Franchi, I.A.

Greenwood, R.G.

Kearsley, A.T.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsAre CK chondrites really a distinct group or justequilibrated CVs?Conference or Workshop ItemHow to cite:Greenwood, R.G.; Kearsley, A.T. and Franchi, I.A. (2003). Are CK chondrites really a distinct group or justequilibrated CVs? In: 66th Annual Meteoritical Society Meeting, 28 Jul - 1 Aug 2003, Munster, Germany.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/metsoc2003/pdf/5179.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukARE CK CHONDRITES REALLY A DISTINCT GROUP OR JUST EQUILIBRATED CVs? R C Greenwood1, A T Kearsley2 and I A Franchi1: 1PSSRI, Open University, Walton Hall, Milton Keynes, MK7 6AA, U.K. 2Oxford Brookes University and Dept of Mineralogy, The Natural History Museum, London, SW7 5BD, UK.  Introduction:  The CK group, as presently defined [1,2], is dominated by equilibrated meteorites, whereas all other carbona-ceous chondrites groups contain only unequilibrated material [3]. This is in marked contrast to ordinary chondrites, in which equili-brated and unequilibrated meteorites form a continuum within each of the main groups (H, L, LL). As part of a classification exercise on newly-recovered CK-like material it became apparent to us that the distinction between the CK and CV groups is far from clear-cut and that the relationship between them may need to be reevaluated.  Experimental Techniques: Backscattered electron imagery, X-ray mapping and mineral analyses were performed on a JEOL 840 scanning electron microscope with an Oxford Instruments exL en-ergy dispersive spectrometer (20kV accelerating voltage, 2 nA probe current): Oxygen isotope analyses were undertaken by infrared la-ser-assisted fluorination [4]. Samples studied: Karoonda (CK4 fall), Dar al Gani 250 (CK4/5), Dar al Gani 275 (CK4/5), recently-recovered CK6-like chondrite. Results and Discussion: CK chondrites are highly oxidized me-teorites having high modal abundances of magnetite and trace amounts of  Fe,Ni-metal  [1,2]. The main classification criteria used to define the group are: (i) low chondrule to matrix ratio, (ii) chon-drule size intermediate between the CO and CV groups, (iii) absence of coarse-grained rims around chondrules, (iv) low abundance of refractory inclusions compared to CO and CV groups, (v) low C content, (vi) refractory lithophile abundances intermediate between CO and CV groups, (vii) O-isotope compositions overlapping those of the CO and CV groups.  While we do not dispute the validity of most of these features, our own observations suggest that they are mainly a reflection of the higher metamorphic grade of most CKs compared to other carbona-ceous chondrites. Thus, the apparently low content of refractory inclusions is most likely a product of alteration and subsequent metamorphism, as shown by the similarity between the bulk refrac-tory lithophile abundances in CKs and the CAI-rich CV group  [1]. Chondrule abundance; or perhaps more correctly apparent chondrule abundance, will also decrease with increasing grade. Coarse-grained chondrule rims are present in at least some CKs, (e.g. DAG 250) but due to recrystallisation are far less obvious than in the lower grade CV3 chondrites. A number of meteorites have been classified as being CK3 (Camel Donga 003, Watson 002, DAG 431, Dhofar 015). It is in-structive to note that these meteorites generally do not show typical features of the CK group, often having abundant chondrules and/or refractory inclusions and oxygen isotope compositions that plot away from other equilibrated members of the CK group. In many respects these meteorites resemble CV3 chondrites of the oxidized subgroup.  Conclusions:  The distinction between CK and CV3 (oxidized subgroup) chondrites may be more apparent than real and may re-flect varying metamorphic conditions rather than more fundamental compositional differences.  References: [1] Kallemeyn G.W. et al (1991) in GCA 55, 881-892. [2] Geiger T. and Bischoff A. (1995) Planet. Space Sci., 43, 485-495. [3] Brearley A. J and Jones R. H. (1998) in Planetary Materi-als, Papike J.J.(ed). [4] Miller M.F. et al. (1999)  Rapid Commun. Mass Spectrom. 13, 1211-1217.  66th Annual Meteoritical Society Meeting (2003) 5179.pdf

Are CK chondrites really a distinct group or just equilibrated CVs?

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Are CK chondrites really a distinct group or just equilibrated CVs?

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