[meteorite-list] The metachondrite question answered

From: Frank Prochaska <fprochaska_at_meteoritecentral.com>
Date: Fri Jul 29 13:53:34 2005
Message-ID: <0IKE00KZGHOTCDMF_at_vms040.mailsrvcs.net>

        Well, here is my stab at it. This sorta goes back to posts on the
list a while back regarding the difference between achondrites, impact
melts, etc.

        First a little background.
        If a rock (meteorite) has a bulk composition that is basically
unchanged from that of the solar nebula, it's a chondrite. It may or may
not have chondrules. It may have completely or partially melted at some
time. It may have been altered somewhat by aqueous or thermal processes.
But, if the bulk composition is basically unchanged, it's a chondrite.
        If you take such a rock and either melt it or partially melt it and
let it recrystallize into rocks with different compositions, you now have
differentiated rocks. There are two basic ways this happens. If you melt
the whole thing and then cool it slowly, higher temperature minerals will
crystallize first. Given enough time before others minerals crystallize and
get in the way, these minerals will settle out and accumulate on the floor
of the magma chamber. On a bigger scale this is also how you get a fully
differentiated body with a core, etc. Then lower temperature minerals will
crystallize and you end up with a layered structure. If you sample the
entire body, you will still get a chondritic composition. The Earth should
have an overall chonrdritic composition if we were able to properly sample
the whole thing. However, normally you can only hold in your hand and
analyze a fragment of that whole structure. For example, an earth mantle or
crust rock or a meteorite that someone picked up. This would be how (we
assume) that the eucrites and diogenites formed, for example. Cumulate
eucrites are eucrites where you can see the crystal textures that form based
on this accumulation model, as another example.
        Another way to create a differentiated rock is to partially melt the
chondrite and draw this melt off to crystallize somewhere else. The high
temperature minerals that didn't melt are left to form a new rock in place,
while the melt of lower temp minerals recrystallizes somewhere else. Again,
if you analyzed the entire mass of both rocks, you'd get a chondritic bulk
composition. However, looking at samples of the individual rocks, you get
something else; they are differentiated. If you have a rock that is
differentiated yet not terribly altered from the original chondritic
precursor, it will generally be referred to as a primitive achondrite. More
dramatically altered rocks (the HED group for example) are considered more
'evolved' I guess is the way to put it.

        Another background piece. While we talk about a chondritic
composition for the solar nebula, there was clearly some variation,
particularly with regard to volatile elements. The closer to the Sun, the
fewer of the lighter volatile elements. By the same token, the isotopic
ratios of elements varied. The Earth/moon system has a bulk oxygen isotopic
ratio that is different from Mars, which is different than meteorites, etc.
It is sometimes assumed that each body, or at least bodies that formed at
different distances from the Sun, have different oxygen isotope ratios.

        Now for the abstract that is the actual subject of your question.
        The abstract basically describes the work that this group did on
primitive achondrites. They are assuming that these rocks represent mantle
rocks or early samples of mantle rocks. They group these into categories of
meteorites that share oxygen isotope ratios, along with other classes of
meteorites with similar ratios, assuming then that these meteorites came
from the same parent body or at least from bodies that formed in the same
area of 'collection zone' in the solar system.
        Almost of as an aside, they propose that these primitive achondrites
be called "metachondrites" apparently because they assume they are formed
metamorphically (through heat, pressure, etc.) from chondritic precursor
materials.

        That is the end of my impartial 'translation' based on what I know
of geology. My personal slant is that I think "primitive achondrite" is a
more useful description. I don't think calling a rock a metachondrite
because it was derived from a chondrite is terribly helpful, because until
we identify a meteorite that made it here from some other solar nebula
besides our own, every rock is derived from a chondrite. On the other hand,
I think primitive achondrite tells me a lot about a rock and how it relates
to the other classes of meteorites that I'm familiar with, without any
additional information. However, this is obviously just my opinion.

        Anyone else on the list care to elaborate or see a correction to be
made in my stab at the geology here?



Frank Prochaska





-----Original Message-----
From: meteorite-list-bounces_at_meteoritecentral.com
[mailto:meteorite-list-bounces_at_meteoritecentral.com] On Behalf Of Tom
Knudson
Sent: Friday, July 29, 2005 8:41 AM
To: met list
Subject: [meteorite-list] The metachondrite question answered

Hey List, I found out what a metachondrite is, I guess, if someone wants to
convert it to english, well it's in english, but it is all latin to me!!!
 : )



Metachondrites: Recrystallized and/OR Residual MANTLE Rocks From Multiple,
LARGE Chondritic Parent Bodies. A. J. Irving1, T. E. Bunch2, D. Rumble, III3
and T. E. Larson4, 1Earth & Space Sciences, University of Washington,
Seattle, WA 98195 irving_at_ess.washington.edu; 2Dept. of Geology, Northern
Arizona University, Flagstaff, AZ 86011; 3Geophysical Laboratory,
Washington, DC 20015; 4Los Alamos National Laboratory, NM 87545.


Although the concept that multiple, relatively large, and differentiated
planetary bodies existed in the early asteroid belt is not new [1], only
recently has evidence from meteorite samples has been marshalled to support
this idea [2]. The recovery of new specimens from Northwest Africa has made
it possible to forensically reconstruct such planetary bodies from fragments
representing core, mantle, crust and regolithic rocks. This relies on the
assumption that such fragments will share common oxygen isotopic signatures.
Some specimens are highly recrystallized rocks devoid of chondrules which
possibly represent mantle samples. The term primitive achondrite has been
applied to such rocks; yet, if they are texturally evolved rocks from
chondritic precursors, we suggest that metachondrite is a better term.

Metachondrite Groups: At least five different groups of metachondrites can
be recognized, and each can be affiliated with a specific chondrite class
utilizing oxygen isotopes:

CV NWA 3133, NWA 1839 [2]

CR NWA 3100, Tafassasset, LEW 88763 [2]

CH Lodranites, acapulcoites [3]

NWA 1463, NWA 1058 Winonaites (+ IAB irons)

H NWA 2353, NWA 2635, NWA 3145 (+ IIE irons)

Unique chondrites NWA 1463 [4] and NWA 1058 [5] may represent the regolith
of the winonaite parent body [3]. Since these specimens contain obvious
chondrules, they should not be termed achondrites (despite a likely genetic
relationship).

Metachondrites From the H Chondrite Parent Body: NWA 2353 (paired with NWA
3145) and NWA 2635 have polygonal-granular textures, no chondrules and,
respectively: mean grainsize (0.2; 0.5 mm), olivine (Fa17.9-18.7, FeO/MnO =
34-38; Fa18.9, FeO/MnO = 35), orthopyroxene (Fs15.6Wo3.1 to Fs16.6Wo4.2,
FeO/MnO = 19-26; Fs16.8Wo2.9, FeO/MnO = 20), plagioclase (An12.3Or6.7 to
An27.4Or2.8; An15.1Or4.7), with accessory metal, chromite, merrillite and
troilite. Clinopyroxene (Fs7.4Wo43.4 to Fs8.5Wo40.4, FeO/MnO = 16-22) occurs
only in NWA 2353/3145. Their oxygen isotopic compositions (d18O = 5.51,
5.10; d17O = 3.31, 3.16; D17O = +0.440, +0.510 per mil for NWA 2353; d18O =
3.23, 2.98; d17O = 5.03, 4.37; D17O = +0.575, +0.676 per mil for NWA 2635)
overlap those of H chondrites [6] and IIE irons [7].

References: [1] Wetherill G. 1992 Icarus, 100, 307-325; Chambers J. and
Wetherill G. 2001 MAPS, 36, 381 [2] Irving A. et al. 2004 EOS, 85, #P31C-02;
Bunch T. et al. 2005 LPS XXXVI, #2308 [3] Rumble D. et al. 2005 68th Met.
Soc. Mtg., #5138 [4] Benedix G. et al. 2003 66th Met. Soc. Mtg., #5125 [5]
Russell S. et al. 2003 Met. Bull. 87 [6] Clayton R. et al. 1991 GCA, 55,
2317-2337 [7] Clayton R. and Mayeda T. 1996 GCA, 60, 1999-2018.

Thanks, Tom
peregrineflier <><


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Received on Fri 29 Jul 2005 01:53:11 PM PDT