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TENERE-1 (Provisional)

standby for te-1 photoCR7 or Meta-CR (CR-like in MetBull 86; CR-an in MetBull DB)
Found March 2000, 20 ° 45.8′ N., 10 ° 26.5′ E.

A single mass of 3,636 g was found by a German team in the Ténéré region of the Sahara Desert in north-central Niger, specifically, at a location known as Grein. Provisionally named Te-1, it was classified by J. Otto and A. Ruh (Universitat Freiburg) as a metal-rich, coarse-grained, primitive achondrite. Olivine grains are mostly 0.1–0.4 mm in size, but larger grains occur. They commonly exhibit triple-junctions, consistent with recrystallization. Large poikilitic pyroxene grains are present, as well as small agglomerates of crystals, sometimes called ‘Sammelkristalle’, which usually form during melting and recrystallization processes. Unlike chondrules, these structures are composed primarily of plagioclase poikilitically enclosing minor olivines and pyroxenes, and are often accompanied by FeNi-metal. Te-1 is a freshly fallen meteorite with a weathering grade of W0, and it has a shock stage of S1–2.This primitive achondrite has a chemical and mineral composition unlike that of any other meteorite. It has an O-isotopic composition distinct from any other achondrite group, plotting within the CR-field, and interestingly, very near to that of the ungrouped basaltic meteorite NWA 011. Oxygen isotopes are similar to those of the lodranite/acapulcoite parent body but are not an exact match. The mineral composition and noble gas content of Te-1 are very similar to that of the brachinites and the brachinite-like meteorite, Divnoe; moreover, the olivine and pyroxene compositions are nearly identical to those of Brachina. Furthermore, the composition of chromite and metal in Te-1 is also indicative of a very close relationship with Divnoe. These varied characteristics are most consistent with the grouping of Te-1 as a brachinite-like primitive achondrite. See the Tafassasset page for further information.

Te-1 has a CRE age of 45 m.y. The specimen shown above is a 1.72 g partial slice with fresh fusion crust on one end. The photo below shows the main mass of Te-1 with an end slice removed.

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Tafassasset

standby for tafassasset photoPrimitive Achondrite, ungrouped: carbonaceous chondrite-related (CR [2000], CR-like [2002], CR-an [2006], Primitive achondrite in MetBull 105 [2017])
Found: February 14, 2000, Coordinates: 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.
Diagram credit: Sanborn et al., 45th LPSC, #2032 (2014) <!–

 

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Diagram credit: Sanborn et al., 45th LPSC, #2032 (2014) –>

 

17O vs. ε54Cr and ε50Ti for CR Carbonaceous Achondrites
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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)


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NWA 10503

Meta-CV or -ung
‘CX’ trend
(Achondrite ungrouped in MetBull 105)

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Purchased May 2015
no coordinates recorded A single 407 g meteorite was found in the Sahara Desert, possibly near Tan Tan, Morocco, and subsequently purchased by F. Kuntz in Zagora, Morocco on behalf of Planetary Studies Foundation in Galena, Illinois. An analysis was conducted at the University of Washington in Seattle (A. Irving and S. Kuehner), and NWA 10503 was determined to be a texturally-evolved meteorite exhibiting ~120° triple junctions with no relict chondrules visible. The elevated silicate FeO/MnO ratios are higher than those for ordinary chondrites and are consistent with a carbonaceous chondrite classification. In May 2016, a lot of smaller stones having a combined weight of 215 g was purchased by B. Hoefnagels. This group of stones was designated NWA 10859, and results of petrographic and isotopic analyses (A. Irving and S. Kuehner, UWS; K. Ziegler, UNM) led to the determination that they are paired with NWA 10503.

On an oxygen three-isotope diagram the values plot away from all other analyzed meteorites and along an extension of the trend line for the ungrouped pallasite Milton (K. Ziegler, UNM; see diagram below). NWA 10503 was initially classified as an ungrouped carbonaceous metachondrite that might be related to the Milton pallasite. standby for nwa 10503 o-isotope diagram
click on image for a magnified view

Diagram credit: Irving et al., 79th MetSoc, #6461 (2016) In an effort to better resolve potential genetic relationships that might exist among these meteorites, a Cr-isotopic analysis was conducted by Sanborn et al. (2018) for NWA 10503 as well as for olivine from the Milton pallasite. It is demonstrated on a coupled Δ17O vs. ε54Cr diagram (shown below) that both meteorites plot among the CV clan and plausibly share a genetic relationship. Chromium vs. Oxygen Isotope Plot
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click on diagram for a magnified view

Diagram credit: Sanborn et al., 49th LPSC, #1780 (2018) In an effort to further resolve differences between the CV and CK chondrite groups, Yin and Sanborn (2019) analyzed Cr isotopes in a significant number and broad range of meteorites. Their study included samples from each of the three CV subgroups (oxA, oxB, Red), anomalous CV3 chondrites, a C3-ungrouped, several CK members, and other potential CV-related meteorites including NWA 10503 and Milton (see top diagram below). It is demonstrated that the CV and CK meteorites are clearly resolved into two distinct isotopic reservoirs. In addition, it is shown by the ε54Cr values that NWA 10503 plots among the CV-related meteorites. A coupled Δ17O vs. ε54Cr diagram plotting all of the meteorites in the study is shown in the bottom diagram below. Cr Isotope Weighted Average For CV and CK Chondrites
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O–Cr Diagram For CV and CK Chondrites
CK: orange shades; CV: green shades; Achondrites: open
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click on photo for a magnified view

Diagrams credit: Yin and Sanborn et al., 50th LPSC, #3023 (2019)
In a study of two newly recovered ungrouped carbonaceous meteorites, the unequilibrated chondrite NWA 11961 and the dunitic breccia NWA 12264, Irving et al. (2019) further populated the O-isotopic trend line previously defined by NWA 10503 and the Milton pallasite; they termed this the ‘CX’ trend. However, Cr isotope data obtained for all of these meteorites have resolved both NWA 11961 and NWA 12264 as potential new carbonaceous parent bodies distinct from that of NWA 10503 and Milton, the latter previously considered possible members of the CV-clan (see diagrams below). ‘CX’ Oxygen Isotope Trend Line

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O–Cr Diagram for ‘CX’ Trend Meteorites
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Diagrams credit: Irving et al., 50th LPSC, #2542 (2019)
Another ungrouped metachondrite with affinities to carbonaceous chondrites is NWA 2788. This meteorite has a metamorphic texture exhibiting ~120° triple junctions, elevated Fe/Mn and Ca/Na ratios, and an O-isotopic composition that plots very near to the TFL. Bunch et al. (2006) suggest that if a chondrule-bearing representative of this parent body is found and identified in the future it should be termed a ‘CT chondrite’ (see NWA 2788 photos, abstracts [1, 2], and isotopic plot).

Northwest Africa 10503 is a somewhat friable meteorite with features indicating a low degree of terrestrial weathering and a low shock stage. Two views of a 3.79 g fragment of NWA 10503 are shown above. In the top photo below are some of the larger stones representing the paired NWA 10859, while two stones of NWA 10503 with green-colored fusion crust are shown in the bottom photo. standby for nwa 10859 group photo
Photos courtesy of Ben Hoefnagels

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Photos courtesy of Fabien Kuntz


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NWA 7030

Meta-LL <!–(Achondrite-ung submitted to MetBull)
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click on photo for a magnified view Found 2011
no coordinates recorded A single fusion crusted stone weighing 224 g was found in Northwest Africa and subsequently sold to meteorite dealer G. Hupé in Zagora, Morocco. A sample was submitted for analysis and classification to the University of Washington in Seattle (A. Irving and S. Kuehner), and NWA 7030 was determined to be a metachondrite with similarities to the LL chondrite group.

Northwest Africa 7030 is a texturally evolved meteorite that lacks any relict chondrules, and it exhibits an overall texture akin to a fine-grained igneous cumulate. The predominant lithology was described as consisting of olivine, clinopyroxene, sodic plagioclase, chromite, troilite and taenite. A less prevalent greenish-colored, igneous-textured lithology is also present. Its composition is different from that of the primary lithology in that it contains interstitial potassic-feldspar (sanidine) along with albitic plagioclase, and it contains much less troilite. Another difference is the presence of orthopyroxene and rare kamacite in the minor lithology (Fernandes et al., 2014).

The O-isotopic composition of NWA 7030 was analyzed at Okayama University (R. Tanaka) and it was shown to plot within the LL chondrite field. Bulk density measurements for NWA 7030 were conducted at the University of Central Florida (D. Britt [UCF]; B. Macke and G. Consolmagno [Vatican]) employing both pycnometer and glass bead methods (results forthcoming).

Although NWA 7030 was shown to have affinities to the LL-chondrite group, it has many anomalous features that make it unique, and therefore, could be considered as ungrouped. The K–Ar age was determined for the minor lithology, and the results indicate the occurrence of two separate impact heating/degassing events that have disturbed this isotopic chronometer: one at ~4.12 b.y. and the other at ~2.39 b.y. (Fernandes et al., 2014).

The specimen of NWA 7030 shown above is a 0.77 g end section. The top photo below shows the fresh fusion-crusted mass, while the bottom photo shows the interior of a 15.62 g slice with both lithologies—photos courtesy of Greg Hupé.



Photos courtesy of Greg Hupé—Nature’s Vault


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NWA 6348

Meta-L
(L7 in MetBull 102 [submitted as Achondrite-ung])
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Found 2010
no coordinates recorded A stone weighing 134 g was found in the Sahara and sold to Chladni’s Heirs. A sample was submitted for analysis and classification to the University of Washington in Seattle (A. Irving), and NWA 6348 was determined to be a metachondrite (Irving et al., 2005) with affinities to the L chondrite group. The meteorite has a shock stage of S3 and a weathering grade of W2.

Northwest Africa 6348 is a texturally evolved meteorite with grains of variable sizes, but which do not appear to constitute separate clasts. Although no chondrules were identified, some fine-grained areas might represent relict chondrules. Mineral constituents include olivine, orthopyroxene, clinopyroxene, sodic plagioclase, and opaque phases.

Northwest Africa 6348 has been classified by A. Irving as the first metachondrite from the L chondrite parent body. The term metachondrite was proposed to describe those achondrites which are texturally evolved chondrites with completely recrystallized textures resulting from high degrees of metamorphism or partial melting. They lack chondrules and have elemental abundance ratios and O-isotopic compositions that show affinities to existing chondrite groups. Meteorites described as metachondrites have been found representing the CV, CR, H, L, and LL groups, while unique metachondrite NWA 2788 is considered by Bunch et al. (2006) to be associated with an unknown carbonaceous chondrite parent body; if a chondrule-bearing representative of this parent body is found and identified in the future, it was suggested that it be termed a ‘CT chondrite’ (see NWA 2788 photos and abstract #P51E-1246). In addition, some groups of achondrites having chondrule-bearing members might also be more appropriately described as metachondrites, including the acapulcoites and winonaites as demonstrated by Monument Draw and NWA 725, respectively. These chondrule-bearing members have been referred to as ‘AC chondrites’ or ‘W chondrites’ for those associated with acapulcoites and winonaites, respectively. The photo shown above is a 2.9 g specimen of NWA 6348.