine-rich material was likely derived from harzburgitic and dunitic lithologies exposed on the surface of Vesta. Further investigation employing the Antarctic DOM 10 howardite pairing group was conducted by Hahn et al. (2018). They sought to identify Mg-rich harzburgitic (distinguished from diogenitic) silicates (Mg# >80 and >85 for olivine and pyroxene, respectively) that represent HED mantle material. From results of a comprehensive geochemical analysis, they contend that these Mg-rich fragments are not related to cumulate diogenites, but instead are more consistent with a mantle residue that was affected by a late infiltration of metasomatic melt. In addition, they determined that QUE 93148 also likely represents a mantle residue from the HED parent body. The larger degree of partial melting (~35–55%) required to produce the observed Mg-rich lithologies, considered to be mantle residua, is attributed by Hahn et al. (2018) to a hybrid magma ocean model that combines aspects of the magma ocean model of Mandler and Elkins-Tanton (2013) and the shallow magma ocean model of Neumann et al. (2014) (see diagrams B and D below). standby for magma ocean diagrams
click on image for a magnified view

Diagram credit: Hahn et al., MAPS, vol. 53, #3, p. 541 (2018)
‘Mg-rich harzburgites from Vesta: Mantle residua or cumulates from planetary differentiation?’
(http://dx.doi.org/10.1111/maps.13036)
Further information regarding the origin of the dunitic clasts in our collections can be found on the Vaca Muerta page. To see an alternative classification system for the diogenites and dunites based on mineralogical and petrographical features, proposed by Beck and McSween (2010) and modified by Wittke et al. (2011), click here. The photo shown above is a 0.52g fragment of NWA 3329. The photo below is an excellent petrographic thin section micrograph of NWA 3329, shown courtesy of Peter Marmet. standby for nwa 3329 ts photo
click on image for a magnified view
Photo courtesy of Peter Marmet


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NWA 2968

Diogenite
Dunite
(Ungrouped achondrite in MetBull 91)

standby for northwest africa 2968 photo
Purchased November 2005
no coordinates recorded

Through the untiring efforts of nomads searching the vast Sahara Desert region of Algeria, a remarkable new meteorite type has been recovered. Only 268 g of small broken fragments of a coarse-grained, dark brown meteorite was found, and these fragments were subsequently sold in Erfoud, Morocco to meteorite dealer B. Reed. The small size range of these fragments (17 mm to at least 25 mm wide) is due to fracturing along compression and shear zones. Numerous pieces of this meteorite were submitted for analysis and classification (T. Bunch and J. Wittke, NAU; A. Irving, UWS; D. Rumble III, CIW).

Northwest Africa 2968 is a cumulate, olivine-rich (>95 vol%), dunitic rock, containing minor amounts of orthopyroxene and FeNi-metal, along with troilite and pyrrhotite which primarily fill fractures. Olivines exhibit shock features including domain offsets, mosaicism, and undulatory extinction. The FeO/MnO ratios and O-isotopic compositions of NWA 2968 are consistent with an origin from the howardite/eucrite/diogenite/ (HED) parent body, widely accepted to be the asteroid 4 Vesta. The averaged Δ17O value of –0.23 (±0.02) plots within the field of the HED group (Scott et al., 2009); see a linearized O-isotope plot (Miller, 2002). However, in contrast to the olivine and orthopyroxene in known howardites or diogenites, these minerals are considerably more highly magnesian in NWA 2968 (92.5 and 93, respectively), and likely crystallized from a less evolved parental melt. The composition and mineralogy of NWA 2968 is consistent with a mantle or lower crustal origin on a differentiated body, in a formation region analogous to that of the chassignites on Mars. standby for greenwood diagram
Diagram credit: Greenwood et al., 2015
For an explanation of the diagram components see the open access article in GCA, vol. 169, p. 130 (2015)
Geochemistry and oxygen isotope composition of main-group pallasites and olivine-rich clasts in mesosiderites:
Implications for the “Great Dunite Shortage ” and HED-mesosiderite connection’
(https://doi.org/10.1016/j.gca.2015.07.023)

standby for o-isotopic diagram
Diagram credit: Greenwood et al., 2017
For an explanation of the diagram components see the open access article in Chemie der Erde – Geochemistry, vol. 77, p. 25 (2017)
‘Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies’
(http://dx.doi.org/10.1016/j.chemer.2016.09.005)
It is still unresolved whether such dunitic lithologies represent higher-level cumulates or if they are instead mantle material (Greenwood et al., 2015). Mandler and Elkins-Tanton (2013) proposed a formation scenario for such dunites that involves a two-stage crystallization process: first, an equilibrium crystallization process from the late-stage liquid after 60–70% solidification of the global magma ocean; second, a fractional crystallization process within an ascended, high-level (crustal) pluton composed of the former extracted residual melt, ultimately resulting in the formation of a thin lower-crustal dunite layer along with more shallow olivine diogenite, diogenite, and cumulate eucrite lithologies. On the other hand, if the dunitic clasts are actually derived from mantle material, a scenario is required to explain how such material was incorporated into the regolith. However, it was argued by Barrat and Yamaguchi (2014) that magma chamber processes are unable to explain the chemical diversity of the diogenites (e.g., the range of heavy-REE ratios in diogenitic orthopyroxenes), and that neither assimilation of wallrock nor incorporation of a trapped melt component can account for this diversity. They contend that the diversity is more likely the result of variability in the respective initial parental melt compositions.

It is noteworthy that through continuing studies of MIL 03443, which is a cumulate, monomict, brecciated dunite previously classified as a mesosiderite clast, strong evidence has been developed for an origin on the HED parent body, and a relationship to diogenites specifically (Mittlefehldt, 2008; Beck et al., 2011). This evidence includes FeO/MnO and Δ17O values (see plot from Greenwood et al., 2015), the occurrence of olivine melt inclusions, and the abundances of pyrrhotite, Ni and Co. MIL 03443 has been shown to represent a fractional cumulate rather than a mantle restite (Beck et al., 2011). In a similar way, O-isotopic and trace element data for the unique 1.1 g olivine-rich (dunitic/harzburgitic?) achondrite QUE 93148 have led to the suggestion that it might be derived from the deep mantle of the HED parent body (Goodrich and Righter, 2000; C. Floss, 2003). However, due to its lower Co and Ni abundances than what would otherwise be expected for an olivine-rich mantle lithology or magma ocean cumulate, QUE 93148 could have actually originated on a distinct planetary body such as that of the main-group pallasites (Shearer et al., 2008; Shearer et al., 2010). Two other possible HED-related dunites, NWA 5784 and NWA 5968, will require further study to accurately assess their classification.

Notably, Beck et al. (2012) identified the first olivine-rich melt material present in the howardites of the PCA 02009 pairing group. This olivine-rich material was likely derived from harzburgitic and dunitic lithologies exposed on the surface of Vesta. Further investigation employing the Antarctic DOM 10 howardite pairing group was conducted by Hahn et al. (2018). They sought to identify Mg-rich harzburgitic (distinguished from diogenitic) silicates (Mg# >80 and >85 for olivine and pyroxene, respectively) that represent HED mantle material. From results of a comprehensive geochemical analysis, they contend that these Mg-rich fragments are not related to cumulate diogenites, but instead are more consistent with a mantle residue that was affected by a late infiltration of metasomatic melt. In addition, they determined that QUE 93148 also likely represents a mantle residue from the HED parent body. The larger degree of partial melting (~35–55%) required to produce the observed Mg-rich lithologies, considered to be mantle residua, is attributed by Hahn et al. (2018) to a hybrid magma ocean model that combines aspects of the magma ocean model of Mandler and Elkins-Tanton (2013) and the shallow magma ocean model of Neumann et al. (2014) (see diagrams B and D below). standby for magma ocean diagrams
click on diagrams for a magnified view

Diagram credit: Hahn et al., MAPS, vol. 53, #3, p. 541 (2018)
‘Mg-rich harzburgites from Vesta: Mantle residua or cumulates from planetary differentiation?’
(http://dx.doi.org/10.1111/maps.13036)
For further information about this dunitic meteorite and its potential pairing relationships see the NWA 3329 page. An alternative classification system for the diogenites based on mineralogical and petrographical features has been proposed by Beck and McSween (2010), and modified by Wittke et al. (2011). The photo shown above is a 5.8 g fragment of NWA 2968.


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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