NORTHWEST AFRICA 12319


Achondrite, ungrouped
Lherzolite breccia
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Found 2018
no coordinates recorded

Thirteen partially crusted fragments and other broken bits having a combined weight of 35.3 g were found in 2018 and subsequently purchased by A. Mazur. Samples were sent for analysis and classification to the Bartoschewitz Meteorite Laboratory (R. Bartoschewitz) and the Geological and Mineralogical Museum at the University of Kiel (P. Appel and B. Mader). Following an O-isotopic analysis conducted at the University of Göttingen (A. Pack), it was determined that NWA 12319 should be classified as an ungrouped achondrite.

In 2015, prior to the purchase and classification of NWA 12319, a 148 g stone designated NWA 12217 was purchased by J. Piatek. The meteorite was analyzed at the University of New Mexico (C. Agee), including an oxygen isotope analysis (K. Ziegler), and it was classified as an ungrouped achondrite, dunite breccia (see photo below).

Cut Slice of NWA 12217
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Photo credit: Vaci et al., 50th LPSC, #1175 (2019)

Besides NWA 12217, a 3,930 g stone designated NWA 12562 was found in 2017 and subsequently purchased in May 2018 by Z. Ke. The meteorite was analyzed at the Central South University in China (X. Gu) and the University of New Mexico (Z. Vaci and C. Agee; oxygen isotopes by K. Ziegler), and it was also classified as an ungrouped achondrite, dunite breccia, although its reported modal olivine content of less than 90% would indicate a lherzolite breccia similar to NWA 12319. The MetBull description of NWA 12562 shows that it shares many similarities with NWA 12217 (see photo below).

Complete Mass of NWA 12562
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Photo courtesy of Zuokai Ke

Other material in the possession of A. Habibi, including a 338 g individual, a crusted fragment weighing 129 g, and numerous small fragments weighing together 114.5 g, are part of this pairing group but have not yet been given a NWA-series designation. A probable paired stone designated NWA 13954 with a weight of 4,500 g was purchased by J. Piatek in 2020. This mass has a reported modal olivine content of 90% consistent with a lherzolite. In addition, the 174 g NWA 14993 is likely a member of this pairing group.

Even though some textural differences exist among these independently classified meteorites, and while they encompass a broad range of major element and O- and Cr-isotopic compositions (see diagrams below), Vaci et al. (2021) contend that they represent a common pairing group from a unique parent body. The authors note the slightly higher Δ17O value for the NWA 12217 meteorite compared to the others, and ascribe this deviation to the incorporation of an ordinary chondrite component to the parental source.

Oxygen and Chromium Isotope Systematics for NWA 12217, 12319, 12562, and 13954
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Diagram credit: Vaci et al., 84th MetSoc, #6067 (2021)

ε54Cr–Δ17O Diagram for Meteorites Related to V-type Asteroids
NWA 12217, 12319, and 12562; EET 92023; Main-Group Pallasites
(note that chromites in IIIAB Sacramento Mountains show no genetic linkage)
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Diagram credit: Vaci et al., Nature Communications, vol. 12, #5443 (2021, open access link)
Adapted by Dey and Yin, 53rd LPSC, #2428 (2022)

Utilizing a coupled ε54Cr vs. Δ17O diagram, Anand et al. (2022) demonstrated that a possible genetic link may exist between the dunite/lherzolite cumulate breccias NWA 12217, NWA 12319, and NWA 12562 and the ungrouped andesitic achondrite Erg Chech 002, and/or the unbrecciated anomalous eucrite EET 92023, and/or the main-group pallasites (MGP). If any of these various meteorites do share a genetic link, the differences in their respective O and Cr isotopes may possibly be due to varying degrees of contamination of their parental V-type asteroid (possibly Vesta) by an H-type chondritic impactor (see diagram below).

ε54Cr–Δ17O Diagram for Erg Chech 002 and Possible Relatives
(note that chromites in IIIAB Cape York plot with HEDs but distant from EC 002 and MGP)
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Diagram credit: Anand et al., vol. 57, #11, p. 2011, fig. 6b (2022, open access link)
'53Mn–53Cr chronology and ε54Cr-Δ17O genealogy of Erg Chech 002: the oldest andesite in the Solar System'
(https://doi.org/10.48550/arXiv.2209.05381)
(MAPS: https://doi.org/10.1111/maps.13916)

Northwest Africa 12217, NWA 12319, and NWA 12562 have similar petrographic characteristics and have been described as fragmental breccias. Despite the fact that there is significant grain-scale compositional variability, these breccias are considered to reflect a monomict rock type (Vaci et al., 2019, #6459; Vaci et al., 2021). The meteorites are composed primarily of large (up to cm-sized), angular, cream-colored olivine grains that are crosscut by dark shock fractures. The olivines are embedded in and possibly sintered together with darker gray comminuted material (down to µm-sized). Olivine has a modal abundance of 85–93% (NWA 12562 and NWA 12217, respectively) consistent with a lherzolite (40–90%) or dunite (>90%) in the BGS Rock Classification Scheme (see diagram). Other phases present in minor or trace abundances include pyroxene, troilite, chromite, FeNi-metal, tridymite, andesine and alkali feldspar, schreibersite, pentlandite, merrillite, fluorapatite, and ilmenite. Secondary Cr-bearing symplectites associated with silicates were observed in each of the meteorite breccias (NWA 12217, 12319, and 12562). These symplectites were determined by Vaci et al. (2021) to be nearly identical to the still enigmatic symplectites associated with Mg-rich harzburgite clasts in howardites.

On an Fe/Mn vs. Fe/Mg coupled diagram, Vaci et al. (2019) demonstrated that NWA 12217 olivines have values that are consistent with formation as a cumulate from an igneous differentiation process involving fractional crystallization, rather than formation as a partial melt residue (see diagram below). Importantly, the Mg-rich harzburgite clasts in howardites overlap with the NWA 12217, 12319, and 12562 breccias.

Fe/Mg vs. Fe/Mn for NWA 12217, 12319, and 12562
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Diagram credit: Vaci et al., Nature Communications, vol. 12, #5443 (2021, open access link)
'Olivine-rich achondrites from Vesta and the missing mantle problem'
(https://doi.org/10.1038/s41467-021-25808-9)

Pang and Du (2021) identified a clast in NWA 12319 that contains phosphoran olivine as part of a mineral assemblage composed of troilite, chromite, nickelphosphide, merrillite, and orthopyroxene. They infer that this P-rich olivine, which has a similar high forsterite content (Fo86–89) as that observed in some pallasites, was formed by fractional crystallization of residual melt at the core-mantle boundary.

It is noteworthy that the unique 1.1 g olivine-rich (dunitic/harzburgitic?) achondrite QUE 93148, originally classified as a lodranite, has O-isotope and trace element data which has led some investigators (Goodrich and Righter, 2000; C. Floss, 2003) to suggest it might instead be derived from the deep mantle of the HED parent body. 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, some other teams (Shearer et al., 2008; Shearer et al., 2010) suggest that QUE 93148 originated on a distinct planetary body such as that of the main-group pallasites. Furthermore, Hahn et al. (2018) studied orthopyroxene–chromite symplectites in QUE 93148 as well as its geochemistry, and they contend that the meteorite is probably a mantle residue derived from a high degree of melting and is likely related to harzburgite clasts present in some howardites. Yet, the symplectites in QUE 93148 have been compared by Vaci et al. (2020, #1751) to those in NWA 12217 and NWA 12562, and they argue that the similar mineralogy and petrology of these meteorites suggest a possible genetic link.

Vaci et al. (2021) suggest that the many mineralogical, petrological, and isotopic similarities that exist among these olivine-rich achondrites, as well as between them and the normal HEDs (diogenite olivine and howardite harzburgite clasts in particular), indicate they all likely originated on Vesta or the Vestoids. They also contemplate that further dunite/lherzolite meteorite recoveries should only be a matter of time. The specimen of NWA 12319 shown above is a 0.87 g fragment. This specimen is also shown below with two larger fragments and the main mass representing this NWA classification.

NWA 12319 Fragments: (l–r) 4.28 g, main mass, 0.87 g, 3.74 g
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Photo courtesy of Alan Mazur