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: <[email protected]>
To: <[email protected]>
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.
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