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NWA 2388

CK6
standby for northwest africa 2388 photo
Purchased September 2004
no coordinates recorded Four fusion-crusted stones having a combined weight of 916 g were purchased from a Moroccan dealer at the 2004 Denver Show. A sample was submitted to Northern Arizona University (T. Bunch and J. Wittke), and NWA 2388 was classified as a CK6 carbonaceous chondrite with sparse relict chondrules within a recrystallized, equilibrated groundmass. The meteorite has experienced moderate weathering and a low shock stage, and contains abundant plagioclase and magnetite. The CK6 chondrites are the most oxidized meteorites, formed under oxidation conditions of ~QFM+2.5 (McCoy et al., 2018).

After in-depth analyses of many CV and CK meteorites having a wide range of petrologic types was conducted by Wasson et al. (2013), they presented a reasoned argument for merging the CK and CV groups into a single unified group. The geochemical and petrological justification for such a reclassification of the CK chondrites, along with details of their proposed taxonomic scheme, can be found on the Dhofar 015 page. Subsequent studies have demonstrated a high likelihood for separate parent bodies. One such study conducted by Dunn et al. (2016) compared magnetite in a number of CK and CV chondrites. They presented geochemical, mineralogical, and petrographic evidence which is more consistent with separate CV and CK parent bodies. Another study conducted by Yin et al. (2017) utilized a coupled Δ17O vs. ε54Cr diagram to plot several CK and CV chondrites. Through this technique they demonstrated that these two meteorite groups derive from separate parent bodies. Details of these studies can also be found on the Dhofar 015 page. The photo above shows a 1.4 g partial slice of NWA 2388.


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NWA 1694

CK3
standby for northwest africa 1694 photo
Found October 2002
no coordinates recorded A single fusion-crusted stone weighing 47.1 g was found in the Sahara Desert and was subsequently purchased in Safsaf, Morocco by American collectors. Classification was conducted at Northern Arizona University (T. Bunch and J. Wittke). This very fresh find (W0/1) is classified as an unshocked (S1) CK3. Northwest Africa 1694 is composed of 47 vol% fine-grained matrix, 44 vol% chondrules (up to 1.2 mm), 6 vol% sulfides (containing inclusions of chlorapatite and silica glass), 2 vol% Cr-rich magnetite, and 1 vol% CAIs. Although this meteorite is similar to NWA 772, the two are not considered to be paired.

After in-depth analyses of many CV and CK meteorites having a wide range of petrologic types was conducted by Wasson et al. (2013), they presented a reasoned argument for merging the CK and CV groups into a single unified group. The geochemical and petrological justification for such a reclassification of the CK chondrites, along with details of their proposed taxonomic scheme, can be found on the Dhofar 015 page. Subsequent studies have demonstrated a high likelihood for separate parent bodies. One such study conducted by Dunn et al. (2016) compared magnetite in a number of CK and CV chondrites. They presented geochemical, mineralogical, and petrographic evidence which is more consistent with separate CV and CK parent bodies. Another study conducted by Yin et al. (2017) utilized a coupled Δ17O vs. ε54Cr diagram to plot several CK and CV chondrites. Through this technique they demonstrated that these two meteorite groups derive from separate parent bodies. Details of these studies can also be found on the Dhofar 015 page.

Our collections contain less than 20 different meteorites representating unequilibrated CK material. The specimen shown above is a 2.2 g partial slice of CK3 NWA 1694, however, a subtype has not yet been ascertained.


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NWA 735

CK4
standby for northwest africa 735 photo
Purchased January 1999
30° 37′ N., 4° 07′ W. 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 highly oxidized meteorites of the more equilibrated CV members and the R chondrite group. 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.

Meteorites in the CK group were generally classified as such based on their lower abundances of refractory lithophile elements and their lack of CAIs and coarse-grained rims around chondrules compared to members of the CV group. After in-depth analyses of many CV and CK meteorites having a wide range of petrologic types was conducted by Wasson et al. (2013), they presented a reasoned argument for merging the CK and CV groups into a single unified group. The geochemical and petrological justification for such a reclassification of the CK chondrites, along with details of their proposed taxonomic scheme, can be found on the Dhofar 015 page. Subsequent studies have demonstrated a high likelihood for separate parent bodies. One such study conducted by Dunn et al. (2016) compared magnetite in a number of CK and CV chondrites. They presented geochemical, mineralogical, and petrographic evidence which is more consistent with separate CV and CK parent bodies. Another study conducted by Yin et al. (2017) utilized a coupled Δ17O vs. ε54Cr diagram to plot several CK and CV chondrites. Through this technique they demonstrated that these two meteorite groups derive from separate parent bodies. Details of these studies can also be found on the Dhofar 015 page. The photo above shows a 3.69 g slice of NWA 735.


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NWA 060

CK5
standby for northwest africa 060 photo
Purchased August 2000
no coordinates recorded A single 604 g stone was purchased in a market in Erfoud, Morocco by American collectors in August 2000. Black fusion crust still covers the portion of the meteorite that was exposed above the desert sands, while the buried portion is encrusted with desert caliche. The meteorite was classified by Alan Rubin at UCLA as a CK5. Typical of CK chondrites, NWA 060 contains ~75 vol% matrix, 15 vol% chondrules, and 0.5 vol% CAIs (Huber et al., 2006). A few vol% of magnetite is present as grains in the matrix, as curvilinear trails inside of silicates, and as components of small nodules. Minor sulfides, primarily pentlandite and pyrite, also occur. Regions exhibiting crushing were documented by Wasson et al. (2013).

A weathering index (wi) was developed by Rubin and Huber (2005) for the highly oxidized meteorites, such as those of the R chondrite and equilibrated CV 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 060 was determined to have an index of wi-5 (severely weathered).

After in-depth analyses of many CV and CK meteorites having a wide range of petrologic types was conducted by Wasson et al. (2013), they presented a reasoned argument for merging the CK and CV groups into a single unified group. The geochemical and petrological justification for such a reclassification of the CK chondrites, along with details of their proposed taxonomic scheme, can be found on the Dhofar 015 page. Subsequent studies have demonstrated a high likelihood for separate parent bodies. One such study conducted by Dunn et al. (2016) compared magnetite in a number of CK and CV chondrites. They presented geochemical, mineralogical, and petrographic evidence which is more consistent with separate CV and CK parent bodies. Another study conducted by Yin et al. (2017) utilized a coupled Δ17O vs. ε54Cr diagram to plot several CK and CV chondrites. Through this technique they demonstrated that these two meteorite groups derive from separate parent bodies. Details of these studies can also be found on the Dhofar 015 page. The photo above shows a 0.7 g partial slice of NWA 060, while that below shows the complete mass as found. standby for northwest africa 060 photo
Photo courtesy of Michael Cottingham, © 2000


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Dho 015

CK4
standby for dhofar 015 photo
Found January 21, 2000
18° 38.6′ N., 54° 25.8′ E. A single black, 184 g, fusion-crusted stone 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 CK3–4 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 ~50–70 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.

As with 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: 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 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 CV–CK group relationships by Davidson et al. (2012) led to the identification of several parameters which 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. 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 CV4–6.

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 5343—unspecified
NWA 1559—3.6 or 3.7
NWA 2043—unspecified
DaG 431—3.8
Dhofar 015—likely 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 ⇒ CK4–6. 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)
standby for cv vs. ck diagram
CK3 chondrites are open triangles, CK4–6 chondrites are open squares, Dhofar 015 is an open diamond, and CV chondrites are solid circles.
Diagram credit: Dunn et al., MAPS vol. 51, #9, p. 1711 (2016)
‘Magnetite in the unequilibrated CK chondrites: Implications for metamorphism and new insights into the relationship between the CV and CK chondrites’
(http://dx.doi.org/10.1111/maps.12691)
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). A comparitive analysis was conducted by Dunn et al. (2018) between the most unequilibrated CK chondrite known, NWA 5343 (3.6/3.7), and its possible CV-type precursor represented by the reduced CV3.3 Vigarano. They found that both the porosity and the texture of matrix olivine in NWA 5343 were inconsistent with metamorphism of Vigarano-like material. 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. Therefore, Dunn et al. (2016) contend that Dhofar 015 should be classified as an equilibrated CK4. Magnetite Composition Among CV and CK Chondrites (B)
standby for cv vs. ck diagrams
CK3 chondrites are open triangles, CK4–6 chondrites are open squares, Dhofar 015 is an open diamond, and CV chondrites are solid circles.
Diagrams credit: Dunn et al., MAPS vol. 51, #9, pp. 1712–1713 (2016)
‘Magnetite in the unequilibrated CK chondrites: Implications for metamorphism and new insights into the relationship between the CV and CK chondrites’
(http://dx.doi.org/10.1111/maps.12691)
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, 2019) for a broad sampling of CK and CV chondrites. When combined with previous analyses, it was determined that CK chondrites have an average ε54Cr value of +0.65 (±0.04), 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, and also the NWA 6047 page). Cr Isotope Weighted Average For CV and CK Chondrites
standby for cv and ck cr isotope diagram
click on photo for a magnified view

O–Cr Diagram For CV and CK Chondrites
CK: orange shades; CV: green shades; Achondrites: open
standby for cv and ck cr isotope diagram
click on photo for a magnified view

Diagrams credit: Yin and Sanborn et al., 50th LPSC, #3023 (2019)
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.

standby for dhofar 015 photo