EL3/6, impact-melt rock
"Fossil" or "Paleo" Meteorite*
(EL6/7 in MetBull 91; aubrite pairing in MetBull 92) Revised classifications have been submitted to NomCom and MetBull
click on photo for a magnified view
Purchased August 2005
27° 30' N., 12° 30' W.
Numerous individual pieces of an extensively weathered "relict"* or fossil meteorite, weighing together as much as 3,000 kg, were found in Western Sahara, reportedly near the village of Al Haggounia. Stones of various sizes were found both on top of the surface and buried under the soil in geological strata associated with both quaternary limestones and cretaceous limestones. This is evidence that the fall occurred later than the deposition of these strata (Chennaoui et al., 2007). The 14C age of this meteorite was determined by ChennaouiAoudjehane et al. (2009) to be 23 (±2) t.y., in agreement with geological evidence. Northwest Africa 2965 has undergone heavy oxidation since its arrival on Earth as attested by the many calcite veins intruding the stone; it has a weathering grade of W2W5.
A large portion of this meteorite was purchased by a collector at the 2006 Tucson Gem and Mineral Show and in subsequent purchases from a Moroccan source. The vast majority of Northwest Africa 2965 is surface material which is very highly weathered and dark brown in color with a porosity of 40%. It contains an abundance of very dark fractures filled with oxidation products such as goethite. Although primary minerals are present (e.g., enstatite, plagioclase, troilite, daubreelite, alabandite, oldhamite, and others), secondary minerals occur throughout, including those within the ubiquitous mm- to cm-sized pores, considered by some to have formed by weathering of metal phases. However, in his study of the Al Haggounia 001 pairing, Rubin (2016) attributed the presence of these vesicles (~6.8 vol%) to impact-induced evaporation of sulfides, in a similar manner to those in the aubrite Mayo Belwa. Employing X-ray µCT and SEM imaging for samples of Al Haggounia 001, Manzari et al. (2021) identified both open and closed, sub-mm-sized pores as well as bubbles on sulfide grains, which supports the postulation by Rubin (2016) of impact-induced evaporation of sulfur. Rubin (2016) also reasoned that the sparsity of metal observed in some parts of the mass, especially in the less weathered bluish-gray portions, is the result of metalsulfide melt drainage into nearby regions as represented by Al Haggounia 001 with its large component of limonite (32.6 vol%) replacing FeNi-metal (0.29 vol%) along with sulfide (4.0 vol%).
A very low abundance of radial pyroxene chondrules was observed (<5 vol%), in addition to fine-grained, rounded enstatite and plagioclase aggregates. The chondrules contain a NaAlSirich glass phase consistent with an unequilibrated chondrite. Recognizing this scarce population of chondrules and the other unequilibrated features of this meteorite has enabled investigators to classify this "fossil" meteorite. Northwest Africa 2965 was initially analyzed at Northern Arizona University (T. Bunch and J. Wittke), and due to its apparent lack of chondrules and its fine-grained igneous-like matrix, it was determined to be a recrystallized EL6/7 chondrite. An alternate classification of this enstatite meteorite as the first enstatite metachondrite, a newly proposed metamorphic category defined by Irving et al. (2005), was also considered. Subsequent to this study, additional material reported to be from the same meteorite, but having a less altered bluish color, was studied at the University of Washington, Seattle (A. Irving and S. Kuehner). This material was classified as an EL3 chondrite (the likely paired NWA 2828 was classified as an aubrite in MetBull 91).
This meteorite contains only trace amounts of FeNi-metal, but some portions near the top strata bear a dark goethite-rich rind consistent with leaching of Fe from the interior of the stone during an extended terrestrial residence in a wetland location. Analyses of the trace FeNi-metal and of rare kamacite inclusions identified within enstatite grains of NWA 2828 revealed a Si content consistent with EL chondrites, but different from aubrites (Irving et al., 2010). The investigation also determined that while major elements are depleted, trace siderophile elements still have abundances typical of E chondrites which were possibly preserved through an electroplating process. According to investigator T. Bunch (pers. comm.), this meteorite contains very tiny prismatic enstatite crystals with µm-sized oblate-to-spherical glass inclusions that might be presolar condensates. Also present are vermicular carbon, well-formed and poorly-formed "graphite", and unresolved carbon grains (<2 µm in size). Graphite contains grains of sinoite along with the silica polymorphs cristobalite and tridymite, which are thought to be products of rapid cooling during an impact melting event (Leili et al., 2019).
Prior to the analysis of NWA 2965, a 171.5 g stone with an identical appearance was analyzed at the Lunar & Planetary Laboratory, University of Arizona (Lowe, Hill, Domanik, and Lauretta), and the Southwest Meteorite Laboratory, Payson, Arizona (Killgore). The details of their analysis and the reasons for their ultimate classification can be found in the abstract NWA 2736: An unusual new graphite-bearing aubrite. They concluded that NWA 2736 has an igneous texture and is best described as an unusual enstatite achondrite (aubrite) rather than an enstatite metachondrite or enstatite impact melt. Notably, they compare the impact-shock features to those of Happy Canyon, which is typically classified as a highly metamorphosed EL chondrite rather than an aubrite.
Utilizing a larger volume of this meteorite to provide a better representation of its various components, investigators from NAU and UWS released results of an exhaustive study at the Fall 2006 Meeting of the American Geophysical Union. They suggest that all of the similarities found among NWA 2965, NWA 2736, and NWA 2828 make it likely that these independently classified stones, along with NWA 4232 and several others, are paired. In further support of a pairing, they determined that all of these stones share a common subsurface excavation site in Algeria delimiting a 40-km-long strewn field. Based on visual evidence, the previously classified EL6 chondrite NWA 002, a meteorite purchased in Morocco in 1999 having an appearance virtually identical to NWA 2965, may also belong to this pairing group.
Successive excavations at the strewn field led to the recovery of several large masses that were studied at Université Pierre & Marie Curie, Paris (A. Jambon, O. Boudouma, and D. Badia) under the name Al Haggounia 001. A classification of aubrite was assigned to this material in light of its enstatite and plagioclase composition (MetBull 92), but the documented existence of terrestrially weathered chondrules in this meteorite should disqualify this classification (see The Al Haggounia "Fossil or Paleo" Meteorite Problem).
Subsequent studies of Al Haggounia by Devaux et al. (2011) found that the ordering of carbon in the matrix indicates that it has experienced significant metamorphism. This crystalline carbon and the overall textures of this material prompted them to classify this meteorite as petrologic type 6. Contrariwise, the lack of recrystallization/equilibration in this meteorite has been cited by some investigators as evidence against a petrologic type as high as 6. It was proposed by T. Bunch (pers. comm.) that this meteorite could represent an unequilibrated primitive regolith that formed without chaotic, turbulent mixing on an E-type asteroid. However, in his study of the Al Haggounia 001 pairing, A. Rubin (2016) noted multiple features that are indicative of an impact-melt breccia, including the following: i) vesicles produced by troilite evaporation and preserved through quenching; ii) euhedral lath-like graphite grains; iii) kamacite-rich veins; iv) melt globules; v) partially resorbed chondrules; vi) enstatite nucleation on relict silicate grains and relict chondrules; vii) shock-induced silicate darkening; viii) shock deformation and mosaicism of silicates; ix) quenched feldspathic glass containing trapped, rounded enstatite inclusions; x) depletion of siderophile elements; xi) depletion in elements associated with various sulfides.
Images of distinct chondrules present in this meteorite can be seen on J. Kashuba's NWA 2965 page. In their study of multiple samples of this meteorite, Leili et al. (2018, #6263) identified a wide range of chondrule types including rimmed macrochondrules/clasts up to 11 × 6 mm in size. The specimen of NWA 2965 shown above is a 30 g slice exemplifying the disparate weathering grades in contact with each other (e.g., a small portion of the less weathered, bluish-colored clast is attached on the right side. The top photo below is a close-up of a 7.4 g slice of NWA 2965 exhibiting a dense web of oxide-filled fractures and an oval enstatite or plagioclase aggregate. The bottom photo shows a 4.1 g partial end section exhibiting a distinct bluish coloration representing the less altered material of this extensive find.
*Relict meteorites, defined as those highly altered meteorites which are >95% replaced by secondary phases since their fall, comprise a new category adopted by the Committee on Meteorite Nomenclature in October 2006 (see sec. 7.7 in 'Guidelines For Meteorite Nomenclature').