[meteorite-list] Silicated Iron vs. Winonaite (Part 2)

From: Jason Utas <meteoritekid_at_meteoritecentral.com>
Date: Tue, 30 Mar 2010 02:54:22 -0700
Message-ID: <93aaac891003300254yfb6703fxe3c11d66256f7e4a_at_mail.gmail.com>

Indeed - thanks, Bernd for the informative post.

But the question still remains; while the papers you noted establish a
clear connection between IAB irons and winonaites (not to mention
IIICD irons, though that class is now being toted as a pair of IAB
sub-classes in some circles), I'm still not seeing any clear
distinction between the two; only one paper makes note of the fact
that the CRE dates are different for irons versus winonaites, and that
seems to be the only real distinction between them. While this
reflects different dates for their respective liberation-impacts from
their common parent body, I don't believe that CRE dating has ever
actually been used to distinguish between meteorite classes before.
Probably because it says little to nothing about how a given meteorite
initially formed, and our current class system, from what I
understand, is an attempt to group meteorites based on just that -
parent bodies and where meteorites formed in the early solar system
(or later on, if the information has been erased through metamorphism,
etc).

I'm wondering if any scientist on the list would care to comment on
these classifications and the way in which they might draw a
distinction between the two; it really wasn't addressed in the
literature that Bernd posted, unless CRE dating is used as the
determinant.

Thanks, Regards,
Jason

On Tue, Mar 30, 2010 at 2:40 AM, Jeff Kuyken <info at meteorites.com.au> wrote:
> Thanks so much for that thorough and very interesting summary Bernd. It
> would still be interesting to know exactly where the line is drawn between
> the silicated irons and the Winonaites. Take a look at these for example:
>
> http://www.meteoriteguy.com/catalog/nwa4024.htm
>
> http://www.arizonaskiesmeteorites.com/AZ_Skies_Links/NWA_2680/index.html
>
> http://www.polandmet.com/_nwa5980.htm
>
> Cheers,
>
> Jeff
>
> ----- Original Message ----- From: <bernd.pauli at paulinet.de>
> To: <Meteorite-list at meteoritecentral.com>
> Sent: Tuesday, March 30, 2010 3:57 AM
> Subject: [meteorite-list] Silicated Iron vs. Winonaite (Part 2)
>
>
>> Hello again Jeff, Jason, and List!
>>
>> Further, more specific information culled from the Benedix et al.
>> article in MAPS about IAB, IIICD inclusions and winonaites.
>>
>> Cheers,
>>
>> Bernd
>>
>>
>> ------------------------------------------------------------------------------------
>>
>> BENEDIX G.K. et al. (2000) A petrologic study of the IAB iron meteorites:
>> Constraints on the formation of the IAB-Winonaite parent body
>> (MAPS 35-6, 2000, pp. 1127-1141):
>>
>> 1) most IAB and IIICD inclusions are roughly chondritic in mineralogy and
>> composition
>>
>> 2) most IAB and IIICD inclusions have nonchondritic, recrystallized
>> textures,
>> ?similar to winonaites in O-isotopic and mineral compositions
>>
>> 3) seemingly contradictory presence of relatively primitive silicate
>> inclusions embedded
>> ?in dense metal that was presumably molten at the time of mixing
>>
>> 4) three models for the formation of these inclusions:
>>
>> ?- formation by impact-induced large-scale selective melting and
>> ? ?mixing in the megaregoliths on a chondritic parent body
>>
>> ?- formation by parent-body-wide partial melting and fractional
>> ? ?crystallization during formation of a S-rich core
>>
>> ?- inhomogeneous segregation of silicates and metal
>>
>> 5) petrologic, textural, and isotopic evidence suggest formation from
>> heterogeneous chondritic
>> ?precursor materials by partial melting, brecciation, and metamorphism
>> (Benedix et al., 1998).
>>
>> 6) mineralogies and mineral compositions of silicates overlap between
>> ?winonaites and silicate inclusions in IAB iron meteorites
>>
>> 7) most textures of the IAB and IIICD chondritic clasts
>> ?are nearly identical to the textures of winonaites
>>
>> 8) different cosmic-ray exposure ages for winonaites (0.02-0.08 Ga) and
>> IAB iron
>> ?meteorites (0.4-1.0 Ga) reflect liberation from the parent body in
>> different events.
>>
>> 9) same parent body for IAB iron-winonaite meteorites *and* IIICD iron
>> meteorites?
>>
>> ?- oxygen-isotopic compositions of silicates from inclusions in IAB and
>> ? ?IIICD iron and winonaite meteorites are essentially indistinguishable
>>
>> ?- inclusions broadly similar in mineralogy to those in IIICD iron
>> ? ?meteorites are found among the IAB iron-winonaite meteorites
>>
>> But important differences exist in mineral compositions:
>>
>> ?- higher Fs contents of the pyroxene compositions of inclusions in IIICD
>> meteorites
>> ?- plagioclase compositions more albitic* than those in IAB iron-winonaite
>> meteorites
>>
>> *albite = the sodium end-member of the plagioclase series (NaAlSi3O8).
>> ?anorthite = the calcium end-member of the plagioclase series
>> (CaAl2Si2O8).
>>
>> Benedix et al. state there is a strong link between IAB iron and winonaite
>> meteorites
>> but question such a strong link exists between IAB iron and winonaite
>> meteorites and
>> IIICD iron meteorites.
>>
>> But they also advise caution because these apparent differences might
>> simply be
>> sampling biases so that further recoveries of additional meteorites are
>> necessary
>> to exlude or include the IIICD iron meteorites.
>>
>> ______________________________________________
>> Visit the Archives at
>> http://www.meteoritecentral.com/mailing-list-archives.html
>> Meteorite-list mailing list
>> Meteorite-list at meteoritecentral.com
>> http://six.pairlist.net/mailman/listinfo/meteorite-list
>>
>>
>
> ______________________________________________
> Visit the Archives at
> http://www.meteoritecentral.com/mailing-list-archives.html
> Meteorite-list mailing list
> Meteorite-list at meteoritecentral.com
> http://six.pairlist.net/mailman/listinfo/meteorite-list
>
Received on Tue 30 Mar 2010 05:54:22 AM PDT