[meteorite-list] Carancas in the news PLUS GRA 06128/9

Wed Feb 27 19:11:19 EST 2008

Hi, Adam,

    They say:
"‘New’ Parent Body. The GRA meteorites’ combination
of isotopic and mineral compositions is unlike those of any
other meteorite. sigma-18O 􀀂 +6‰ and delta-17O 􀀂 - 0.05‰
are distinct from other meteorites except the primitive
achondrite NWA 2788 [7]. Mineral compositions in GRA
are distinct from those of other achondrite groups and
NWA 2788 (although Fe/Mn in olivine is like Earth’s)"

Of course NWA 2788 isn't 75% Oligoclase;
the only thing "proved" is a common oxygen-isotope
source pool.

http://www.agu.org/meetings/fm06/fm06-sessions/fm06_P51E.html
P51E-1246
Evidence for a Carbonaceous Chondrite Parent Body With Near-TFL Oxygen 
Isotopes From Unique Metachondrite Northwest Africa 2788

* Bunch, T E (tbear1 at cableone.net) , Dept. of Geology, Northern Arizona 
University, Flagstaff, AZ 86011, United States
Irving, A J (irving at ess.washington.edu) , Dept. of Earth and Spaces 
Sciences, University of Washington, Seattle, WA 98195, United States
Rumble, D (rumble at gl.ciw.edu) , Geophysical Laboratory, Carnegie 
Institution, Washington, DC 20015, United States
Korotev, R L (korotev at wustl.edu) , Dept. of Earth and Planetary Sciences, 
Washington University, St. Louis, MO 63130, United States
Hup\'{e}, G M (gmhupe at tampabay.rr.com)

{\bf Metachondrites}: Metachondrites are newly recognized groups of stony 
meteorites that lack chondrules, but which have elemental and oxygen 
isotopic compositions and textures suggesting that they have been 
transformed by metamorphism or partial melting from precursor ordinary and 
carbonaceous chondrites on relatively large parent bodies [1]. The best 
known examples have affinities to CR (e.g., LEW 88763), CV (e.g., NWA 3133), 
H, L and LL chondrites; conversely there is evidence that winonaites and 
acapulcoites also are metachondrites derived from chondritic precursors 
(represented by rare chondrites such as NWA 1463 and Monument Draw). With 
increased sampling of new meteorites from both hot and cold desert regions, 
there is an emerging realization that the early solar system was populated 
with many relatively large differentiated planetary bodies complete with 
metallic cores, silicate mantles and chondritic regoliths of various types. 
The affinity of a particular metachondrite to a specific chondrite class 
relies mainly on oxygen isotopic analysis combined with distinctive 
elemental ratios in bulk rocks and constituent minerals (notably Fe/Mn and 
Ca/Na ratios, which are quite different for ordinary vs. various 
carbonaceous chondrite classes). {\bf Northwest Africa 2788}: This specimen 
exhibits a metamorphic texture with triple grain junctions (grain size is 
mostly < 0.5 mm, a few grains reach nearly 1 mm), and is composed of 
orthopyroxene (63 vol.%, Fs$_{18.0}$Wo$_{1.3}$, FeO/MnO = 30), olivine (27 
vol.%, Fa$_{21.4}$, FeO/MnO = 57-61), clinopyroxene (5 vol.%, 
Fs$_{7.4}$Wo$_{49.8}$, TiO$_{2}$ = 0.74 wt.%, Cr$_{2}$O$_{3}$ = 0.63 wt.%, 
FeO/MnO = 19), plagioclase (5 vol.%, An$_{53.9}$Or$_{3}$), and accessory 
merrillite, troilite and metal. Replicate oxygen isotopic analyses of 
acid-washed bulk samples by laser fluorination gave $\delta$$^{18}$O = 
6.004, 6.082; $\delta$$^{17}$O = 3.082, 3.102; $\Delta$$^{17}$O 
= -0.076, -0.097 per mil (for TFL slope of 0.526); these values plot close 
to but below the TFL. The elevated Fe/Mn ratios in the mafic silicates 
coupled with the relatively calcic plagioclase are hallmarks of carbonaceous 
chondrite bulk compositions [2], but the oxygen isotopic compositions are 
quite unlike those of any known chondrite class. Bulk rock abundances by 
INAA relative to Allende analyzed simultaneously are 0.89 times for Fe and 
~1.2 times for Cr, REE and Hf. We conclude that NWA 2788 is a unique 
metachondrite sampled from a new, perhaps now-disaggregated parent body 
accreted from a distinctive oxygen isotopic reservoir. It is possible that 
this body also may harbor or have harbored chondrule-bearing carbonaceous 
regolith materials ('CT chondrites') that are yet to be found or recognized. 
[1] Irving A. J. et al. (2005) {\it 68th Met. Soc. Mtg.}, \#5218; Bunch T. 
E. et al. (2005) {\it LPS XXXVI}, \#2308 [2] Wasson J. T. and Kallemyn G. W. 
(1988) {\it Phil. Trans. Roy. Soc. Lond.} {\bf A325}, 535-544.

This whole metachondrite notion is a slippery one:
Melt a chondrite -- get a metachondrite. Say, melt a chondrite?
Isn't that how you get... a planet? Ok, or a differentiated
body, but it has to be just barely melted, enough to
eliminate the chondrules but not change the composition.
Problem is that chondrules are tough little buggers and
not that easy to melt. It could happen, of course, rarely,
and there are not many so-called metachondrites, so that
fits. Maybe a metachondrite is chipped off a body when
it just begins to melt, or off a body that just melts a little...

It takes special pleading to explain, you see.

That's why my classification is:
"Weird Puppy"

Sterling
------------------------------------------------------------------
----- Original Message ----- 
From: "Adam Hupe" <raremeteorites at yahoo.com>
To: "Adam" <meteorite-list at meteoritecentral.com>
Sent: Wednesday, February 27, 2008 5:36 PM
Subject: Re: [meteorite-list] Carancas in the news PLUS GRA 06128/9

I thought oxygen isotopes proved the GRA 06128/9
stones to be part of the Brachinite parent body so
calling them ungrouped seems wrong to me.  Cool, but
not Earth-shattering by any means.

Best Regards,

Adam

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