[meteorite-list] metachondrite

From: Thunder Stone <stanleygregr_at_meteoritecentral.com>
Date: Mon, 25 Oct 2010 10:19:50 -0700
Message-ID: <SNT117-W25A356EC1E8C3303B54E64D2410_at_phx.gbl>

All:

I wondering if at some time as more of these meta or'7-s' are found, there could be classification system implemented like there currently is for the 3-type.? There is 3.0 through 3.9 based on further analysis; how about a 6.1 through 6.9, were the higher number represent more metamorphism, and other unique changes in the re-crystallization.

Just a thought.

Greg S.

----------------------------------------
> Date: Sun, 24 Oct 2010 17:56:03 -0400
> From: jgrossman at usgs.gov
> CC: meteorite-list at meteoritecentral.com
> Subject: Re: [meteorite-list] metachondrite
>
> I don't think it's appropriate to say that somebody who does not like a
> term suggested in a couple of abstracts is a "purist who sh[ies] away
> from any thinking out of the conventional box"! As far as I understand
> the term, there do not seem to be any concepts behind "metachondrite"
> that are outside the boxes of the existing terminology. It just appears
> to me to be a new word for something we already have terms to describe.
> So it comes down to a question of semantics.
>
> "Type 7" chondrites have not been "excommunicated" either. It has been a
> problematic term because some of the rocks that led to the coining of
> this term turned out to be impact melt rocks, which is a different
> phenomenon from the thermal metamorphism we are trying to describe with
> petrologic types. Once you take these out, there are a few rocks that
> arguably are metamorphosed to slightly higher temperatures than normal
> type 6 chondrites, with the primary indicator of this being high CaO
> content of pyroxene. I have no strong objection to these being called
> type 7, although I'm not convinced that the term is particularly
> necessary. Still, if we're sure we're not dealing with impact melted
> rocks or rocks that have experienced partial melting, then I think the
> term is ok. That's why some of these got through the nomcom. Once again,
> this is not an "outside the box" term, just a minor extension of the
> usual classification scheme (when used carefully).
>
> And finally, as I said before, the word "primitive" in PAC refers to the
> composition, which is close to chondritic compared to, say, that of a
> eucrite, aubrite, or angrite. The same word is frequently used by
> geologists to describe the Earth's mantle before it gets depleted in
> certain elements by generation of melts. The word is perfectly fine in
> this context.
>
> Jeff
>
>
> On 2010-10-24 4:20 PM, Ted Bunch wrote:
> > Jeff and the LIST members:
> >
> > Yes, Van Schmus and Wood (V & W) did a classic study over 40 years
> > ago, they worked with the available classic OC meteorites and made a
> > workable classification scheme. That was 43 years ago and things
> > change, especially the 10s of thousands of non-classic meteorites
> > available for study and advanced instrumentation including the now
> > common usage of SEM BSE imagery and elemental distribution mapping. V
> > & W probably never saw a completely recrystallized OC, they are not
> > even common now.
> >
> > Investigators started seeing OCs that had absolutely NO relict
> > chondrules or chondrule fragments. So, apparently, the V & W scheme
> > became inadequate over time and the "petrologic" classification scheme
> > was unofficially amended with L7, LL7, H7, etc. Seemed reasonable to
> > us. But, not to the purists who shy away from any thinking out of the
> > conventional box. "Sevens" were excommunicated, a few got through a
> > couple of NOM COMs. Because "sevens" are completely recrystallized
> > with some elemental loss, we thought that metachondrite was
> > appropriate, but not until our paper on metachondrites goes through
> > the peer review process - if you are clever handicappers, don't bet on
> > this horse. I think we have the same chance as did Joan of Arc with
> > her Inquisition.
> >
> > Metamorphism of chondrites took place over hundreds of thousands of
> > years at elevated temperatures and not necessarily under closed
> > systems. With this concept in mind --
> >
> > What is a "primitive achondrite"? Actually, they are
> > metamorphosed/recrystallized rocks that are not the same, even
> > compositionally, as their parent rocks. Primitive? I think not. One
> > dictionary definition of primitive is ?not derived from other things?.
> > Acapulcoites and lodranites _are derived from other things, hence not
> > primitive.
> >
> > _A primitive meteorite to me means a CI or may be a CM1, although even
> > these types sustained aqueous alteration. However, Orgueil, even with
> > aqueous alteration, appears to have retained its elemental solar
> > abundances.
> >
> > Because of the huge number of meteorites that have been discovered
> > over the last 20 years, it is time to re-invent other classification
> > schemes for other meteorite classes, namely Martians, in addition to
> > OCs and ?primitives?. A recent paper in MAPs has suggested a new
> > scheme for diogenites.
> >
> > We need to get our heads out of the sand box and address these issues.
> > Moreover, in a few years, the few members of the ?ungrouped ?
> > meteorites will probably reach the necessary quota of 5 or more to be
> > eligible for a new class.
> >
> > Motivation and patience will prevail,
> >
> > Ted
> >
> > Ted Bunch
> >
> >
> >
> > On 10/24/10 8:45 AM, "Jeff Grossman" wrote:
> >
> > > Here is the opposing view:
> > >
> > > The definition of type 6 chondrites comes from one of the classic papers
> > > in meteoritics, Van Schmus and Wood (1967): "As mentioned above, type 6
> > > contains the most recrystallized chondrites. These chondrites show
> > > extensive-to-complete obliteration of the primary textures (Fig. 11),
> > > extensive evidence of recrystallization of the original olivine and
> > > pyroxene crystals, and good-to-excellent development of plagioclase..."
> > >
> > > Note that this definition includes chondrites that are so recrystallized
> > > that chondrules are no longer recognizable. A number of us petrologists
> > > think that most "type 7" chondrites fit comfortably within this
> > > definition of type 6, and consider the former term to be superfluous.
> > >
> > > With continued heating, chondrites eventually begin to partially melt
> > > (reaching the Fe-FeS eutectic temperature and eventually the point where
> > > a feldspathic silicate liquid can form). At this point, differentiation
> > > can begin, as these liquids separate from residual solids. When some
> > > differentiation occurs, it can produce rocks that are nearly chondritic
> > > in composition, but depleted in elements that went into these liquids.
> > > That is the definition of "primitive achondrite": "primitive" refers to
> > > the composition, which is still close to chondritic, and "achondrite"
> > > refers to the lack of chondrules, although rare chondrule outlines
> > > sometimes persist.
> > >
> > > The new term "metachondrite" has not been defined in the formal
> > > literature, but it seems to overlap with what I would call the "high
> > > end" of type 6 and primitive achondrites. I think Ted Bunch reads this
> > > list, so he can chime in if this is not correct. Personally, I see no
> > > benefit to this term, as I am comfortable calling those with chondritic
> > > compositions "type 6 chondrites," and I am uncomfortable calling those
> > > which have differentiated "chondrites," even with the prefix "meta-".
> > >
> > > Jeff
> > >
> > > On 2010-10-24 10:16 AM, Chladnis Heirs wrote:
> > >> Hi Steve,
> > >>
> > >> no.. a metachondrite is a chondrite without chondrules :-)
> > >>
> > >> The "meta" comes from "metamorph".
> > >> A metamorphosis means, that a rock is changed in its structure or its
> > >> composition into a different rock, but remaining in a solid state, while
> > >> this happens. This change can be caused by different agents, like heat,
> > >> pressure, liquids or contact with other rocks.
> > >>
> > >> Well, with the metachondrites (or 7ers or PACs, where they were/are
> > sorted
> > >> in too)
> > >> their chemistry and their isotopes are similar with or the same as the
> > >> respective chondrite groups.
> > >> L-metachondrites with L; H-metachondrites with H, LL-...ect.
> > >>
> > >> Though - they are free of chondrules.
> > >>
> > >> But: they show evidence of recrystallization and in some of these
> > stones of
> > >> that kind, one finds remainders left and relicts, which once were
> > >> chondrules.
> > >> Therefore it's clear, they were once chondrites (of the respective
> > groups H,
> > >> LL, CV, CR now with NWA 6348 the first one, named to come from the
> > L-group)
> > >> and they are directly derived from the chondrites.
> > >>
> > >> So. With the chondrites, you have always those numbers behind the H,
> > L, ect.
> > >> - the petrological grade. 3,4,5,6.
> > >>
> > >> In the type-3 chondrites, you have the full garden of these funny balls
> > >> called chondrules, sharply defined sitting in the matrix.
> > >> If you heat now the affair, the crystal growing will take place, and
> > from
> > >> the 4, to 5, to the 6ers the chondrules and the matrix are more and more
> > >> grown together, the chondrules get less sharper defined, they got
> > often also
> > >> more sparsely.
> > >> And a second thing happens, the constituents of such a 3er, they are
> > >> chemically and physically different from each other. But like all in
> > >> universe, they want to be balanced, they desire an equilibrium. If
> > you heat
> > >> now such a rock, the ions can roam in the rock. And the constituents
> > start
> > >> to assimilate to each other or to get in a chemical balance.
> > >> So the type-3 chondrites are those, which were least heated and are
> > the most
> > >> unchanged preserved once. Called unequilibrated, while from type 4
> > on they
> > >> are called equilibrated.
> > >>
> > >> Well. So the chondrites evolve and are more and more changed from
> > type-3 to
> > >> type 6.
> > >> But with the new finds from the deserts, we got some rocks, which
> > showed,
> > >> that type 6 is not the end!
> > >> That there are also chondrites, which were more heated or were that
> > process
> > >> continued - so that in the end they had lost all their chondrules,
> > and got
> > >> the most equilibrated ones and fully recrystallized.
> > >>
> > >> Two main heat sources you have for such parent bodies. Heating due
> > impacts,
> > >> where the kinetic energy of the impactor is relieved in deformation
> > of the
> > >> target rock and heat. The other one is after the formation of such a
> > >> celestial body, the radioactive decay of its instable and heavy
> > elements.
> > >> The larger such a body is, the more of that stuff it has, the hotter
> > it can
> > >> get - up to the complete melting of the body. And the larger such an
> > body is
> > >> the longer it can keep the heat - Earth e.g. is large enough, that
> > it was
> > >> quite still warm down there, where the Chilean miners were sitting.
> > >>
> > >> Impacts, you know what happens, if the smack is hefty, then the rock
> > beneath
> > >> simply melts - making these IMB, impact melt breccias meteorites,
> > >> usually quite black, homogenous and with textureless glasses..
> > >> The metachondrites are different from them.
> > >> If you take that idea with that heating by decay, then you can
> > suppose, that
> > >> they once sat more deeper, closer to the core of their parent body
> > than the
> > >> 3ers, 4ers,... where it got hotter and where it was longer hot.
> > >>
> > >> Now, cause 6 was obviously not the end, Dodd in the 1970ies thought
> > it is
> > >> necessary to have also a 7.
> > >>
> > >> Problem: Most of these crazy stones have no chondrules left.
> > >> Therefore some say: A chondrite is called a chondrite because it has
> > >> chondrules!
> > >> A stone, that has no chondrules has to be called: achondrite!!
> > >>
> > >> Therfore a 7er-chondrite would be per definition not allowed.
> > >> And because the stuff is directly derived from chondrites, which are the
> > >> most primitive matter we have,
> > >> we put these stones into the group, we already have, where the
> > ACAPs, LODs,
> > >> WINs are already sitting in
> > >> and call them "primitive achondrites".
> > >>
> > >> And that is somewhat unhappy.
> > >> Achondrites we have all that stuff from differentiated, non-chondritic
> > >> parent bodies,
> > >> like the Vesta matters eucrites, diogenites, howardites, but also the
> > >> aubrites, ureilites, brachinites, angrites, Martians, Lunars...
> > >>
> > >> But from these stones, we know exactly from their composition that
> > they were
> > >> chondrites.
> > >> And "primitive" doesn't fit so well neither - because they aren't that
> > >> primitive but among the chondrites the most metamorph, most evolved,
> > most
> > >> equilibrated ones of all.
> > >> And they are not an unchanged primary meteorite, they are a product
> > of them.
> > >> The opposite of "primitive".
> > >>
> > >> The term "Metachondrite" is there more exact, it says: Look I was a
> > >> chondrite - I am a metamorph chondrite.
> > >> So in the name the genesis of the rock is already visible.
> > >> And it allows to be more specific,
> > >> as one hasn't to lump all these rocks, from ACAPs, WINs and all these
> > >> diverse "ex-chondrites" together under a PAC label although they are so
> > >> heterogeneous and stemming from so many different parent bodies.
> > >> But they easily can be finer specified, in attaching their individual
> > >> prefix: L-metachondrite, H-metachondrite, CV-metachondrite ect.
> > >> And everyone knows immediately, what the stone is about. Not so if
> > you have
> > >> only a "PAC" standing there.
> > >>
> > >>
> > >> Hmmmm, supersimply one could say, metachondrites are chondrites
> > which were
> > >> longer in the oven.
> > >>
> > >>
> > >> We hope that was relatively correct?
> > >> If not, please experts, correct us!
> > >>
> > >>
> > >> Aaaand because we were so nice, to try to explain that,
> > >> and because Eric and Steve could be now hungry for such a whack and
> > >> extremely rare representative of such a meteorite,
> > >> we shall be allowed,
> > >> to note shortly, that from NWA 6438 prov.
> > >> we still have left the
> > >>
> > >> 5.868g-slice
> > >> http://www.chladnis-heirs.com/nwa6348-5.868g.JPG
> > >>
> > >> and the 6.655g corner-cut.
> > >> http://www.chladnis-heirs.com/nwa6348-6.655g-part-encut.JPG
> > >>
> > >> Both from the cleaner not so stained part, nicely fresher.
> > >>
> > >> And that the main mass of that first L-meta is now free again,
> > >> Accepting offers:
> > >> http://www.chladnis-heirs.com/nwa6348-48.119g-end-cut.JPG
> > >>
> > >>
> > >> Btw. David has added NWA 6348 already to his systematics and wrote an
> > >> article about.
> > >> http://www.meteoritestudies.com/
> > >>
> > >> (Click in the side menu on "Metachondrites").
> > >>
> > >> Best!
> > >> Martin& Stefan
> > >>
> > >>
> > >> PS: Eric wrote:
> > >> "My understanding is a chondrite with multiple lithologies."
> > >>
> > >> That would be regolith breccias or polymict breccias,
> > >> where different fragments of rocks were mixed together and solified by
> > >> various impacts on the surface of an asteroid.
> > >>
> > >>
> > >>
> > >>
> > >> -----Urspr?ngliche Nachricht-----
> > >> Von: meteorite-list-bounces at meteoritecentral.com
> > >> [mailto:meteorite-list-bounces at meteoritecentral.com] Im Auftrag von
> > Steve
> > >> Dunklee
> > >> Gesendet: Freitag, 22. Oktober 2010 18:09
> > >> An: jgrossman at usgs.gov; meteorite-list at meteoritecentral.com
> > >> Betreff: Re: [meteorite-list] metachondrite
> > >>
> > >> So by what I have read metachondrites are material that was once
> > chondrite
> > >> based on tfL what this suggests is you can take condrite material
> > and have
> > >> it accrete on a large parent body and make meta chondrite. I think
> > it would
> > >> be material that formed closer to the sun cooling quickly on one
> > side giving
> > >> it chondritic attributes and o isotopes closer to the tfL. Cheers Steve
> > >> Dunklee
> > >>
> > >>
> > >>
> > >>
> > >> ______________________________________________
> > >> Visit the Archives at
> > >> http://www.meteoritecentral.com/mailing-list-archives.html
> > >> Meteorite-list mailing list
> > >> Meteorite-list at meteoritecentral.com
> > >> http://six.pairlist.net/mailman/listinfo/meteorite-list
> > >>
> > >
>
>
> --
> Dr. Jeffrey N. Grossman phone: (703) 648-6184
> US Geological Survey fax: (703) 648-6383
> 954 National Center
> Reston, VA 20192, USA
>
>
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Received on Mon 25 Oct 2010 01:19:50 PM PDT