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, in addition to the low abundance of coarse-grained igneous rims around chondrules in the CK group compared to the CV group; this was considered to be a result of a higher degree of metamorphic recrystallization in CK chondrites. Igneous rims were subsequently identified in ~60% of chondrules observed in CK34 meteorites by Chaumard and Devouard (2016). The group was named for the observed fall in Karoonda, Australia, which was classified as a CK4.
Historically, CK chondrites constituted a heterogeneous group of meteorites that had refractory lithophile abundances intermediate between those of the CO and CV groups, and that had a significant abundance of altered refractory inclusions. The CK group members have O-isotopic compositions that overlap those of the CV group, and the two groups also overlap in both textural and compositional variation. CK chondrites generally have low chondrule to matrix ratios, with matrix composing ~5070 vol%, which is higher than most CV-group members. However, Chaumard and Devouard (2016) reported a large range in chondrule abundances among CK chondrites in their study, which they attributed to re-equilibration during metamorphism. In a comparison of chondrule sizes between the CK and CV meteorites, both groups show similar ranges. While the CV group has low petrologic grades, members of the CK group have been equilibrated to higher petrologic grades of ~3.5 and above. It was first proposed by Greenwood et al. (2009) that these two meteorite groups might represent a single, thermally stratified "onion-shell-like" parent body, and subsequent studies by many investigators have provided valuable evidence towards the resolution of this question.
Like the oxidized CV group, the CK group has a high oxidation state 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. 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 matrix olivines (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 determined that CK chondrites have an olivine chemistry that is correlated with the textural equilibration of the matrix grains. Moreover, Greenwood et al. (2010) found that both the CK and CV chondrites contain magnetites which are compositionally similar, and that major and trace elements overlap between the groups. In addition, in their studies of discrimination diagrams, Isa et al. (2012), 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 instead, consider it more likely that these meteorites constitute a metamorphic continuum derived from the more unequilibrated CV subgroup members.
It was previously recognized that the structural order of insoluble polyaromatic organic matter is irreversibly transformed by thermal metamorphism (carbonization through graphitization) to a commensurate degree across meteorite chemical classes (Bonal et al., 2005; 2007). A positive correlation exists between the particular maturation grade of organic matter and the peak metamorphic temperature of the meteorite, and the latter is 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:
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 the team suggested that a classification revision be adopted in which the CK group is considered a part of the oxidized CV subgrouping and designated CV3oxK.
Runyon and Dunn (2011) investigated Cr2O3 vs. MgO, and TiO2 and NiO in magnetite and olivine, and arrived at a different conclusion from that espoused by Greenwood et al. for a common CV-CK parent body. Their results do not support unambiguously a metamorphic sequence progressing from oxidized CV to unequilibrated and equilibrated CK meteorites. In addition, a study of volatile element abundances by Isa et al. (2011) demonstrated no consistency with a scenario of increasing metamorphism from the oxidized CV to the unequilibrated and equilibrated CK meteorites. Furthermore, 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 contain abundant magnetite. In a similar way, studies of CVCK group relationships by Davidson et al. (2012) led to the identification of several parameters which are inconsistent with a common CVCK origin, including differences in chondrule Fa content and Fe/Mn ratios, and differences in FeO and Cr2O3 contents in opaque phases. Another study of consequence involving elemental analyses in CK chondrites was conducted by Ebihara et al. (2012). They determined that a positive correlation exists between REE abundance and petrologic type, and also that REE abundances in low petrologic type CK chondrites was unlike abundances in Allende. Therefore, a scenario reflecting separate parent bodies for the CV and CK chondrites was deemed most consistent with the data. They suggest that the CK parent body was relatively small in size and had a typical onion-shell structure, with the highest metamorphic grade and the high-temperature phases residing nearest the center.
In their analyses 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; Kereszturi et al. (2015) observed that melting events also played a role in the destruction of chondrules and homogenation of the CK texture. In their in-depth study of CK chondrites, Chaumard and Devouard (2016) found a large range of peak metamorphic temperatures and a lack of correlation with petrologic types, inconsistent with a thermally stratified "onion-shell" structure. They determined that CV and CK chondrites were heated at different temperatures for various durations ranging from tens of years to tens of thousand years, and argue that the heating is most consistent with solar radiative heating over a relatively long-term for an object having a close perihelion (e.g., <0.1 AU), rather than heating by radiogenic decay or impact shock.
Although Wasson et al. (2013) believe the oxidized subgroups of the CV chondrites were originally derived from material related to the reduced subgroup, the exact oxidation pathway is unknown. Therefore they propose a classification scheme in accord with that of Greenwood et al. (2009) in which unequilibrated CK chondrites should be termed CV3oxK, while the equilibrated meteorites should be designated CV46.
Since the proposal to combine the CV and CK groups into one metamorphic continuum was introduced, researchers have applied to the CK3 meteorites many of the same metamorphic indicators previously used to resolve the degree of metamorphism among type 3 ordinary chondrites (Dunn, 2013; Bruck and Dunn, 2014; Dunn and Gross, 2015). Among these metamorphic indicators are 1) Cr2O3 content in olivine from type-II chondrules, 2) percent mean deviation (PMD) of Fa in olivine from type-II chondrules, 3) NiO in olivine from type-II chondrules, and 4) average Cr2O3 and NiO content in magnetite. From their results, they concluded that the CK3 chondrites in their study constitute a metamorphic sequence increasing as follows:
NWA 15593.6 or 3.7
Dhofar 015likely 3.9
Continuing the effort to better resolve the degree of metamorphism among the CK chondrites, and to determine whether or not a genetic relationship exists between the CV and CK groups, Dunn et al. (2016) analyzed the magnetite composition in seven unequilibrated CK meteorites and in one that is equilibrated. Utilizing coupled diagrams which compare magnetite oxide abundances among CV and CK chondrites (e.g., MgO vs. Cr2O3, TiO2, NiO, and Al2O3), they established geochemical and mineralogical evidence (e.g., oxygen fugacities, peak temperatures, magnetite compositions), as well as petrographic evidence (e.g., chondrule size and abundance), which is most consistent with separate CV and CK parent bodies. For example, the magnetite compositions of CV chondrites should be more representative of those among the unequilibrated rather than the equilibrated CK chondrites, given a metamorphic sequence based on increasing oxidation as follows: CV3red ⇒ CV3ox ⇒ CK3 ⇒ CK46. However, in the coupled diagrams presented by Dunn et al. (2016) this expectation is not realized (see sample diagram below).
Magnetite Composition Among CV and CK Chondrites (A)
Diagram credit: Dunn et al., MAPS vol. 51, #9, p. 1711 (2016)
CK3 chondrites are open triangles, CK46 chondrites are open squares, Dhofar 015 is an open diamond, and CV chondrites are solid circles.
The results of this new study based on individual mineral chemistries are contrary to those of previous studies involving bulk compositional analyses, likely due to the heterogeneous nature of these meteorite groups (Dunn et al., 2016). Resolution between the CV and CK chondrite groups is apparent utilizing magnetite compositional diagrams (see diagrams below). In addition, Dhofar 015 is clearly distinguished from the group of unequilibrated CK chondrites; although Dhofar 015 has chondrule textures, matrix textures, and a feldspar compositional range consistent with petrologic type CK3.9 (Ivanova et al., 2000), its fayalite value (Fa32) and its magnetite composition places it among the group of equilibrated CK chondrites, and Dunn et al. (2016) contend that it should be classified as an equilibrated CK4.
Magnetite Composition Among CV and CK Chondrites (B)
Diagrams credit: Dunn et al., MAPS vol. 51, #9, pp. 17121713 (2016)
CK3 chondrites are open triangles, CK46 chondrites are open squares, Dhofar 015 is an open diamond, and CV chondrites are solid circles.
Previous studies (e.g., Sanborn et al., 2014) have established that a coupled Δ17O vs. ε54Cr diagram is one of the best diagnostic tools for determining genetic relationships between meteorites. New Cr-isotopic analyses were conducted by Yin et al. (2017) for two new equilibrated CK chondrites (NWA 7461 and 7704) along with CV3 Allende, and when combined with previous analyses it was determined that CK chondrites have an average ε54Cr value of +0.66 (±0.06), while the CV chondrites have an average value of +0.88 (±0.06). the results are consistent with an origin from two distinct parent bodies (see diagrams below).
Diagrams credit: Yin et al., 48th LPSC, #1771 (2017)
Our collections contain less than 20 different meteorites representating unequilibrated CK material. 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.