CK3
Found January 21, 2000
18° 38.6' N., 54° 25.8' E.
A single, black, fusion-crusted stone weighing 184 g was found in Oman. This Karoonda-type carbonaceous chondrite has a dark-gray, porous, friable, fine-grained matrix (65 vol%), containing chondrules (up to 1 mm) and plagioclase-rich objects, but lacking FeNi-metal. The chondrules contain primary glass and have well-defined boundaries, indicative of a low degree of thermal metamorphism. Likewise, the plagioclase-rich objects have well-defined boundaries and have a wide compositional range of feldspar, further indication of a low petrologic type. Other properties inherent in this meteorite suggest that it experienced oxidizing conditions during nebular condensation. The very low S content and lack of troilite is consistent with the removal of S as an oxide during nebular processes.
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 are a heterogeneous group containing high refractory lithophile abundances intermediate between those of the CO and CV groups, which contain a significant abundance of altered refractory inclusions. The CK group has overlapping O-isotopic compositions with the CV group, as well as an overlapping compositional variation. CK members also have low chondrule to matrix ratios, with matrix comprising ~50–70 vol%, higher than most CV-group members, but likely forming a continuum between the groups. Chondrule sizes between the CK and CV groups show similar ranges. Contrary to CV chondrules, CK chondrules lack coarse-grained rims, likely a result of higher metamorphic recrystallization; while the CV group has low petrologic grades below 3.3, the CK group has been equilibrated to petrologic grade 3.5 and above. It has been proposed by Greenwood et al. (2009) that these two meteorite groups may represent a single, thermally stratified, "onion-shell-like" parent body.
The CK group, like the oxidized CV group, has a high oxidation state that 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, which spans the complete range of 3–6. Experiments have demonstrated that sub-µm- to µm-sized vesicles and µm-sized inclusions are produced during partial melting events, caused by low shock pressures (<25 GPa) and high temperatures (>600°C).
The typical features of the CK group listed above were 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, but more closely resemble the oxidized CV3 chondrites.
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. In their study of the CK and CV chondrites, Greenwood et al. (2010) found that magnetites are compositionally similar and that major and trace elements overlap. Taking these findings into consideration, these investigators suggest that the CK group may not represent a separate parent body, but instead, constitute a metamorphic continuum beginning with the oxidized CV subgroup.
In an in-depth geochemical, mineralogical, and isotopic study of the characteristics of the CK and CV groups, Greenwood et al. (2009) provided detailed evidence for such a common parent body scenario. They revealed that both groups show similar CRE age clusters of ~9 and ~29 m.y., and suggest that the Eos asteroid family may have been formed by the disruption 1.3 b.y. ago of this common ~218 km diameter parent body. This asteroid family is located in the outer asteroid belt 2.95–3.13 AU in which C class asteroids reside. With all of this data consistent with a common asteroidal origin, the team suggests that a classification revision should be adopted in which the CK group is considered part of the oxidized CV subgrouping and designated CVoxK, or CV4–6oxK for equilibrated samples.
Our collections contain only a few representatives of unmetamorphosed, type-3 CK material. The first meteorite identified as such was Camel Donga 003, a 36.6 g meteorite discovered in June 1990. The next, the 83.8 g Watson 002, was found in October 1991. An anomalous member, the 353 g DaG 431, was identified in 1998. In 2000, two additional members were recovered in separate hot deserts—Dhofar 015 in January, and the 71 g NWA 772 in October. More recently, the 47.1 g NWA 1694 and the 284 g NWA 1559 were identified.
Dhofar 015 is a relatively fresh meteorite with a weathering grade of W1 on the Wlotzka (1993) scale, with a shock stage of S3. The photo above shows the interior side of a 0.47 g specimen of Dhofar 015, while that below shows the fusion-crusted side.
