[meteorite-list] The metachondrite question answered

From: Tom Knudson <peregrineflier_at_meteoritecentral.com>
Date: Fri Jul 29 12:41:29 2005
Message-ID: <005701c5945c$5df7f340$2d107918_at_Michelle>

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 <><
Received on Fri 29 Jul 2005 12:41:25 PM PDT