NORTHWEST AFRICA 3133

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 at 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. Its constituents are primarily olivine (46 vol%) and orthopyroxene (28 vol%), along with plagioclase, Cr-diopside, chromite, Na–Mg-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.

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 O-isotope plot). Northwest Africa 3133, along with the probable paired stones NWA 2643 (26 g) and NWA 2825 (664 g), has FeO/MnO ratios in olivine and pyroxene which are higher than those in known ordinary chondrites, and it has other elemental ratios which are most consistent with a carbonaceous chondrite composition (Schoenbeck et al., 2006). These factors led to a tentative revised classification for this meteorite by NAU investigators of ungrouped primitive achondrite.

Subsequent to this reclassification, a proposed revision was made to the nomenclature previously used to describe those primitive achondrites determined to be texturally evolved chondrites; the term ‘metachondrite’ was suggested by Irving et al. (2005) to describe those chondrule-free, stony meteorites which are texturally-evolved chondrites exhibiting completely recrystallized textures resulting from high degrees of metamorphism or partial melting, and which have elemental ratios and O-isotopic compositions demonstrating affinities to existing ordinary and carbonaceous chondrite groups (e.g., CV, CR, H, L, and LL).

The NWA 3133 metachondrite has an elevated 54Cr content that supports the inference of an initially large precursor carbonaceous chondrite parent body of undefined type. It is thought to have experienced differentiation resulting in a body consisting of a metallic core, mantle, and chondritic crust. Utilizing the Mn–Cr age chronometer, anchored to the angrite NWA 4801, an absolute crystallization age of 4,561.5 (±0.4) m.y. has been developed (Shukolyukov et al., 2011). In a similar case, the Pb–Pb age, anchored to the angrite NWA 4801, gives an absolute age of 4,558.0 (±0.13) m.y. NWA 3133 had an early thermal history beginning ~6 m.y. after CAI formation.

This planetesimal possibly comprised the Eagle Station pallasites at the core-mantle boundary and the NWA 176 and Bocaiuva silicated irons in the mantle, along with an Allende-like chondritic regolith (Irving et al., 2004). Although Cr systematics are not consistent with the CV parent body, all of these meteorites have O-isotopes (as well as other specific elemental isotopes) consistent with the CV parent body. Further evidence of 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, through an internal core dynamo (Weiss et al., 2010). Notably, spectroscopic data suggest that the Eos asteroid family, shown to contain a diversity of asteroid types including achondrites, may represent the remnants of the CV chondrite parent body.

Information about another metachondrite with CV affinities can be found on the NWA 1839 page. The photo shown above is a 1.41 g partial slice of NWA 3133, the first such distinguished metachondrite.

---