NORTHWEST AFRICA 4765 (provisional)

A single, partially crusted meteorite fragment weighing 42 g (19 g recorded for NWA-series designation) was found in Northwest Africa, and a portion was sold through A. Habibi to meteorite collector S. Ralew in Erfoud, Morocco. A type specimen was submitted for analysis and classification to the Museum für Naturkunde (A. Greshake and M. Kurz), and it was determined that NWA 4765 is a CM1 carbonaceous chondrite, the first of its kind to be found outside of Antarctica. An analysis of the O-isotopes was conducted by the Open University (Franchi and Greenwood).

This meteorite has been exposed to extensive parent-body aqueous alteration, and all mafic silicates have been converted to phyllosilicates. Chondrules have been severely altered and converted to pseudomorphs composed of phyllosilicate, while various secondary minerals such as carbonates and pyrrhotite are now abundant. Fine-grained accretionary rims, considered by some to be rims formed after parent body accretion through impact-compaction of fine-grained, porous matrix material (Trigo-Rodriguez et al., 2006), surround some discrete coarse-grained objects. Aqueous alteration processes also contributed to the replacement of melilite with phyllosilicate in refractory inclusions, and to the subsequent disruption of the inclusions (Rubin, 2007). Northwest Africa 4765 has experienced a low degree of shock and exhibits minor terrestrial weathering.

The degree to which various CM chondrites have been aqueously altered has been determined through such factors as the water/rock ratio, the temperature, and the duration of the alteration process. A new aqueous alteration sequence for CM chondrites was proposed by Rubin et al. (2005, 2007) in which the most altered meteorites, classified as CM1, would become CM2.0, while the less altered members proportionately increasing in petrologic type up to CM2.6; lower degrees of aqueous alteration have not been identified in the CM group thus far. Based on eight major diagnostic parameters of progressive alteration, the team classified many CM chondrites while invoking a hypothetical precursor lithology having a petrologic type of 3.0. This precursor lithology is broadly similar to the anhydrous, CM-related, type-3.0 Acfer 094, or perhaps to a CO3.0 such as ALHA77307. Following are the major parameters developed by Rubin et al. (2007), Rubin (2007), and de Leuw et al. (2008) for estimating the degree of alteration of the CM chondrites:

early to intermediate stage alteration processes:

1. hydration of fine-grained matrix material to form phyllosilicates, which gradually consists of Mg-rich serpentines
2. conversion of primary igneous glass in chondrules to phyllosilicate
3. production of large PCP clumps (serpentine-tochilinite intergrowths)
4. precipitation of sulfides within cavities

processes occurring throughout the alteration sequence:

5. oxidation of FeNi-metal
6. alteration of chondrule mafic phenocrysts
7. compositional changes in PCP; e.g., S depletion
8. formation of increasingly complex carbonates within sulfide-lined cavities
9. compositional changes in sulfides
10. replacement of primary melilite by secondary alteration products
11. fragmentation/disintegration of refractory inclusions

An aqueous alteration sequence for some CM group members from most to least aqueously altered follows (Rubin et al., 2007):

MET 01070 [2.0]
LAP 02277 [2.0]
QUE 93005 [2.1]
Cold Bokkeveld [2.2]
Nogoya [2.2]
QUE 99355 [2.3]
Mighei [~2.3]
Y-791198 [2.4]
Murray [2.4/2.5]
Murchison [2.5]
Kivesvaara [2.5]
QUE 97990 [2.6] (the least altered known)

Representatives of the earliest stages of aqueous alteration (2.9–2.7) on the CM parent body have not yet been discovered, although the unaltered, type CM3.0 precursor material was probably similar to CM-related Acfer 094.

Further details of the formation of CM chondrites can be found on the Murchison page. The specimen of NWA 4765 shown above is a 0.803 g partial slice. A photo of the main mass is shown below.

Photo courtesy of Chladni's Heirs—S. Ralew & M. Altmann

Photos below courtesy of Aziz Habibi

Click on photo for a magnified view.