[meteorite-list] metachondrite

From: Jeff Grossman <jgrossman_at_meteoritecentral.com>
Date: Sun, 24 Oct 2010 11:45:26 -0400
Message-ID: <4CC45496.5030402_at_usgs.gov>

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 11:45:26 AM PDT