NORTHWEST AFRICA 735

Several pieces of this meteorite having a combined weight of 161 g were purchased in Zagora, Morocco by American collector A. Lang. Northwest Africa 735 was classified by Dr. T. Mickouchi at the University of Tokyo as a CK4 carbonaceous chondrite with a shock stage of S3. Heavy weathering consistent with a W3 on the Wlotzka weathering scale (1993) has obscured many of the chondrules in this meteorite. A more useful weathering index (wi) was developed by Rubin and Huber (2005) for the oxidized meteorites such as the CK and R chondrite groups. This index is based on the modal abundance of brown-stained silicates as visually determined on a thin section in transmitted light at ~100× magnification.

The CK chondrites, closely related to the CV chondrites, were designated as a separate group in 1990. It was named for the observed fall in Karoonda, Australia, which was classified as a CK4 (a second CK4 fall occurred in Kobe, Japan in 1999). CK chondrites have refractory lithophile abundances intermediate between those of the CO and CV groups, as well as overlapping O-isotope compositions. CK members also have low chondrule to matrix ratios, with chondrules comprising ~10–15 vol%. Chondrule sizes are intermediate between those of the CO and CV groups, and they lack coarse-grained rims. Other CK characteristics include a very low content of both C and refractory inclusions. Plagioclase in NWA 735 may be calcic (An40–68), and olivine contains NiO, magnetite, and herzynitic spinel.

Also important is the high oxidation of this group, which has resulted in a very low content of FeNi-metal and a correspondingly high content of magnetite and sulfides. The dispersion of these sub-µm- to µm-sized magnetite and sulfide (pentlandite) grains within vesicles of like size has produced the silicate darkening in all metamorphic grades, spanning the complete range of 3–6. Experiments have demonstrated that sub-µm- to µm-sized vesicles and micro-inclusions are produced during shock-melting of fine-grained olivine grains in the matrices (Hashiguchi et al., 2008). These shock events occurred under conditions of low shock pressures (<25 GPa) and high temperatures (>600°C).

The typical features of the CK group listed above have recently been re-evaluated by Greenwood et al. (2003, 2009), and it was proposed that the predominantly equilibrated members of the CK group were consistent with metamorphic progression of the CV group. It was further suggested that the few unequilibrated CK members, such as Dhofar 015, do not exhibit the typical features of CK chondrites, and more closely resemble the oxidized CV3 chondrites. In that vein, a petrologic study was conducted by Chaumard et al. (2009) comparing the CK chondrites to the oxidized subgroup of CV chondrites. They found that matrix, chondrule and CAI abundances in CK chondrites are similar to those characteristics in some oxidized CV members. Moreover, dark inclusions commonly present in the CV group are also abundant in the CK group. They also determined that CK chondrites have an olivine chemistry that is correlated with the textural equilibration of the matrix. Taking these findings into consideration, the investigators suggest that the CK group may represent a metamorphic continuum beginning with the oxidized CV subgroup rather than representing a separate parent body. This hypothesis was further elucidated by Greenwood et al. (2009), and it was proposed that the CK group should be included as a subgroup of the oxidized CV group, designated as CV_oxK.

More studies are needed to elucidate the true relationship between these carbonaceous chondrite groups. The photo above shows a 3.69 g slice of NWA 735.

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