[meteorite-list] Fusion crusts on stony meteorites

[Armando Afonso]

porphyritic?
Are you sure?
The meteorites cool down in a single stage, I believe.
AA

----- Original Message ----- 
From: <bernd.pauli at paulinet.de>
To: <Meteorite-list at meteoritecentral.com>
Sent: Thursday, March 22, 2007 5:35 PM
Subject: [meteorite-list] Fusion crusts on stony meteorites



> 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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