[meteorite-list] Dr. Jeffrey N. Grossman Chondrule paper in Science

From: Jeff Grossman <jgrossman_at_meteoritecentral.com>
Date: Thu, 26 Jun 2008 06:23:57 -0400
Message-ID: <OFAF0A7CD3.1409308C-ON85257474.003950B6_at_usgs.gov>

People who study the origin of chondrules try to select chondrites in
which the effects of secondary shock and heating are
minimal. Semarkona, in our paper, is a good choice because it is
petrologic type 3.01 and only lightly shocked. Except for a small
amount of glass in the outer parts of some chondrules that was
affected by squeous alteration, the silicate phases are pristine and
in the same condition as when the chondrule originally formed (not
necessarily by condensation, however).

Shock and metamorphism can certainly affect the structure of
chondrules in a variety of ways, although I'm not sure what you're
seeing that could be described as growth from chemical reactions
within the chondrule.

Jeff

At 12:34 AM 6/26/2008, STARSANDSCOPES at aol.com wrote:
>Hi list, I just read Jeff's paper on chondrule and chondrite formation.
>I'm still trying to get my brain around some of the points.
>
>Well done!!! A whole new way of looking at everything!
>
>I had a question related to the paper but just a little off topic. It is
>directed to Jeff but any one with knowledge in this area should jump in.
>
>The many structures found in chondrules, are they formed during the
>condensation process at formation or later during subsequent
>heating and shock events?
>
>Perhaps both? I often see structures within chondrules that give the
>appearance (at least) of having grown out of chemical reactions within the
>chondrule. Structures that I can not imagine shock forces having caused.
>
>Tom Phillips
>
>In a message dated 6/25/2008 7:27:00 A.M. Mountain Daylight Time,
>jbaxter112 at pol.net writes:
>Hi Jeff,
>
>Congratulations on the cover story in Science on formation conditions of
>chondrules. Your color photograph of a Semarkona cross section is
>fabulous.
>
>Cheers,
>Jim Baxter
>
> > I think it is fairly clear that the glass in chondrites, which forms in
> > chondrules because of their rapid cooling from a partially molten
> > state, is stable on the time-scale of the age of the solar
> > system. In the most primitive chondrites, the ones unaffected by
> > reheating or alteration on asteroids, the glass is preserved in
> > pristine condition to this day. In metamorphosed chondrites, glass may
> > survive in protected areas of type 3.9-4 material, but the
> > reheating caused most of the glass to crystallize into feldspar early
> > in solar system history. In aqueously altered chondrites, like CMs, the
> > glass was mostly replaced by phyllosilicates and other phases due to
> > the chemical action of water on the asteroid. Water is apparently a key
> > ingredient in devitrifying silicate glasses, especially
> > important in earth rocks.
> >
> > The image on Tom's website is almost certainly one of dendrites
> > (probably olivine) in what was once glass. These dendrites were the
> > result of rapid crystallization during cooling of a chondrule
> > melt. Because this is a metamorphosed chondrite, the glass is now most
> > likely replaced by fine-grained feldspathic material.
> >
> > Jeff
> >
> > At 12:24 AM 6/25/2008, STARSANDSCOPES at aol.com wrote:
> >>Hi, Several years ago I ran onto an unusual chondrule in JaH 055
> >> that looks like glass but it is forming in crystals. I have had
> >> various explanations presented to me and all involved "Glass" This
> >> might be "On topic"? If any one is up to taking a look and sharing
> >> their observations, I would greatly appreciate it. Just go to my
> >> Meteorite Times Micrograph
> >>Gallery
> >>http://www.meteorite.com/meteorite-gallery/meteorites-alpha_frame.htm
> >> and select alphabetical sorting, JaH 055, and then
> >>crystal structure. These shots were produced using incident
> >>(reflected light). Thanks, Tom Phillips In a message dated
> >>6/24/2008 10:02:55 P.M. Mountain Daylight Time, cynapse at charter.net
> >> writes: Have any studies been done on "decay" of glasses in
> >>meteorites into crystaline configurations? Is there a mesurable
> >> rate, or does it not happen? This story brought that to mind-- if
> >> impact-generated glasses in meteorites HAVE NOT "decayed" into
> >>crystaline material in 4 billion years, it's fairly good evidence
> >> that it won't happen "in billions of years", as the story speculates.
> >
> > Dr. Jeffrey N. Grossman phone: (703) 648-6184
> > US Geological Survey fax: (703) 648-6383
> > 954 National Center
> > Reston, VA 20192, USA
> >
> >
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>
>
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Dr. Jeffrey N. Grossman phone: (703) 648-6184
US Geological Survey fax: (703) 648-6383
954 National Center
Reston, VA 20192, USA
Received on Thu 26 Jun 2008 06:23:57 AM PDT