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

From: bernd.pauli at paulinet.de <bernd.pauli_at_meteoritecentral.com>
Date: 29 Mar 2010 16:57:13 UT
Message-ID: <DIIE.000000C1000047F5_at_paulinet.de>

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

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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.
Received on Mon 29 Mar 2010 12:57:13 PM PDT