[meteorite-list] metachondrite

From: Chladnis Heirs <news_at_meteoritecentral.com>
Date: Sun, 24 Oct 2010 16:16:13 +0200
Message-ID: <001201cb7386$0465bf60$0d313e20$_at_com>

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
Received on Sun 24 Oct 2010 10:16:13 AM PDT