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) due to the fact that chondrule pseudomorphs are present in the least altered CM members. The most altered members, previously classified as CM1, will become CM2.0, with 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, ungrouped (probably CO-related; Simon and Grossman, 2015), type 3.0 Acfer 094, or perhaps the CO3.0 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:
hydration of fine-grained matrix material to form phyllosilicates, which gradually consists of Mg-rich serpentines
conversion of primary igneous glass in chondrules to phyllosilicate
production of large PCP clumps, now determined to be tochilinitecronstedtite intergrowths (TCI)
precipitation of sulfides within cavities
processes occurring throughout the alteration sequence:
formation of increasingly complex carbonates within sulfide-lined cavities
compositional changes in sulfides
replacement of primary melilite by secondary alteration products
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]
QUE 99355 [2.3]
QUE 97990 [2.6]
Representatives of the earliest stages of aqueous alteration (2.92.7) on the CM parent body have not yet been discovered, although the unaltered, type CM3.0 precursor material was probably similar to the ungrouped (probably CO-related; Simon and Grossman, 2015) Acfer 094.
Current studies suggest that both cometary dust and meteorites should be produced from the disruption of Jupiter-family comets which originate in the Kuiper belt. Studies have shown that Antarctic micrometeorites have a similar carbonaceous chondrite:ordinary chondrite ratio ((~7:1) as the composition of zodiacal dust (Meier, 2014). Based on observational evidence and current modeling, it is thought that comets should be dark in color and have a low density and strength, a high porosity, a solar ratio of elements, an elevated ratio of C, H, O, and N, a high interstellar grain content, anhydrous and highly unequilibrated silicates, few to no chondrules, and a low cosmic-ray exposure age (<10 m.y.). Both the CI and CM groups of meteorites exhibit characteristics that are consistent with the above descriptions.
Orbital data obtained from several carbonaceous chondrites (e.g., CI Orgueil [eyewitness plotting]; CMs Maribo and Sutter's Mill [instrument recording]) are a good match to the orbits expected from the disruption of Jupiter-family comets, but are unlike the orbits of ordinary chondrites and most other asteroidal objects (Meier, 2014). Both the orbital eccentricity and semimajor axis for Maribo is nearly identical to those of Comet Encke and the associated Taurid swarm of objects (Haack et al., 2011). On the other hand, a CRE age study of CM chondrites conducted by Meier et al. (2016) shows a possible relationship exists to the asteroid breakup event ~8.3 m.y. ago that formed the Ch/C/Cg-type members of the Veritas family. In addition to the large abundance of 3He-enriched interplanetary dust discovered in 8.2 m.y.-old deep-sea drill cores, ~1/6 of all CM meteorites have 21Ne-based CRE ages that are consistent with derivation from this catastrophic breakup, while others with significantly younger CRE ages could represent secondary collisions among the Veritas fragments.
Based on reflectance spectra in the range of 0.4 to 2.4 μm captured by the OSIRIS-REx spacecraft, as well as utilizing meteorite analogs, it has been speculated that asteroid (101955) 1999 RQ36 is composed of CM1-like material. Further details of the formation of CM chondrites can be found on the Murchison page. The specimen of NWA 4765 shown above is a 0.8 g partial slice. A photo of the main mass is shown below.
Photo courtesy of Chladni's HeirsS. Ralew & M. Altmann
Photos shown below courtesy of Aziz Habibi