NORTHWEST AFRICA 521

A single 376 g stone was purchased in a market in Morocco by American collectors in 2001. Northwest Africa 521 was classified by Dr. Alan Rubin at UCLA as a CK4 carbonaceous chondrite with a shock stage of S3. Although initially a weathering grade of W1 was assigned to 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; NWA 521 was determined to have an index of wi-5, or severely weathered.

Northwest Africa 735, an unpaired CK4 meteorite with a weathering grade of W3, was purchased in Zagora almost two years earlier. Two other CK meteorites with a transitional petrologic grade of 4/5 have also been found in the Sahara, DaG 250 and DaG 275. Based on cosmic-ray exposure age data these two meteorites are not paired; DaG 250 has a CRE age of 27 m.y. while DaG 275 has a CRE age of 35.2 m.y. In addition, the noble gas data does not indicate that they are paired. Age data for NWA 521 and 735 has not yet been published.

The CK chondrites, closely related to the CV and CO 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.

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 caused pronounced 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 partial melting events; events caused by 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), 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. More studies are needed to elucidate the true nature of this group. The photo above shows a 1.9 g partial slice of NWA 521.

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