DHOFAR 015

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, although recognized as closely related to the CV chondrites in bulk O-isotopic composition, mineralogy, and petrology, were designated as a separate group in 1990 based on their lower abundances of refractory lithophiles and CAIs, and their lack of coarse-grained igneous rims around chondrules compared to CV-group members, likely a result of higher metamorphic recrystallization. The group 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).

Historically, CK chondrites constituted a heterogeneous group of meteorites that had refractory lithophile abundances intermediate between those of the CO and CV groups, and had a significant abundance of altered refractory inclusions. The CK group members had overlapping O-isotopic compositions with the CV group, as well as overlapping textural and compositional variation. They also had low chondrule to matrix ratios, with matrix comprising ~50–70 vol%; higher than most CV-group members. In a comparison of chondrule sizes between the CK and CV groups, they show similar ranges. While the CV group has low petrologic grades, members of the CK group have been equilibrated to higher petrologic grades of ~3.8 and above. It was first 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. The magnetite grains present in both CK and CV group members have been metasomatically altered by fluids having similar O-isotopic compositions (Davidson et al., 2013). Other experiments have demonstrated that sub-µm- to µm-sized vesicles and micron-sized 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 were re-evaluated by Greenwood et al. (2003), and it was further established that the predominantly equilibrated members of the CK group were consistent with metamorphic progression of the CV group. It was 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, 2011) 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 features in some oxidized CV members. Moreover, dark inclusions commonly present in the CV group are also abundant in the CK group. In their studies they also determined that CK chondrites have an olivine chemistry that is correlated with the textural equilibration of the matrix. Greenwood et al. (2010) also found that the CK and CV chondrites contain magnetites that are compositionally similar, and that major and trace elements overlap between the groups. In studies of discrimination diagrams by Isa et al. (2012), they found that no significant nebular-based distinctions exist between the CV and CK groups. Taking these findings into consideration, these investigators suggest that the CK group may not represent a separate parent body, but more likely constitutes a metamorphic continuum derived from the more unequilibrated CV subgroups.

It was previously recognized that the structural order of insoluble polyaromatic organic matter is irreversibly transformed by thermal metamorphism (from carbonization to graphitization) to a commensurate degree across chemical classes (Bonal et al., 2005; 2007). A correlation exists between this maturation grade of organic matter and the peak metamorphic temperature of the meteorite, and this is then directly associated with the petrologic type. In their Raman spectrographic study of maturation grade vs. petrologic type for select CV and CK chondrites, Chaumard et al. (2013) found that the transition from carbonization to graphitization in these chondrites occurs at the petrologic type for Allende (>3.6; Raman method). From their data, they concluded that the CV and CK chondrites in their study constitute a metamorphic sequence increasing as follows:

Allende (carbonization)
NWA 779 (graphitization)
Tanezrouft 057 (graphitization)
NWA 2900 (graphitization)
NWA 1559 (graphitization)

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 being consistent with a common asteroidal origin, the team suggests that a classification revision should be adopted in which the CK group is considered a part of the oxidized CV subgrouping and designated CV3oxK.

In contrast to the proposal of Greenwood et al., Runyon and Dunn (2011) studied Cr2O3 vs. MgO as well as TiO2 and NiO in magnetite and olivine, discovering that either the results do not support a metamorphic sequence from the oxidized CV to the unequilibrated and equilibrated CK groups, or that the results were ambiguous. In addition, a study of volatile element abundances demonstrated no consistency with a scenario of increasing metamorphism from the oxidized CV to the unequilibrated and equilibrated CK groups (Isa et al., 2011). Moreover, in their study of metamorphosed clasts in CV chondrites, Jogo et al. (2011) determined that CK chondrites have higher NiO contents, have plagioclase that exhibits a unique An distribution, and have abundant magnetite. In their studies of the CV–CK group relationships, Davidson et al. (2012) identified several parameters that are inconsistent with a common CV–CK origin, including differences in chondrule Fa content and Fe/Mn ratios, and differences in FeO and Cr2O3 contents in opaque phases.

In their study of CV and CK chondrites spanning the entire petrographic range, Wasson et al. (2013) demonstrated that the lack of CAIs and igneous rims, as well as the observed elemental fractionations were consistent with a more extensive metamorphic history, including impact-generated crushing, metasomatic oxidation, volatile loss, and recrystallization. Although they believe that the oxidized subgroups of the CV chondrites were originally derived from material related to the reduced subgroup, the exact oxidation pathway is unknown, and therefore they propose a classification scheme in accord with that of Greenwood et al. (2009) in that unequilibrated CK chondrites should be termed CV3oxK, and the equilibrated meteorites should be designated CV4–6.

Our collections contain less than 20 different meteorites representating unequilibrated CVoxK material. The first meteorite identified as such was Camel Donga 003, a 36.6 g meteorite discovered in June 1990. Several anomalous members have also been recognized, including the 353 g DaG 431. Additional CVoxK members have been recovered, mostly in the hot deserts of Northwest Africa (e.g., the 47.1 g NWA 1694), Oman, and Australia, and one each from Antarctica and Texas.

Dhofar 015 is a relatively fresh meteorite with a weathering grade of W1 on the Wlotzka (1993) scale, and a shock stage of S3. Meteorites constituting the equilibrated CV group have an average porosity of 14%, virtually the same as that measured for the unequilibrated CV group (ave. 14.6%; Macke et al., 2011). The photo above shows the interior side of a 0.47 g specimen of Dhofar 015 while that below shows the fusion-crusted side.

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