Meta-CV/CV7 (CV8; Irving et al., 2019)
(Primitive achondrite in MetBull 89; revised to CV7 in MetBull 108)
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 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).
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 2653 (51 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. Northwest Africa 3133 and the paired NWA 2653 were ultimately classified as the first CV7 in MetBull 108.
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 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, AlTi-bearing chromite, NaMg-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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Diagram credit: Sanborn et al., 49th LPSC, #1780 (2018)
Utilizing the MnCr 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 PbPb 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 coremantle 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/type 7), and a primitive, insulating chondritic regolith which has experienced impact-gardening and metasomatism. 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). See 'The Breakup of Antaeus' for additional details.
A further metamorphic category in the textural continuumtype 8has been proposed by Irving et al. (2019) to distinguish between those highly metamorphosed meteorites in which relict chondrules can still be discerned (e.g., NWA 12272 [LL7]) and those which exhibit a completely recrystallized texture (e.g., NWA 3133 [CV7]). The designation of type 8 was also suggested for other chondrite groups with members having similar completely recrystallized textures, including CK (e.g., NWA 8186 [Achon-ung]), CR (e.g., NWA 12455), and H (e.g., NWA 4226 [H7]), as well as certain meteorites within the acapulcoite and winonaite groups (see Irving et al., 2019 #6399). The photo shown above is a 1.41 g partial slice of NWA 3133, the first meteorite designated as a metachondrite and now officially classified as a CV7. The photo below is an excellent petrographic thin section micrograph of paired stone NWA 2653, shown courtesy of Peter Marmet.
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Photo courtesy of Peter Marmet