AW: [meteorite-list] Tafassasset classification

From: Norbert Classen <trifid_at_meteoritecentral.com>
Date: Thu Apr 22 10:06:55 2004
Message-ID: <NCBBKMGDKLMGIBALJOFCMENBCOAA.trifid_at_timewarp.de>

Hi Frank, John, and List,

While discussing Tafassasset, Dr. Otto explained to me in much
detail the fundamental difference between chondrites and
achondrites - something that is quite obvious in viewing various
thin sections. First, achondrites - including primitive achondrites
- exhibit much larger crystals than any chondrite due to igneous
melting and recrystalization processes. The matrix of a type 6 or
type 7 chondrite is very fine grained compaired to primitive
achondrites. True chondritic matter has never been melted by
igneous processes, and it looks vastly different from achondritic
matter through the microscope.

Tafassasset is a very good example - it shows a coarse-grained
crytalline matrix that exhibitis all features of a total igneous
recrystalization of it's chondritic precursor - one of these
features are the triple-junctions of the olivine, usually found
in brachinites. Dr. Otto showed me various thin sections of
chondrites, including CR's, and there is nothing even remotely
resembling this coarse grained, recrystalized texture. However,
it is very similar to Divnoe, Zag (b), and the brachinites.

To put it short: the crystal size and texture of a primitive
achondrite are very different from type 7 chondrites - even
I could see this without much guideance. However, the elemental
compositions and REE patterns of primitive achondritic matter
remain more or less chondritic - and hence these meteorites have
been defined as primitive achondrites. But since they have been
completely recrystalized it's impossible to say that they have
been an CR, CO, or CV etc. before recrystallization. You can't
take some similarities, such as the O-isotopic composition and
the approximate amount of metal, to justify such a wild guess.
With the same logic you could guess that the aubrites might be
lunar or terrestrial meteorites.

Now, what to make of the "relict chondrules" in Tafassasset -
and what about relict chondrules in general? Dr. Otto explained
to me that relict chondrules have escaped the process of
recrystalization and remained more or less intact in a few
primitive achondrites, such as NWA 725 (Tissemoumine), and
Monument Draw, both acapulcoites. The structures in Tafassasset,
however, look much more like something Dr. Otto calls
"Sammelkristalle" - I don't find the proper translation, sorry -
something like agglomerates of crystals that usually form during
melting and recrystalization processes; Dr. Otto even showed me
similar agglomerates in thin sections of terrestrial igneous rocks.

Well, does that mean that these structures can't be interpreted
as relict chondrules? Yes and no. Dr. Otto said that he has
seen these structures, too, but there's one fundamental fact that
makes him doubt that they could be relict chondrules. Most of these
structures are primarily composed of plagioclase poikilitically
enclosing minor olivines and pyroxenes, often accompanied by
nickel-iron metal. The predominance of plagioclase in these
agglomerates is strange since chondrules usually aren't composed of
plagioclase. There would be just one possibility - glas-rich
or glas chondrules that have been transformed into plagioclase
during the melt process. But, according to Dr. Otto, this is
rather improbable and nothing more than a wild guess. It's
much more probable that these structures are indeed "Sammel-
kristalle", and no relict chondrules. Anyway, even if it should
be relict chondrules in the end, Tafassasset should be regarded
per definition as a quite typical primitive achondrite because
of its completely recrystalized, coarse-grained texture.

Now, I hope this helps to explain Dr. Otto's point of view, and
the difference between type 7 chondrites and primitive achondrites.
To me, all of this sounded more than convincing. I just hope
that I've been able to present his view coherently in my poor
English. And sorry for this lengthy email...

Best regards,
Norbert


> -----Original Message-----
> Von: meteorite-list-admin_at_meteoritecentral.com
> [mailto:meteorite-list-admin_at_meteoritecentral.com]Im Auftrag von fcressy
> Gesendet: Donnerstag, 3. Oktober 2002 06:08
> An: meteorite-list_at_meteoritecentral.com; John Divelbiss
> Betreff: Re: [meteorite-list] Tafassasset classification
>
>
> Hi John and all,
>
> I believe that, in general, primative achondrites are no longer chondrites
> in that the rock has rxperienced melting and chondrules are no longer
> present. The rock has recrystallized and it has a crystalline texture.
> However, compositionally they remain the same as the parent chondrite
> material. So texturally they are achondrites, compositionally they are
> chondrites. If I remember correctly from my acapulcoite samples, a lot of
> metal is present between the crystals. In a regular differented
> achondrite,
> the metal has separated out as has other material and the resultant
> composition of the rock is significanrtly different from the parent body.
> As for the difference between a petrographic grade 7 and a primative
> achondrite, I agree that this line is fuzzy. Grade 7 chondrites
> should still
> have relict chondrules present. However, I believe that at least one
> acapulcoite (NWA725) also contains relict chondrules.
> This is my understanding of the subject. Hope it helps a bit but feel free
> to correct me if I'm not understanding this correctly.
> Regards,
> Frank
>
> ----- Original Message -----
> From: John Divelbiss <j.divelbiss_at_worldnet.att.net>
>
> > Frank,
> >
> > I understand the description of the primitive achondrite...including the
> > types you mention.
> >
> > From that statement in Nortons book, they are still chondrites...just
> > overdone a bit. What line did they cross to be called
> achondrites? And how
> > does it relate to Tafassasset being also called a CR7?
> >
> > John
> >
> > ----- Original Message -----
> > From: "fcressy" <fcressy_at_prodigy.net>
> > To: "John Divelbiss" <j.divelbiss_at_worldnet.att.net>; "Norbert Classen"
> > <trifid_at_timewarp.de>; "Bernd Pauli HD"
> > <bernd.pauli_at_lehrer.uni-karlsruhe.de>; "Svend Buhl" <svendbuhl@web.de>
> > Cc: <meteorite-list_at_meteoritecentral.com>
> > Sent: Wednesday, October 02, 2002 10:55 PM
> > Subject: Re: [meteorite-list] Tafassasset classification
> >
> >
> > > Hello John and all,
> > > Regarding your primative achondrite question, Norton's
> "Encyclopedia of
> > > Meteorites" has a page dedicated to the subject (P. 165). He
> states that
> > > "primative achondrites have achondrite textures but still retain
> something
> > > of their chondritic composition." and that they "are transitional
> between"
> > > chondrites and achondrites. As examples of primative achondrites, he
> uses
> > > acapulcoites, lodranites and winonaites.
> > > Hope this helps.
> > > Frank
> > >
> > >
> > >
> > >
> > > ----- Original Message -----
> > > From: John Divelbiss <j.divelbiss_at_worldnet.att.net>
> > > To: Norbert Classen <trifid_at_timewarp.de>; Bernd Pauli HD
> > > <bernd.pauli_at_lehrer.uni-karlsruhe.de>; Svend Buhl <svendbuhl@web.de>
> > > Cc: <meteorite-list_at_meteoritecentral.com>
> > > Sent: Wednesday, October 02, 2002 5:05 PM
> > > Subject: Re: [meteorite-list] Tafassasset classification
> > >
> > >
> > > > Hello All,
> > > >
> > > > Primitive is primitive! Help me to understand the differences
> > > please.
> > > >
> > > > Is the argument about where the line(s) should be drawn between a
> > > chondrite
> > > > (that is "really" differentiated...like a 7) versus a primitive
> > achondrite
> > > > (that is not quite differentiated enough to be called a achondrite)?
> > > >
> > > > Is there really a 7 category and why?
> > > >
> > > > Are primitive achondrites now considered differentiated enough to be
> > > > distinguished from a highly changed chondrite? small versus coarse
> > grains
> > > in
> > > > matrix?
> > > >
> > > > How is this line drawn? mineral makeup, level of glass, age, grain
> > > > size/changes, etc. ?
> > > >
> > > > Lots of questions with this story.
> > > >
> > > > Alain would have something to say about all this. Ann
> Black, is there
> a
> > > > position written from his corner?
> > > >
> > > >
> > > > John
> > > >
>
>
>
>
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Received on Thu 03 Oct 2002 08:06:12 AM PDT