CR7 or Meta-CR
(CR , CR-like , CR-an , Primitive achondrite in MetBull 86)
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 (and 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 with Tafassasset makes their pairing obvious; this suggests that this fall was composed of a heterogeneous assemblage. The meteorite NWA 5131 is 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 chondrules and chondrule rims in Tafassasset have been reported by the French research team. These chondrules, some metal-bearing, would be most consistent with a porphyritic olivinepyroxene type. A related anomalous achondrite, RBT 04239, contains relict porphyritic chondrules, some of which are ~1.2 mm in diameter. Tafassasset is also consistent with a CR chondrite relationship with respect to its high abundance of siderophile elements, including its high FeNi-metal content of 810 vol% compared to ~7.4 vol% in CR chondrites (Nehru et al., 2010).
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 olivinepyroxene 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.
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 has been 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 the continuum which includes the CR6 chondrites NWA 7317 (and pairings), NWA 3100, and NWA 2994 (and pairings) has been invoked by Bunch et al (2008) to explain the recrystallized poikiloblastic texture in Tafassasset, and therefore the term metachondrite might be most appropriate for this meteorite. They also argue 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.
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 in 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 PbPb 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 MnCr 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 AlMg 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.
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 it 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.
In a new 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 ~IW1. 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 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 which 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.
Diagram credit: Sanborn et al., 45th LPSC, #2032 (2014)
Diagram credit: Sanborn et al., 45th LPSC, #2032 (2014)