[meteorite-list] Fusion crusts on stony meteorites

From: jbaxter112 at pol.net <jbaxter112_at_meteoritecentral.com>
Date: Thu, 22 Mar 2007 21:58:51 -0400 (EDT)
Message-ID: <60523.10.250.10.1.1174615131.squirrel_at_webmail.medscape.com>

Hello Berndt et al.,

I thought you and the list members might find interesting a phenomenon
that was shown to me by Marcin Cimala. In cutting an LL5 he found areas
where thick crust had built up and actually incorporated within the crust
small angular fragments of relatively unaltered meteorite. Here are scans
of a slice I obtained from Marcin:

http://img.photobucket.com/albums/v614/CaneySprings/NWA2826LL528gmMarcinCimala.jpg

http://img.photobucket.com/albums/v614/CaneySprings/NWA2826LL528gmMarcinCimalacloseup.jpg

I assume that these fragments were dislodged late in flight while the
crust was still liquid but too late to be melted or thermally altered.

Best Wishes,
Jim Baxter

> GENGE M.J. and GRADY M. (1999) The fusion crusts of stony
> meteorites: implications for the atmospheric reprocessing of extra-
> terrestrial materials (MAPS 34-3, 1999, 341-356):
>
> a) Fusion crusts develop on *all* meteorites during their passage of the
> atmosphere ...
>
> b) Most chondrite fusion crusts are porphyritic and dominated by
> olivine, glass and accessory magnetite whereas those of the achondrites
> are mainly glassy.
>
> c) Chondrite fusion crusts contain sulphide droplets with high-Ni
> contents (>55 wt%).
>
> d) The partially melted substrate of OCs, underlying the outer melted
> crusts, are dominated by silicate glass and composite metal, sulphide
> and Cr-bearing Fe-oxide droplets that form as coexisting immiscible
> liquids.
>
> e) Enstatite chondrite substrates contain Cr- and Mn- bearing sulphides.
>
> f) The substrates of the CCs comprise a sulphide-enriched layer of
> matrix.
>
> The compositions of melted crusts are similar to those of the bulk
> meteorite. Differences from whole rock, however, suggest that three main
> processes control their chemical evolution:
>
> 1) the loss and reaction of immiscible Fe-rich liquids,
> 2) mixing between substrate partial melts and bulk melts of the melted
> crust, 3) the loss of volatile components by evaporation and degassing.
>
> Conclusions, p. 356:
>
> The fusion crusts of stony meteorites exhibit considerable variation
> with meteorite type and compositions and textures that are largely
> dependent on the bulk chemical and mineralogical properties of the
> meteorite. Four processes are important in the chemical evolution of
> fusion crusts:
>
> 1) the separation and reaction of Fe-rich sulphide, metal, and oxide
> liquids, 2) reaction with atmospheric O,
> 3) mixing of incompatible element-rich partial melts from the substrate
> with bulk melted crust liquid, 4) the loss of volatile elements by e v
> a p o r a t i o n and d e g a s s i n g.
>
> Best wishes,
>
> Bernd
>
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Received on Thu 22 Mar 2007 09:58:51 PM PDT