TAFASSASSET

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 hosted 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 off. Relict chondrules and chondrule rims in Tafassasset have been reported by the French research team. These chondrule relicts would be most consistent with a porphyritic olivine–pyroxene type; some are metal-bearing. 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 8–10 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 investigators, that have retained the textures of an earlier, pre-metamorphic stage. Small, abundant troilite grains present in the recrystallized olivine-pyroxene matrix within Tafassasset are similar to those found in CR chondrites. The O-isotope plot is also within the CR field and away from the majority of brachinites, with the exception of the anomalous brachinite LEW 88763. Still, the plagioclase composition and other silicate abundances in Tafassasset are most similar to those of the brachinites.

In a similar manner to several CR6 meteorites, Tafassasset has 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.

An advanced stage of metamorphism compared to that exhibited by 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 may 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 with 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, while some differences were found to exist in its equilibration temperature, O-isotopic composition (though it is similar to the anomalous brachinite LEW 88763), and high metal abundance. 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). They also found that its Cr systematics are the same as those in Renazzo, and that its 54Cr excess is the first such occurrence in a non-carbonaceous meteorite (Göpel and Birck, 2010).

Tafassasset (and the paired Te-1) is a recrystallized meteorite that is petrographically consistent with a low-degree partial melt of Renazzo-like precursor material that has retained its metal component. 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 has been characterized by Nehru et al. (2010) as an unusual brachinite derived from a CR-like precursor body that experienced partial differentiation. A Fa vs. Fs plot demonstrates this genetic relationship, as well as a relationship between 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 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. The specimen of Tafassasset pictured above is a 4.45 g partial slice with an edge of preserved fusion crust.