[meteorite-list] metachondrite

From: Jeff Grossman <jgrossman_at_meteoritecentral.com>
Date: Sun, 24 Oct 2010 17:56:03 -0400
Message-ID: <4CC4AB73.1060306_at_usgs.gov>

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" <jgrossman at usgs.gov> 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
> >>
> >>
> >>
> >>
> >> ______________________________________________
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> >>
> >

-- 
Dr. Jeffrey N. Grossman       phone: (703) 648-6184
US Geological Survey          fax:   (703) 648-6383
954 National Center
Reston, VA 20192, USA

Received on Sun 24 Oct 2010 05:56:03 PM PDT