NORTHWEST AFRICA NWA 3100


Ténéréite (typical)*
(Primitive achondrite in MetBull 89)
(carbonaceous chondrite-related)

standby for nwa 3100 photo
Purchased June 2003
no coordinates recorded

A single meteorite weighing 136 g was purchased in Rissani, Morocco. The stone was analyzed at Northern Arizona University (T. Bunch and J. Wittke) and was initially thought to represent a very fine-grained, completely recrystallized L7 chondrite (Bunch et al., 2005). Two very small relict chondrules were identified in the thin sections studied. This meteorite has been shocked to stage S1 and terrestrially weathered to grade W2.

*Previously, Floss (2000) and Patzer et al. (2003) proposed that the acapulcoite/lodranite meteorites should be divided based on metamorphic stage:
  1. primitive acapulcoites: near-chondritic (Se >12–13 ppm [degree of sulfide extraction])
  2. typical acapulcoites: Fe–Ni–FeS melting and some loss of sulfide (Se ~5–12 ppm)
  3. transitional acapulcoites: sulfide depletion and some loss of plagioclase (Se <5 ppm)
  4. lodranites: sulfide, metal, and plagioclase depletion (K <200 ppm [degree of plagioclase extraction])
  5. enriched acapulcoites (addition of feldspar-rich melt component)
A similar distinction could be made among the winonaites in our collections, as well as among members of the newly proposed group ténéréites (Agee et al., 2020). One of the most "primitive" members identified in this new group is NWA 7317, which contains relict chondrules comparable to a petrologic type 6 chondrite. However, most ténéréites have experienced more extensive thermal metamorphism involving incipient melting and now exhibit highly recrystallized textures, characteristics analogous to the "typical" acapulcoites. Metamorphic progression in other ténéréites involved higher degrees of partial melting and even separation of a basaltic fraction (e.g., NWA 011 pairing group). Samples representing such an advanced metamorphic stage are known as lodranites in the acapulcoite/lodranite metamorphic sequence, while the term "evolved" could be used to represent a similar metamorphic stage in the ténéréite group.

Despite the similarities of NWA 3100 to an L7 chondrite, certain elemental ratios such as Fe/Mn and Ca/Na are inconsistent with those known from any ordinary chondrite group, and therefore an O-isotopic analysis was conducted. Based on the values from the completed analysis, conducted at the University of Western Ontario (T. Larson and F. Longstaffe), it was demonstrated that the O-isotope plot of NWA 3100 falls along the trend line of the CR chondrites. This suggests that NWA 3100 is likely genetically related to the CR carbonaceous chondrite group. Studies of the REE pattern for NWA 3100 also demonstrate a similarity to the CR-related, FeO-rich achondrite LEW 88763 (Bunch et al., 2008 and reference therein); however, new analyses of LEW 88763 by Day et al. (2015) led them to propose its reclassification as an anomalous achondrite, with a possible relationship to the ungrouped achondrite NWA 6704 pairing group.

A cooperative study was undertaken for a number of previously ungrouped achondrites, primitive achondrites, and silicated irons which have O-isotopic compositions that plot along the CR oxygen isotope trend line (Bunch et al., 2005—Northern Arizona University, University of Washington, and University of Western Ontario). From the meteorites that were studied, including NWA 3100, NWA 801, Tafassasset, NWA 011 pairing group, LEW 88763, Sombrerete, and NWA 468, it was suggested that some or all of them may have originated in the core, mantle, crust, and chondritic regolith of a large, at least partially differentiated CR-type parent body which experienced collisional disaggregation.

standby for CR trend line diagram
click on image for a magnified view

Diagram credit: Bunch et al., 36th LPSC, #2308 (2005)
'"Primitive" And Igneous Achondrites Related To The Large And Differentiated CR Parent Body'

standby for nwa pasamonte stone photo
click on image for a magnified view

Diagram credit: Wittke et al., 74th MetSoc, #5222 (2011)
'Northwest Africa 5131: Another Tafassasset-Like Metachondrite Related To The CR Chondrite Parent Body'

Northwest Africa 3100 is a recrystallized, texturally evolved chondrite with an elevated Fe/Mn ratio and Ca-rich plagioclase (features possibly reflecting metasomatism), and an O-isotopic composition that plots within the CR chondrite field. These features may be most appropriately associated with the newly proposed group of carbonaceous metachondrites (Irving et al., 2005). The more highly fractionated, CR-related chondrite NWA 2994 and the equilibrated CR-an (or CR7) Tafassasset, both meteorites with O-isotopic ratios that plot with the CR chondrite group, may each represent lithologies on the CR parent body that experienced a higher degree and/or a longer duration of thermal metamorphism (Bunch et al., 2008) as well as metasomatism. Since relict chondrules have also been reported in the texturally-evolved Tafassasset, NWA 7317, and LEW 88763, and with NomCom presently lacking a type 8 category, they would perhaps be more appropriately designated type 6 as well. However, if the metamorphic continuum were to include type 8 as a completely recrystallized end point as proposed by Irving et al. (2019 #6399), then a type 7 designation for all of these meteorites would be appropriate.

Continued research on this topic has been ongoing (e.g., Bunch et al., 2005; Floss et al., 2005, [MAPS vol. 40, #3]; Irving et al., 2014 [#2465]; Sanborn et al., 2014 [#2032]). As provided in the Sanborn et al. (2014) abstract, a Δ17O vs. ε54Cr diagram is one of the best diagnostic tools for determining genetic relationships among meteorites (see diagrams below). Moreover, Sanborn et al. (2015) demonstrated that ε54Cr values are not affected by aqueous alteration.

standby for o-isotopic diagram
Diagram credit: Sanborn et al., 45th LPSC, #2032 (2014)

17O vs. ε54Cr and ε50Ti for CR-like Achondrites
standby for o-cr diagram
click on image for a magnified view

Diagrams credit: Sanborn et al., GCA, vol. 245, pp. 577–596 (2019)
'Carbonaceous Achondrites Northwest Africa 6704/6693: Milestones for Early Solar System Chronology and Genealogy'
(https://doi.org/10.1016/j.gca.2018.10.004)

It was asserted by Agee et al. (2020) that the similarity in O, Cr, and Ti values among the CR2 carbonaceous chondrites and these ungrouped equilibrated meteorites is coincidental, and that significant geochemical differences (e.g., olivine Fa content and Fe/Mn) and other discrepancies (e.g., petrologic type discontinuity) exist that make a common parent body untenable. They contend that the thermally metamorphosed CC meteorites represent a unique group for which they propose the name 'ténéréites' (see list and diagrams below).

standby for ténéréite oxygen isotopes diagram
Diagram credit: Agee et al., 51st LPSC, #2292 (2020)
'Northwest Africa 12869: Primitive Achondrite From the CR2 Parent Body or Memeber of a New Meteorite Group?'

standby for ténéréite fa vs fe/mn diagram
Diagram credit: Dr. Carl Agee, IOM Seminar Sept 1, 2020
'Dr. Carl Agee: Some New Meteorites from the Sahara Desert'

Miller et al. (2021) utilized a coupled ε54Cr vs. Δ17O diagram (see diagram below) to determine the genetic provenance of the ungrouped carbonaceous chondrite AhS 202, which was found as a xenolithic clast in the Almahata Sitta polymict ureilite. Based on its plot, AhS 202 could represent the unmelted chondritic lid surrounding a Ceres-sized (Hamilton et al., 2020) differentiated asteroid possibly associated with the proposed ténéréite group (Agee et al., 2020). Alternatively, AhS 202 may derive from an asteroid that formed in the CR reservoir which was previously unrepresented in our collections.

ε54Cr vs. Δ17O Diagram for AhS 202
standby for o-cr diagram
click on image for a magnified view

Diagram credit: Miller et al., 52nd LPSC, #2360 (2021)
'Stalking a Large Carbonaceous Chondrite Asteroid Using ε54Cr–Δ17O
Isotope Systematics of the Unique Xenolith Almahata Sitta 202'

Continued studies of the many ungrouped meteorites in our collections will eventually lead to the resolution of more disrupted parent bodies which formed in the early Solar System. The specimen of NWA 3100 shown above is a 3.1 g partial slice.