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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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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 3133

Meta-CV
(or CV7; Primitive achondrite in MetBull 89)
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Purchased March 2004
no coordinates recorded Several stones weighing together ~4.2 kg were found in the Sahara Desert and later sold to a collector in Zagora, Morocco in March 2004. This meteorite was analyzed at Northern Arizona University (T. Bunch and J. Wittke) and the University of Washington in Seattle (A. Irving and S. Kuehner), and it was initially determined to be a highly recrystallized (~120° triple junctions), highly equilibrated (W sequestration into metal) L7 chondrite with no relict chondrules apparent. Notwithstanding this L7 classification, a subsequent oxygen isotope study was conducted at the University of Western Ontario (T. Larson) which showed that NWA 3133 plots on the Carbonaceous Chondrite Anhydrous Mineral (CCAM) line clearly within the field of CV chondrites (see the O-isotope plot and diagram below). standby for nwa 10503 o-isotope diagram
click on image for a magnified view

Diagram credit: Irving et al., 79th MetSoc, #6461 (2016) Northwest Africa 3133 and the probable paired stones NWA 2643 (26 g) and NWA 2825 (664 g) have FeO/MnO ratios in olivine and pyroxene which are higher than those in known ordinary chondrites, as well as other elemental ratios more consistent with a carbonaceous chondrite composition (Schoenbeck et al., 2006). These factors led to a tentative revised classification for this meteorite by NAU investigators to that of ungrouped primitive achondrite. A trace element analysis was subsequently conducted by Noronha and Friedrich (2014), and they concluded that NWA 3133 should be classified as a completely recrystallized member of the CV-clan (CV or CK) of carbonaceous chondrites.

A change in the nomenclature used to describe such texturally evolved chondrites as NWA 3133 was suggested by Irving et al. (2005). The term ‘metachondrite’ was proposed to describe those chondrule-free stony meteorites which are texturally-evolved chondrites and that exhibit completely recrystallized textures resulting from high degrees of metamorphism or partial melting, and that have elemental ratios and O-isotopic compositions demonstrating affinities to existing ordinary and carbonaceous chondrite groups (e.g., CV, CO, CR, H, L, and LL).

Constituents of NWA 3133 are primarily olivine (46 vol%) and orthopyroxene (28 vol%), with lesser abundances of plagioclase, Cr-diopside, Al–Ti-bearing chromite, Na–Mg-bearing merrillite, troilite, and FeNi-metal. This meteorite exhibits shock features commensurate with stage S2. It has experienced significant weathering during its terrestrial residence, evidenced by limonite and calcite observed along grain boundaries, which corresponds to grade W3. Terrestrial alteration was also observed in a trace element study conducted by Noronha and Friedrich (2014), especially in the elevated Sr, Ba, U, and Sb abundances and the lower than usual Nb abundances.

The NWA 3133 meteorite has an elevated 54Cr content which supports the inference of an initially large precursor carbonaceous chondrite parent body of undefined type, considered to have experienced differentiation resulting in a body consisting of a metallic core, mantle, and chondritic crust. In their continued analyses of this meteorite, Sanborn et al. (2015) demonstrated that NWA 3133 plots with the CV chondrites, overlapping the CV chondrite Allende, on a Δ17O vs. ε54Cr coupled diagram. 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) Utilizing the Mn–Cr age chronometer anchored to the angrite NWA 4801, an absolute crystallization age of 4.5615 (±0.0004) b.y. has been calculated (Shukolyukov et al., 2011). In a similar case, the Pb–Pb age anchored to the angrite NWA 4801 gives an absolute age of 4.5580 (±0.00013) b.y. NWA 3133 experienced an early thermal history beginning ~6 m.y. after CAI formation.

Prior to its catastrophic disruption, this planetesimal possibly comprised multiple lithological zones including a metallic core (iron), a core–mantle boundary (pallasite), an upper mantle impact-melted zone composed of metal+silicate assemblages (silicated iron), a high-temperature zone (dunite), an intensely thermally metamorphosed stratigraphy (metachondrite), and a primitive, insulating chondritic regolith which has experienced impact-gardening and metasomatism. See ‘The Breakup of Antaeus’ for additional details.

Further evidence for a large differentiated CV planetesimal lies in the fact that Allende acquired a strong unidirectional natural remanent magnetization at least 8 m.y. after CAI formation, reflecting the existence of an internal core dynamo (Weiss et al., 2010). The photo shown above is a 1.41 g partial slice of NWA 3133, the first meteorite distinguished as a metachondrite.