[meteorite-list] Fred's TAN 057 a CK or a CV Chondrite?

From: bernd.pauli_at_paulinet.de <bernd.pauli_at_meteoritecentral.com>
Date: Tue Nov 1 18:00:16 2005
Message-ID: <DIIE.0000000D00003EE0_at_paulinet.de>

David Weir kindly wrote:

"The CK-CV group designation has been the subject of some debate, as can
be seen in the paper by Greenwood et al. (2003):'Are CK Chondrites Really
a Distinct Group or Just Equilibrated CVs?' "

http://www.lpi.usra.edu/meetings/metsoc2003/pdf/5179.pdf

Hello List,

Just in case you did not download this abstract or did not read it in full:
It contains three important passages (plus one typo ;-)

1. Chondrule abundance, or perhaps more correctly, apparent
   chondrule abundance, will also decrease with increasing grade.

I think there are too many chondrules in Fred's TAN 057 to classify it as
a CK5 - especially the "5" is disturbing... "4" ... OK, ...maybe but "5" ??
 
2. The main classification criteria used to define the [CK] group are:

1) low chondrule to matrix ratio; (-)
2) chondrule size intermediate between the CO and CV groups; (+)
3) absence of coarse-grained rims around chondrules; (-)
4) low abundance of refractory inclusions compared to CO and CV groups; (?)
5) low C content; (?)
6) refractory lithophile abundances intermediate between CO and CV groups (?)
7) O-isotope compositions overlapping those of the CO and CV groups (?)

The "+" says I agree, the "-" says I don't agree, and "?" indicates I don't know with
regard to Fred's TAN 057 and its classification as a CK chondrite, but who am I to
disagree with renowned meteoriticists ?!

Anyway, this leads us to:

3. The authors conclude:

The distinction between CK and CV3 (oxidized subgroup) chondrites may be more
apparent than real and may reflect varying metamorphic conditions rather than
more fundamental compositional differences.

Where have we heard that before? Yeah, right! Look here:

NORTON O.R. (1998) RFSII, pp. 196-197: CK Carbonaceous Chondrites

Meteorites that were earlier labeled CV4-5 are now a new class, the CK
carbonaceous chondrites. There are only thirteen known, most found in
Antarctica. The only observed fall of a CK chondrite occurred in 1930 in
Karoonda, South Australia. It is the type specimen for all CK meteorites.
A large mass, the only CK chondrite available to the collector, was found
in Maralinga, Australia, in 1974.

All CK chondrite meteorites show various degrees of metamorphism, displaying
petrologic types 3-6 and some showing shock veins, suggesting an impact history.
The cut surfaces of these meteorites appear blackened with a sooty substance so
that the structure is difficult to distinguish*. The blackening agent is fine part-
icles of magnetite and dark sulfides dispersed in the silicates of the groundmass
and chondrules(+). Like most other carbonaceous chondrites, CK chondrites are highly
oxidized, show no metal grains, and have iron-rich olivine and pyroxene. This group
is similar to CV and CO chondrites but differs in bulk chemistry.

* Is the structure in Fred's TAN 057 so difficult to distinguish ???
(+) Does anyone see any traces of this black, sooty substance in TAN 057?
    Well, after all, don't forget it is "anomalous" from its "head down to
    its toes"!


Cheers and good night,

Bernd
Received on Tue 01 Nov 2005 06:00:13 PM PST