TAFASSASSET


Ténéréite (typical)*
or Tafassite Clan
(CR [2000], CR-like [2002], CR-an [2006], Primitive achondrite in MetBull 105 [2017])
standby for tafassasset photo
Found February 14, 2000
20° 45' 48" N. 10° 26' 30" E.

Twenty-six stones totaling ~110 kg, the two largest weighing ~30 kg, were found by Bernard Dejonghein in the Ténéré Desert in the Agadez region of north-central Niger; all are considered to be paired. This olivine-rich meteorite was classified at the Muséum National d'Histoire Naturelle in France as the first thermally metamorphosed CR chondrite. A separate ~3.6 kg stone found independently in the same vicinity as Tafassasset was provisionally named Te-1 (previous synonym Grein 004), and it was independently analyzed at the Max-Planck-Institut für Chemie in Germany. A bulk compositional analysis of Te-1 found that it differs slightly from Tafassasset in its texture and in certain elemental abundances, but its overall similarity in texture (recrystallized with 120° triple junctions) and elemental composition to Tafassasset makes their pairing obvious. The differences observed suggest this fall was composed of a heterogeneous assemblage. The meteorite NWA 5131 was found to be very similar geochemically and petrologically to Tafassasset.

*Previously, Floss (2000) and Patzer et al. (2003 #1352, 2004) 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.

Although Tafassasset is only slightly weathered to a grade of W0/1, the majority of the fusion crust has been extensively sand-blasted away. Relict metal-bearing chondrules and chondrule rims in Tafassasset were reported by the French research team. This evidence led some to classify the meteorite as CR7 or Meta-CR. However, these features were determined by Breton et al. (2015) to be pockets of molten material containing refractory olivine and immiscible metal, which upon cooling, resemble chondrule textures. In addition, Breton et al. (2015) utilized micro-computed tomography and found a 3-D connected metal network in Tafassasset attesting to temperatures at least above the Fe–FeS eutectic. Furthermore, Tomkins et al. (2020) observed regions of inverted pigeonite in Tafassasset consistent with the Pigeonite Facies which represents the highest metamorphic temperatures for primitive achondrites (temperatures of ~1080°C for Tafassasset; see Systematics Part VI for further details). More recent research results (see below) have determined that the Tafassasset parent asteroid accreted very early, prior to the onset of chondrule formation in the carbonaceous chondrite (CC) reservoir beyond Jupiter.

Plagioclase, chromite, and phosphates present in the matrix of Tafassasset have been attributed to metamorphism of original fine-grained matrix material. By contrast, similar mineral phases are found in areas that define possible relict chondrules, described as poikiloblastic aggregates by some, which have retained the textures of an earlier, pre-metamorphic stage. The abundant small troilite grains present in the recrystallized olivine–pyroxene matrix in Tafassasset are similar to those found in CR chondrites. On an oxygen three-isotope plot, Tafassasset falls within the CR field and away from the majority of brachinites. Nevertheless, the plagioclase composition and other silicate abundances in Tafassasset are most similar to those of brachinites.

Tafassasset has similar O- and Cr-isotopic compositions to the CR chondrites, and is also similar with respect to its high abundance of siderophile elements, including its high FeNi-metal content of 8–10 vol% compared to ~7.4 vol% in CR chondrites (Nehru et al., 2010). However, in their siderophile element study emphasising Hf–W systematics, Archer et al. (2019) contend that the near-zero ε183W values for metal in Tafassasset (–0.06 [±0.17] to 0.02 [±0.2]; Breton et al., 2015) distinguish it from the positive ε183W values for metal in CR chondrites (~0.4 to ~0.6; Archer et al., 2019; Budde et al., 2018 [diagram]), making a genetic relationship (common parent body) doubtful.

Similar to several CR6 meteorites, Tafassasset exhibits a fractionated element signature uncharacteristic for the CR group, including a depletion in refractory lithophile elements, an extremely low Zn concentration, and Al/Mg and Mn/Mg ratios that plot near more evolved achondrites. This fractionation is consistent with an early stage of partial melting involving the mobilization of melts incorporating Si, P, and S, and/or perhaps a late stage of metasomatism. Classification of Tafassasset as an ungrouped primitive achondrite was suggested by the German research team (Zipfel et al., 2002) as the most plausible classification; however, the texturally evolved nature of this meteorite is not consistent with a primitive designation.

A further advancement of metamorphism along a continuum that includes the CR6 chondrites NWA 7317 (and pairings), NWA 3100, and NWA 2994 (and pairings) was invoked by Bunch et al (2008) to explain the recrystallized poikiloblastic texture in Tafassasset, and therefore the term metachondrite was thought to be most appropriate for this meteorite. Considering that relict chondrules have been reported in samples from the texturally-evolved Tafassasset, NWA 3100, NWA 7317, and LEW 88763, and with NomCom (Meteoritical Society Committee on Meteorite Nomenclature) presently lacking a type 8 category, they would perhaps be more appropriately designated CR6. 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.

Bunch et al (2008) also identified a similarity in the O-isotopic compositions among the non-metamorphosed CR chondrites, the metamorphosed CR6 chondrites, Tafassasset, and the igneous achondrite NWA 011 (and pairings), which is consistent with derivation from a common large parent body that experienced internal partial melting while retaining a chondritic regolith.

Tafassasset is a recrystallized meteorite that is petrographically consistent with a low-degree partial melt with a retained metal component that was derived from Renazzo-like precursor source material. It subsequently experienced equilibration processes through an extended period of thermal metamorphism. Tafassasset is considered to be closely related to the brachinites and other FeO-rich primitive achondrites, and the meteorite has been characterized by Nehru et al. (2010) as an unusual brachinite derived from a CR-like precursor body through partial differentiation. A Fa vs. Fs plot demonstrates this genetic relationship, as well as a relationship with the more primitive anomalous achondrites Divnoe and RBT 04239 (Gardner et al., 2007). It should be noted that some investigators including Tomkins et al. (2020) have observed both the presence of relict porphyritic chondrules (up to ~1.2 mm diameter) and a lack of interconnected plagioclase in RBT 04239, and thus they suggest a classification of L6 for that meteorite. A genetic relationship between Tafassasset and the CR group could be excluded based on differences in elemental compositions, noble gas ratios, and solar gas abundances. The CRE age of Tafassasset is also much higher (76.1 ±15.2 m.y.) than that of any CR chondrite (<10 m.y.). Still, it has been suggested by some investigators that all of the differences between Tafassasset and CR chondrites may be the result of an increased degree of metamorphism and/or metasomatism experienced by Tafassasset.

A study in which Tafassasset was compared with the brachinites was undertaken by Nehru et al. (2003). They determined that the texture, modal abundances, and mineral compositions of Tafassasset were very similar to Brachina, although differences were found to exist for Tafassasset with respect to its equilibration temperature, O-isotopic composition, and high abundance of metal. In a similar comparison made by Patzer et al. (2003), it was found that the level of radiogenic 129Xe measured in Tafassasset is similar to that of some brachinites. They also found that the trapped 132Xe component of Tafassasset was lower than that of CR chondrites, and that the 36Ar/132Xe ratio is at least 10× lower than it is in CR chondrites.

As with brachinites, Tafassasset was determined to have an ancient Pb–Pb age of ~4.563 b.y. (Göpel et al., 2009, 2015). It was also determined that its Cr systematics are the same as those for Renazzo, and that its 54Cr excess is the first such occurrence in a carbonaceous achondrite (Göpel and Birck, 2010). The carbonaceous achondrites NWA 011/2976, NWA 6704/6693, and NWA 2994/6901 have since been determined to have similar positive ε54Cr and ε50Ti values (Sanborn et al., 2018). In a study of the Mn–Cr systematics for Tafassasset, Göpel et al. (2015) obtained an absolute age of 4.56351 (+0.00025/–0.00026) b.y. anchored to the D'Orbigny angrite. Based on Al–Mg systematics, Dunlap et al. (2015) calculated an upper limit of <4.5677 b.y. ago for the timing of Al/Mg fractionation during differentiation on the Tafassasset parent body.

In the study of Tafassasset by Breton et al. (2015) they obtained a "most reliable" metal phase Hf–W age of 2.9 (±0.9) m.y. after CAIs, corresponding to the timing of metal–silicate segregation; this corresponds to an absolute age of 4.5644 b.y. By comparison, the Hf–W age of CR chondrites was determined by Budde et al. (2018) to be somewhat younger at 3.63 (±0.62) m.y. after CAIs. It is noteworthy that chondrule formation for CR chondrites was calculated by Schrader et al. (2017) and Amelin et al. (2002) using Al–Mg and Pb–Pb chronometry, respectively. Their studies provided corrected ages of 3.75 (±0.24) and 3.66 (±0.63) m.y. after CAIs, respectively. Moreover, a temporally similar accretion age of 3.5 (±0.5) m.y. after CAIs was determined for CR chondrites by Sugiura and Fujiya (2014). These ages are inconsistent with a common parent body for CR chondrites and Tafassasset.

In a petrographic analysis and O-isotope study conducted by Gardner-Vandy et al. (2012), it was found that samples of Tafassasset have O-isotopic ratios that plot within the CR-chondrite field, and that it was equilibrated at an oxygen fugacity of ~IW–1. They determined that this meteorite experienced a low degree of partial melting on a small parent body without reaching isotopic homogeneity. Overall, Tafassasset was found to be most similar to the ungrouped achondrites LEW 88763 and Divnoe, as well as to the brachinites. The study concluded that Tafassasset is not consistent with partial melting of CR chondrites, although each meteorite appears to have formed within the same oxygen reservoir. In their comprehensive study of Tafassasset, Breton et al. (2015) used thermal modeling to derive a size for the Tafassasset parent body of 30–50 km in diameter, an early timing of accretion at 0.8–1.2 m.y. after CAIs, a partial melting degree of 20–25% due to radiogenic 26Al, and a formation depth for the Tafassasset lithology of 7.3–7.7 km. They also inferred that this relatively small asteroid experienced a high cooling rate of ~300–400 K/m.y. near the closure temperature for the Hf–W chronometer.

In a contrary scenario presented by Nehru et al. (2012, 2014), Tafassasset (and LEW 88763) may represent the residua of a low-degree partial melting event that occurred at some depth within a late-accreted chondritic veneer on a large (~400 km diameter) CR-like differentiated parent body. Subsequent impact excavation of the crust would have exposed the underlying Tafassasset and brachinite lithologies. For more information pertaining to the latter scenario, see the LPSC abstract "Primitive" and igneous achondrites related to the large and differentiated CR parent body by Bunch et al. (2005), the MetSoc abstract Northwest Africa 5131: Another Tafassasset-Like Metachondrite Related To The CR Chondrite Parent Body by Wittke et al. (2011), and the MetSoc abstract Tafassasset and Primitive Achondrites: Records of Planetary Differentiation by Nehru et al. (2014).

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'

Efforts to better resolve the relationship that exists between Tafassasset and other anomalous meteorites continues. As provided by Sanborn et al. (2014), a coupled Δ17O vs. ε54Cr diagram is one of the best diagnostic tools for determining genetic relationships between meteorites. Moreover, Sanborn et al. (2015) demonstrated that ε54Cr values are not affected by aqueous alteration. The diagrams below include Tafassasset, and it is apparent that it plots within the CR chondrite field.

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

ε54Cr and ε50Ti vs. Δ17O 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 Member 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'

Ma et al. (2021, 2022) and Neumann et al. (2021) investigated the suite of ténéréites, for which they proposed the name 'tafassites'. They employed numerical modeling to constrain the formation and thermal history of the parent body, which they found was most consistent with an accretion age of 0.9 (±0.1) m.y. after CAIs—significantly earlier than that of the CR chondrite parent body at 3–4 m.y. after CAIs. In addition, they determined the diameter of the tafassite parent body to be 200–400 km. Moreover, based on stable isotope systematics and the distinct accretion ages obtained for the NWA 011 and NWA 6704 grouplets of 1.5 and 1.7 m.y. after CAIs, respectively, they argued that these meteorites derive from one or more additional parent bodies associated with a common reservoir (see top diagrams below). At the other end of the lumping–splitting spectrum, Jiang et al. (2021) contend that the CR parent body once comprised all of the meteorites that are isotopically and geochemically similar, composing a now disaggregated, at least partially differentiated body with a metallic core, achondritic mantle, and chondritic crust (see schematic illustration below).

ε54Cr vs. Δ17O for Tafassites and the NWA 011 and NWA 6704 grouplets
standby for o-cr diagram
click on image for a magnified view

Diagrams credit: Ma et al., Geochemical Perspectives Letters, vol. 23, pp. 33–37, fig. S-13 (2022 open access link)
'Early formation of primitive achondrites in an outer region of the protoplanetary disc'
(https://doi.org/10.7185/geochemlet.2234)

standby for cr parent body illustration
Schematic illustration credit: Jiang et al., 84th MetSoc, #6062 (2021)

A more comprehensive investigation of the suite of four ungrouped primitive achondrites (NWA 3250, NWA 11112, NWA 12869, and Tafassasset) was undertaken by Jiang et al (2023) with an expanded team having relevant expertise in Cr and O isotope systematics, Mn–Cr chronometry, nucleosynthetic anomalous isotopes, and geothermometry. Employing advanced petrographic and mineralogical techniques, including high resolution X-ray tomographic microscopy, their analyses led to the conclusion that NWA 3250, NWA 11112, and NWA 12869 compose a grouplet of primitive achondrites that derive from a small parent body (tens of km in diameter) which accreted very early (<1 m.y. after CAIs) from a nebular reservoir that would later produce the CR chondrite parent body. Importantly, they determined that Tafassasset should be removed from inclusion in this grouplet due to significant mineralogical differences in comparison with the other three members (see diagrams below). Therefore, a potential 'tafassite clan' comprised of up to 4 parent bodies, each of which formed early in the CR reservoir, may be represented in our collections as (1) Tafassasset grouplet, (2) Jiang et al. grouplet, (3) NWA 011 basalt grouplet, and (4) NWA 6704 orthopyroxene grouplet.

Triple Oxygen Isotopes for CR-like Primitive Achondrites
standby for triple oxygen isotopic diagram
click on image for a magnified view

ε54Cr vs. Δ17O for CR-like Primitive Achondrites
standby for o-cr diagram
click on image for a magnified view

Diagrams credit: Jiang et al., GCA, In Press (2023)
'Tracking and dating incipient melting of a new grouplet of primitive achondrites'
(https://doi.org/10.1016/j.gca.2023.01.022)

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 (~640–1,800 km-diameter as indicated by evident prograde metamorphism involving the amphibole tremolite [Hamilton et al., 2020; Hamilton et al., 2021]; Dodds et al., 2022 [#2158]) 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 and 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'

The specimen of Tafassasset pictured above is a 4.45 g partial slice with an edge of preserved fusion crust. Below is an excellent petrographic thin section micrograph of Tafassasset shown courtesy of Peter Marmet.

standby for tafassasset ts photo
click on photo for a magnified view
Photo courtesy of Peter Marmet