TAFASSASSET


Primitive Achondrite, ungrouped
carbonaceous chondrite-related
(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 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.

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. 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. Still, 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., 2018; 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. They also argued that the similarity in O-isotopic compositions that is observed among the non-metamorphosed CR chondrites, the metamorphosed CR6 chondrites, and Tafassasset, compared to the igneous achondrite NWA 011 (and pairings), is consistent with their derivation from a common large parent body, one which 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). However, 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) ascertained 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 their study, Breton et al. (2015) ascertained a "most reliable" metal phase Hf–W age for Tafassasset 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 also noteworthy that chondrule formation for CR chondrites was also calculated employing Al–Mg and Pb–Pb chronometry by Schrader et al. (2017) and Amelin et al. (2002), 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 dates 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), and the MetSoc abstract Tafassasset and Primitive Achondrites: Records of Planetary Differentiation by Nehru et al. (2014).

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. The specimen of Tafassasset pictured above is a 4.45 g partial slice with an edge of preserved fusion crust.

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

17O vs. ε54Cr and ε50Ti for CR Carbonaceous 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)