[meteorite-list] Chondrule formation mechanism

[E.P. Grondine]

Thanks Darren - 

now this is more like it - hmmmm

--- Darren Garrison <cynapse at charter.net> wrote:

http://www.aspbooks.org/custom/publications/paper/index.phtml?paper_id=2447
 
> Chondrule textures depend on the extent of melting
> of the chondrule precursor- material when cooling 
> starts. 

Kind of begs the question - chodrules formed by
collision, which causes melt - consider if one started
from a steady molten state 

>If "viable nuclei" 

I wonder what these "viable nuclei" are? viable cystal
nuclei=Chondrules?

> remain in the melt, crystallization begins 
> immediately, producing crystals with shapes that 
> approach equilibrium. If not, crystallization does
> not occur until the melt is supersaturated,
resulting
> in more rapid growth rates and the formation of 
> skeletal or dendritic crystals. 

> A chondrule texture thus indicates whether
> nuclei were destroyed, which implies a melting
> temperature above the liquidus temperature for its
> particular composition. The  presence or absence of 
> skeletal or dendritic crystals in chondrules can be 
> used to constrain their peak temperatures, which 
> range from 1400-1850°C. 

hmmm - collision temperatures of 1400-1850°C

> Heating times of less than a second result in 
> aggregates of starting materials coated
> with glass, resembling agglutinates rather than
> objects with typical chondrule textures, suggesting
> that heating times are longer. 

different heating times=two mechanisms for chondrule
formation?  two classes 1)agglutinate 2)original?

> Chondrule textures can be duplicated with a very
> wide range of cooling rates, but if olivine zoning
> is to be matched the cooling rate should be within
> the range 10- 1000°C/hr. The size of overgrowths on 
> relict grains cannot be used to infer cooling rates.


> Chondrules melted in a canonical nebular gas lose 
> sulfur and alkalis in minutes, while iron loss from
> the silicate melt continues over many hours. Mass
> loss and isotopic fractionation can be suppressed if
> the partial pressures of the species of interest are
> high enough in the ambient gas. Chondrule bulk and
> mineral composition arrays can be reproduced to a 
> large extent by evaporation.

Yeah, but chondrules are found in matrices.

> However, condensation of SiO 

and how exactly is this SiO condensation supposed to
occur?

> into the melt can simulate the zonation in some
> chondrules, with pyroxene and a silica polymorph
near > the rims. 

yeah, but could the silica rims have come from the
inside?

> The partial equilibration of chondrule melt with
> noncanonical nebular gas would require heating for
> time periods of hours. 

> Hm.  Effervescence following a sudden release of
> pressure makes me think more of
> a can of Coke.  :-)

We all get our inspiration somewhere - 

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