[meteorite-list] Chondrule formation mechanism

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Sat Oct 21 18:38:33 2006
Message-ID: <v18lj2div6n0cehhmdfdpvtl1iqng6c85n_at_4ax.com>

On Sat, 21 Oct 2006 14:58:23 -0700 (PDT), you wrote:

>Hi Darren -
>
>Thanks - much has been lost in the stroke. I also seem
>to remember a "long slow cooling" involved in
>chondrule formation -
>

Googling "chondrule formation" comes up with lots of stuff, much of it pretty
densly technical. Here's one interesting tidbit (unfortunately you have to buy
the whole article to read it):

 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. If viable nuclei 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. 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. 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. However, condensation of SiO into
the melt can simulate the zonation in some chondrules, with pyroxene and a
silica polymorph near the rims. The partial equilibration of chondrule melt with
noncanonical nebular gas would require heating for time periods of hours.


>I am thinking that effervescence following a sudden
>release of pressure might be a better process
>description -

Hm. Effervescence following a sudden release of pressure makes me think more of
a can of Coke. :-)
Received on Sat 21 Oct 2006 06:38:33 PM PDT