Icarus 221 (2012) 911–924

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Icarus

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Spectral reflectance properties of carbonaceous chondrites: 7. CK chondrites

E.A. Cloutis a,⇑, P. Hudon b,1, T. Hiroi c, M.J. Gaffey da Department of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba, Canada R3B 2E9b Astromaterials Research and Exploration Science Office, NASA Johnson Space Center, Mail Code KR, 2101 NASA Road 1, Houston, TX 77058-3696, USAc Department of Geological Sciences, Brown University, PO Box 1846, Providence, RI 02912-1846, USAd Department of Space Studies, University of North Dakota, PO Box 9008, Grand Forks, ND 58202-9008, USA

a r t i c l e i n f o

Article history:Received 25 June 2012Revised 10 September 2012Accepted 10 September 2012Available online 1 October 2012

Keywords:Asteroids, SurfacesAsteroids, CompositionMeteoritesSpectroscopy

0019-1035/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.icarus.2012.09.017

⇑ Corresponding author. Fax: +1 204 774 4134.E-mail addresses: e.cloutis@uwinnipeg.ca (E.A. Clo

(P. Hudon), takahiro_hiroi@brown.edu (T. Hiroi), gaffe1 Present address: Department of Mining and M

University, 3610 rue Université, Montreal, QC, Canada

a b s t r a c t

The reflectance spectra of 15 CK chondrites have been measured as part of an ongoing study of carbona-ceous chondrite reflectance spectra. The available sample suite includes multiple grain sizes and sampleswith petrologic grades varying from CK4 to CK6. CK reflectance spectra are all characterized by an olivine-associated absorption band in the 1.05 lm region. Compared to pure olivine, CK spectra are darker, have amore subdued olivine absorption band, and are often more blue-sloped. Reflectance at 0.56 lm variesfrom 9.6% to 22.5%, and olivine band depth varies from 6.7% to 31.0%, for powders that include the finestfraction. With increasing grain size, and exclusion of the finest fraction, CK spectra become darker andmore blue sloped, while the olivine absorption band initially becomes deeper and then shallower. Thepresence of calcium–aluminum inclusions (CAIs), whose abundance varies widely in CKs, does not nor-mally lead to the appearance of a well-defined absorption band in the 2.1 lm region, although the overallblue slope of many CKs is likely attributable to Fe-bearing spinel in CK CAIs. The only consistent spectralfeature that relates to metamorphic grade is that CK6 spectra have uniformly deeper olivine absorptionband than CK4–5.5 spectra. This could be related to various factors such as loss/aggregation of opaquesthat may accompany metamorphism. Comparison of CV and thermally metamorphosed carbonaceouschondrite to CK spectra suggests that metamorphism to between �1000 and 1200 �C is required forCV spectra to match CK spectra; CV spectra are uniformly darker and have shallower olivine absorptionbands than CK spectra.

� 2012 Elsevier Inc. All rights reserved.

1. Introduction

In an ongoing series of papers concerning the spectral reflec-tance properties of carbonaceous chondrites (CCs), this paperfocuses on the CK group. The CK chondrites are interesting froma number of perspectives. In comparison to other CC groups, theCKs have experienced varying degrees of thermal metamorphism,with petrologic grades ranging from �3 to �6 (Kallemeyn et al.,1991; Noguchi, 1993; Geiger and Bischoff, 1995; Brearley andJones, 1998), and evidence of fluid-assisted metamorphism insome cases (Brearley, 2009). Thus they provide insights into hownaturally occurring thermal metamorphism affects CCs, supple-menting the results of laboratory studies and other, generallyungrouped CCs that have also been affected by thermal metamor-phism (e.g., Hiroi et al., 1993, 1996).

ll rights reserved.

utis), pierre.hudon@mcgill.cay@space.edu (M.J. Gaffey).aterials Engineering, McGillH3A O5C.

CK chondrites are highly oxidized and generally range betweenpetrologic grades 3 and 6, with high modal abundances of magne-tite – the most common opaque phase – (1.2–8.1 vol.%) comparedto CO, CM, and CV chondrites and high olivine Fa contents (Fa28–33)(Noguchi, 1993; Geiger and Bischoff, 1995; Huber et al., 2006).Metamorphism under oxidizing conditions is also suggested bythe possible presence of Fe3+ in low-calcium pyroxenes and spinels,and abundant Ni in olivines (Noguchi, 1993). Metamorphism alsoleads to Fe enrichment in CAIs (Chaumard et al., 2009).

The CKs have been linked to CVs, and possibly COs, on the basisof a number of criteria. These include compositional, textural, andoxygen isotope similarities to CVs and COs (Kallemeyn et al., 1991).Compositional similarities include elemental abundance patterns,refractory lithophile and siderophile abundances similar to COsand CVs (Kallemeyn et al., 1991). Differences between CK versusCV3OxA groups include lower chondrule/matrix ratios, lower C con-tent, and lower CAI abundances (

Table 1Petrographic characteristics of C-chondrite groups. Source: Brearley and Jones (1998),Neff and Righter (2006), and Rubin (2010).

Group Chondruleabundance(vol.%)

Matrixabundance(vol.%)

Refractoryinclusionabundance(vol.%)

Metalabundance(vol.%)

Chondrulemeandiameter(mm)

CI �1 >99 �1 0 –CM 20 70 5 0.1 0.3CR 50–60 30–50 0.5 5–8 0.7CO 48 34 13 1–5 0.15CV 45 40 10 0–5 1.0CK 15 50–79 4

Table 2Compositional data for CKs included in this study. Source: Geiger and Bischoff (1995) and Neff and Righter (2006).

Meteorite Type Chondrules (vol.%) Opaques (vol.%) CAIs Matrix (vol.%) Magnetite (vol.%) Sulfides H2O Olivine Fa Pyroxene Fs

A-881551 CK6 11 5.3 wt.% 2.0 wt.% 33A-882113 CK4 4.7 wt.% 3.1 wt.% 21.4 22.6ALH 85002 CK4 3.7 0.6 vol.% 29 26DAV 92300 CK4 26 26EET 83311 CK5 6.6 0.5 vol.% 31EET 87526 CK5 29 24EET 87507 CK5 0.2 areal% 3.1

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same metrics for analysis of CKs. These metrics include absolutereflectance at 0.56 lm, highest absolute reflectance, various mea-sures of overall spectral slope, and band depths. Band centers weredetermined by first dividing out a straight line continuum tangentto the spectra on either side of the 1 lm region feature. The centerof this feature was then fit with both third order polynomials usingdifferent numbers of data points as well as the midpoints of a ser-ies of chords spanning the absorption feature.

4. Spectral properties of constituent phases

The main constituents in CK chondrites include olivine(Fa�21–33), magnetite (with a variety of grain sizes), Fe-sulfides,pyroxene (both low- and high-Ca), plagioclase feldspar, CAIs (thatinclude a variety of minerals such as fassaite and Fe-bearing spi-nel), and minor amounts of carbonaceous phases.

CK constituents can be broadly subdivided into two varieties –those that contribute distinct spectral features, and those thataffect overall reflectance and spectral slopes. The constituents thatwill contribute spectral features include olivine, pyroxene, plagio-clase feldspar, magnetite, and CAI minerals. Of these, olivine is themost common constituent in CKs. It is characterized by anabsorption feature centered near 1.06 lm that consists of threeabsorption features due to Fe2+ crystal field transitions in the M2site (near 1.06 lm), and M1 site (near 0.85 and 1.25 lm). Withincreasing Fe2+ content, the center of this feature moves to longerwavelengths, it becomes deeper, and overall reflectance decreases(King and Ridley, 1987). The olivine band center occurs between�1.045 and 1.085 lm. Fig. 1 shows reflectance spectra of someolivine spectra as a function of Fe2+ content.

Fig. 1. Reflectance spectra of CK constituents (

Fig. 2. Reflectance spectra of fine-grained CK chondrites. (a) CK4. (b) CK4/5 andCK5. (c) CK 5/6 and CK6. Grain sizes are indicated for each spectrum.

Fig. 3. Duplicate reflectance spectra of a

Table 4Selected spectral parameters for CK chondrites.

Meteorite Class Grain size(lm)

Reflectance at0.56 lm (%)

0.6/0.5 lmreflectance ratio

Band centernear 1 lm (lm)

Band minimum near1 lm (lm)

Depth of1 lm band (%)

�0.7 lm peak:2.5 lm refl. ratio

RELABfile ID

A-881551a CK6

Fig. 4. Reflectance spectra of different size fractions of CK chondrites. (a) EET 87526. (b) EET 87860. (c) LEW 87009. (d) ALH 85002 – single grain size sequence. (e) LEW 85002– other spectra from multiple investigators. Grain sizes are indicated for each spectrum; ‘‘powder’’ refers to powdered sample of unknown grain size.

E.A. Cloutis et al. / Icarus 221 (2012) 911–924 917

absorption feature in the 1 lm region. Band depths are 10.0–14.5%for the CKs and 2.5–6.0% for the CVOx chondrites. These differencesare likely related, and could be attributable to thermal metamor-phism leading to a loss or aggregation of opaque phases. Thiswould lead to an increase in reflectance and a greater apparentolivine band depth – an increase in olivine abundance is notrequired for its band depth to increase.

The presence of finely-dispersed opaque phases in CK olivinehas been noted by many investigators (e.g., Kallemeyn et al.,1991; Rubin, 1991, 1992; Noguchi, 1993; Geiger and Bischoff,1995). Their origin is uncertain, and different investigators attributethem to shock (Rubin, 1992) or other processes, such as oxidationof olivine (Scott et al., 1992). The higher reflectance and deeper1 lm region absorption band in CKs versus CVs is not consistentwith shock processes, which tends to lower absolute reflectanceand weaken absorption bands, but is consistent with thermal

annealing, which may have occurred after a shock event (Rubin,1992).

5.5. CKs versus heated Allende

Further insights into whether thermal metamorphism canaccount for differences between the CVOx and CK chondrites canbe gleaned from studies of the Allende CV3Ox chondrite subjectedto laboratory heating. Figs. 7a and b show

Fig. 5. Band depth of 1 lm region absorption feature versus metamorphic grade forall CK spectra.

Table 5CVOx chondrites that have been spectrally characterized in previous studies.

Meteorite Subtype Petrologicsubgraderangea

Averagepetrologicsubtypeb

Petrologicsubtypeb

ALH 84028 CV3OxA 3.2 3.2ALH 85006 CV3OxAllende CV3OxA 3.1–3.6 3.2 >3.6Grosnaja CV3OxB 3.0–3.3 3.3 �3.6Mokoia CV3OxA 3.0–3.3 3.2 �3.6Y-86751 CV3OxA–OxB

a Guimon et al. (1995).b Bonal et al. (2006).

Fig. 6. Reflectance spectra of CVOx carbonaceous chondrite powders and CKchondrites. (a)

Fig. 7. Reflectance spectra of heated Allende

Fig. 9. Reflectance spectrum of

E.A. Cloutis et al. / Icarus 221 (2012) 911–924 921

olivine. The center of this band is broadly consistent with the com-position of CK olivine, although it should be noted that the olivineband center varies by only �40 nm from Fa0 to Fa100 (King andRidley, 1987). CAIs, which are present in variable amounts inCKs, do not normally result in a well-defined absorption band thatcan be related to Fe-bearing spinel; i.e., an absorption band cen-tered near 2.1–2.2 lm. However, CAIs may be causing a numberof CK spectra to be blue-sloped beyond �1.5 lm. No good spectralcorrelations have been found to determine metamorphic grade,although it appears that olivine band depths are greatest in theCK6 group. This could be related to various factors such as loss/aggregation of opaques that may accompany metamorphism.

A number of investigators have suggested linkages between CVand CK chondrites (e.g., Greenwood et al., 2003, 2004, 2010;Devouard et al., 2006; Isa et al., 2011). Spectrally it appears thatfine-grained CVs are darker than CKs and have shallower olivineabsorption bands. CVs also generally have more well-definedCAI-associated absorption features in the 2 lm region. If CKs arethermally metamorphosed CVs, heating temperatures of between�1000 and �1200 �C are required for CVs (at least for the case ofAllende) to match CK spectra in terms of overall reflectance andolivine band depth. These temperatures appear to be at odds withmany CK temperature estimates based on mineralogic and petro-logic criteria, which are much lower. CCs that have been naturallythermally metamorphosed (up to �950 �C) are darker than CKspectra. They also do not have well-defined olivine absorptionbands, suggesting that they were not heated to high enough tem-peratures to reduce the spectrum-darkening effects of opaques orto recrystallize from pre-existing phyllosilicates.

With increasing grain size (and excluding the finest fraction),CK spectra become darker and more blue-sloped, and as a resultthe band minimum in the 1 lm region moves to longer wave-lengths. These observations are consistent with the behavior ofother CCs (Johnson and Fanale, 1973).

Acknowledgments

We wish to thank the invaluable and generous assistance pro-vided by many individuals which made this study possible. In par-ticular we thank the US and Japanese Antarctic meteorite programsfor recovering the majority of the samples included in this study.The RELAB facility at Brown University is a multi-user facility oper-ated with support from NASA Planetary Geology and GeophysicsGrant NNG06GJ31G, whose support is gratefully acknowledged.This study was supported by an NSERC Discovery grant to EAC.We also wish to thank Alan Rubin and Beth Clark for their cogentand valuable comments which improved the accuracy and read-ability of this manuscript.

Appendix A. Descriptions of CKs included in this study

A.1. A-881551 (CK6)

This meteorite has been classified as a CK6 chondrite byChikami et al. (1998) on the basis of absence of CAIs, recrystallizedmatrix, blurring of chondrules, low abundance of chondrules(11 vol.%), homogenous olivine (Fa33), a matrix composed largelyof coarsely recrystallized olivine, plagioclase, compositionallyhomogeneous augite, pentlandite, pyrite, and a high abundanceof magnetite with ilmenite and spinel lamellae. It contains5.3 wt.% FeS, 0 wt.% Fe–Ni metal and 2.0 wt.% H2O (Yanai et al.,1995), Olivine composition is Fa32.7–34.8 (average Fa33.5) (Yanaiet al., 1995). Hirota et al. (2002) classified it as a CK6, presumablyon the basis of REE abundances.

A.2. A-882113 (CK4)

This meteorite was classified as C4 by Yanai et al. (1995) andCK4 by Hirota et al. (2002), presumably on the basis of REE abun-dances. Olivine composition is Fa21.4 (range Fa20.0–22.8); pyroxenecomposition is Fs22.6 (range Fs20.0–27.3); high-Ca pyroxene thatwas detected is Fs8.3 Wo45.3 (Yanai et al., 1995). It also contains pla-gioclase feldspar and high-Ca pyroxene (Yanai et al., 1995). It con-tains 4.7 wt.% FeS, 0 wt.% Fe–Ni metal and 3.1 wt.% H2O (Yanaiet al., 1995).

A.3. ALH 85002 (CK4; weathering index-1; weathering grade A)

This meteorite was initially classified as a C4 by Martinez andMason (1986). The interior is light gray with dark rounded inclu-sions and white irregular-shaped inclusions (Martinez and Mason,1986). In thin section it consists largely of olivine, with a littlepyroxene, plagioclase and opaques (largely magnetite) (Martinezand Mason, 1986). A few olivine-bearing chondrules are present;compositions are: olivine Fa29, pyroxene Fs26, and plagioclaseAn54–59 (Martinez and Mason, 1986). Mason (1991) described itas consisting of finely-granular olivine (Fa29), small amounts ofpyroxene (Fs26), plagioclase (An54–59), and opaques (largely magne-tite), and a few olivine-rich chondrules. Olivine and low-Ca pyrox-ene compositions are Fa�30 and Fs23–29 according to Kallemeynet al. (1991). Opaque phases include sulfides and magnetite (Kalle-meyn et al., 1991). It contains 3.7 modal vol.% magnetite and 0.6modal vol.% sulfides (Geiger and Bischoff, 1995). Non-stoichiome-tric magnetite constitutes �20% of the iron modal mineralogy(Fisher and Burns, 1991). C and S contents are 0.006 and1.86 wt.%, respectively (Hartmetz et al., 1989).

A.4. DAV 92300 (CK4; weathering grade A/B)

This meteorite was classified as a CK4 chondrite by Marlow andMason (1993a). Magnetic susceptibility measurements for thismeteorite are consistent with CK chondrites (Rochette et al.,2008). It contains numerous chondrules in a medium-gray fine-grained matrix (Marlow and Mason, 1993a). It contains small(0.01–0.05 mm) olivine grains and minor opaque material with afew chondrules; olivine composition is Fa26; minor pyroxene isFs26 (Marlow and Mason, 1993a). The opaque mineral is largelymagnetite (Marlow and Mason, 1993a).

A.5. EET 83311 (CK5; weathering index-1; weathering grade A/B)

This meteorite consists largely of finely granular olivine (Fa31)and minor magnetite and is medium to dark gray in color (Scoreand Mason, 1987). The interior is medium to dark gray with abarely visible chondritic structure (Score and Mason, 1987). It iscomposed largely of finely granular olivine and a little plagioclaseand opaques (largely magnetite) (Score and Mason, 1987). Itcontains 6.6 modal vol.% magnetite and 0.5 modal vol.% sulfides(Geiger and Bischoff, 1995). It was reclassified from a C4 chondriteto a CK5 chondrite by Score and Lindstrom (1994). Olivine compo-sition is Fa�33 according to Kallemeyn et al. (1991). Opaque phasesinclude sulfides and magnetite (Kallemeyn et al., 1991). C and Scontents are 0.04 and 1.98 wt.%, respectively (Hartmetz et al.,1989).

A.6. EET 87526 (CK5; weathering grade Be)

It was initially classified as a C4 chondrite (Martinez et al.,1988). The interior of this meteorite is fine-grained and light todark gray, with abundant dark inclusions (Martinez et al.,1988). A thin section shows an aggregate of small olivine grains

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and a little opaque material with sparse chondrules; olivine isFa29 and it also contains a little pyroxene (Fs24 Wo1) and plagio-clase (An49); the opaque material is mainly magnetite with a lit-tle pentlandite (Martinez et al., 1988). Non-stoichiometricmagnetite constitutes �20% of the iron modal mineralogy (Fisherand Burns, 1991). C and S contents are 0.007 and 1.9 wt.%,respectively (Hartmetz et al., 1989). It may be paired with EET87507 (Martinez et al., 1988).

A.7. EET 87507 (CK5; weathering index-1; weathering grade B)

It was initially classified as a C4 chondrite (Martinez et al.,1988). The interior of this meteorite is fine-grained and light todark gray, with abundant dark inclusions, and may be paired withEET 87526 (Martinez et al., 1988). See EET 87526 (above) for adescription. It contains 3.1 modal vol.% magnetite and

E.A. Cloutis et al. / Icarus 221 (2012) 911–924 923

plagioclase, and 22.9 vol.% opaque minerals (Okada, 1975). It con-tains 11 vol.% recognizable chondrules (Nakamura et al., 1993). Thematrix is composed of homogenous olivine, plagioclase, and finely-dispersed opaques (Okada, 1975). Olivine (homogeneous: Fa29–30)and plagioclase are the major matrix constituents, with minorlow-Ca pyroxene, magnetite and pentlandite according toNakamura et al. (1993). Both low and high-Ca content pyroxenesare present (Noguchi, 1993). Olivine and low-Ca pyroxene compo-sitions are Fa�29 and Fs26, respectively (Kallemeyn et al., 1991), orFa28–32, average Fa29, and Fs24–26, respectively (Yanai et al., 1995).Olivine composition is homogeneous around Fa34 according toNoguchi (1993), or averages Fa30 (Okada, 1975). Low-Ca pyroxenecompositions average Fs26 Wo2 and high-Ca pyroxenes averageFs10 Wo44; Karoonda has more heterogeneous mafic silicates thanY-693 and EET 87507 (Noguchi, 1993). Chondrule olivine andpyroxene are very homogenous, Fa29–31 and Fs25–28, respectively(Nakamura et al., 1993). Opaque phases include sulfides and mag-netite (Kallemeyn et al., 1991), with magnetite and pentlandite asthe major opaque phases, and a small amount of troilite (Okada,1975). It contains 4.2 wt.% FeS and 0 wt.% Fe–Ni metal (Yanaiet al., 1995). It contains 5.7 modal vol.% magnetite and 0.8 modalvol.% sulfides (Geiger and Bischoff, 1995). Saturation magnetiza-tion measurements indicate that magnetite is the major opaquemineral along with a very small amount of metal (Okada, 1975).Magnetite is abundant as micron-sized grains dispersed in bothchondrules and matrix (Nakamura et al., 1993). It contains0.06 wt.% C and 1.6 wt.% S (Gibson and Yanai, 1979), and0.18 wt.% H2O (Yanai et al., 1995). It exhibits pronounced silicateblackening due to widely dispersed magnetite grains (Nakamuraet al., 1993).

A.14. Y-82102 (CK5; weathering index-0)

Olivine composition is Fa28.3–30.7 (average Fa29.5), and pyroxenecomposition is Fs23, high-Ca pyroxene that is present is Fs9.3 Wo41.3(Yanai et al., 1995). Righter and Neff (2007), Hirota et al. (2002) andRubin and Huber (2005) categorized it as a CK5. It may be pairedwith Y-82103 (Rubin and Huber, 2005).

A.15. Y-82103 (CK5; weathering index-0)

Olivine composition is Fa28.7–34.9 (average Fa29.8), and pyroxenecomposition is Fs25.1, high-Ca pyroxene that is present is Fs17.9Wo22.2 (Yanai et al., 1995). Righter and Neff (2007) and Rubinand Huber (2005) categorized it as a CK5. It may be paired withY-82102 (Rubin and Huber, 2005).

Appendix B. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.icarus.2012.09.017.

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Spectral reflectance properties of carbonaceous chondrites: 7. CK chondrites1 Introduction2 Mineralogy/petrology of CK chondrites2.1 Overview2.2 Matrix2.3 Opaque phases2.4 CAIs2.5 Mafic silicates2.6 Thermal metamorphism2.7 Darkening – shock

3 Experimental procedure4 Spectral properties of constituent phases5 Results5.1 Duplicate CK spectra5.2 Grain size effects5.3 Metamorphic sequence5.4 CK4 versus CV3 (versus CO3)5.5 CKs versus heated Allende5.6 Falls versus finds

6 Discussion6.1 CAI spectral contributions

7 Implications for asteroids8 Summary and conclusionsAcknowledgmentsAppendix A Descriptions of CKs included in this studyA.1 A-881551 (CK6)A.2 A-882113 (CK4)A.3 ALH 85002 (CK4; weathering index-1; weathering grade A)A.4 DAV 92300 (CK4; weathering grade A/B)A.5 EET 83311 (CK5; weathering index-1; weathering grade A/B)A.6 EET 87526 (CK5; weathering grade Be)A.7 EET 87507 (CK5; weathering index-1; weathering grade B)A.8 EET 87860 (CK5/6; weathering index-1; weathering grade A/B)A.9 EET 92002 (CK4; weathering grade A/Be)A.10 Karoonda (CK4; weathering index-0; fall)A.11 LEW 87009 (CK6; weathering index-1; weathering grade Ae)A.12 PCA 91470 (CK4; weathering grade A/B)A.13 Y-693 (CK4/5; weathering index-0)A.14 Y-82102 (CK5; weathering index-0)A.15 Y-82103 (CK5; weathering index-0)

Appendix B Supplementary materialReferences