[meteorite-list] CAI and chondrules

From: Jeff Kuyken <info_at_meteoritecentral.com>
Date: Fri, 2 Oct 2009 20:30:51 +1000
Message-ID: <B81D6FA527B54707A41024C7BD58833D_at_JeffPC>

Thanks very much for posting this Alan. Chondrule formation and the
primitive chondrites have always fascinated me and one of your points in
particular peaked my interest.

"Melting experiments that produce artificial chondrules have provided useful
constraints on chondrule cooling times, which are probably on the order of a
few minutes."

It's not the first time I have heard this. As I have looked over various
sources over the last several years, a common opinion seemed to be that
chondrules formed, cooled and then accreted into larger bodies. But how does
a model with chondrules cooling so fast take this into account:

http://www.meteorites.com.au/features/nwa2892.html

NWA 2892 is my favourite meteorite because of the story it tells and it's
uniqueness. It would seem to me that some of the models may need work. Could
it be that either chondrules were continuing to melt for longer than
commonly thought or the accretion of the larger bodies started earlier?

Thanks,

Jeff



----- Original Message -----
From: "Alan Rubin" <aerubin at ucla.edu>
To: "Alexander Seidel" <gsac at gmx.net>; "Jeff Grossman" <jgrossman at usgs.gov>;
<meteorite-list at meteoritecentral.com>
Sent: Friday, October 02, 2009 6:50 AM
Subject: Re: [meteorite-list] CAI and chondrules


> The origin of chondrules is one of the outstanding problems facing
> meteorite researchers. There is significant disagreement even among
> colleagues who routinely work together on this problem. But be aware,
> nearly everything I say below will be disputed by some folks in the field,
> each highlighting a separate area of contention.
> I think that many researchers would agree that most presently
> existing chondrules formed in the solar nebula by melting pre-existing
> solid objects (let's call them dustballs) that contained chunks of
> pre-existing chondrules and some nebular dust. Many chondrules may have
> experienced more than one melting event. Chondrules with relict grains
> melted more than once; enveloping compound chondrules contain components
> that experienced different melting events; chondrules with igneous rims
> melted more than once. Enveloping compound chondrules with igneous rims
> (rather common in CR chondrites) were melted at least three times. During
> melting it is likely that some volatiles escaped from chondrules and
> condensed onto adjacent dust. Some metal and some sulfide also likely
> escaped from many chondrules during melting. It is also likely that
> barred olivine and cryptocrystalline chondrules were essentially
> completely melted during their final melting event, but that porphyritic
> chondrules were only incompletely melted. Melting episodes were either of
> short duration or occurred in the presence of sufficient amounts of gas to
> avoid fractionation of K isotopes. Melting experiments that produce
> artificial chondrules have provided useful constraints on chondrule
> cooling times, which are probably on the order of a few minutes.
> Chondrules in different chondrite groups vary in size, O-isotopic
> composition, proportion of different textural types, proportions of
> chondrules sporting igneous rims, and the proportion of compound
> chondrules. These differences have only been described to a limited
> extent; much more work needs to be done. Different chondrite groups also
> vary in their CAI and amoeboid olivine inclusion (AOI) populations. It
> isn't clear what the relationship is (if any) between a chondrite group's
> chondrules and CAIs.
> Some researchers favor the idea that chondrules were formed by
> flash-melting mechanisms such as lightning in the nebula; quite a few
> favor melting of pre-existing dustballs by gas dynamic shock waves. A
> vocal minority believes that chondrules formed by planetesimal collisions.
> I think that most chondrules formed in the nebula by repeated
> episodes of flash melting but that some chondrules (e.g., most of those in
> CH chondrites and Bencubbin-like (CB) chondrites) may have formed by
> impact-related processes. It is also possible that rare varieties of
> chondrules in ordinary chondrites (e.g., chromite-bearing chondrules) were
> formed by impact melting.
> Another problem confronting researchers is that chondrules were
> affected by parent-body processes after the chondrules formed and were
> incorporated into asteroids. Such secondary processes include thermal
> metamorphism, impact heating, brecciation, and aqueous alteration. And,
> of course, chondrules in meteorite finds may have been affected by
> terrestrial weathering. We need to look beyond these processes and infer
> what the chondrules were like prior to accretion when they were isolated
> dustballs in the nebula. This is a difficult task, but, as Jeff Grossman
> said, progress is being made.
> Alan Rubin
>
>
> ----- Original Message -----
> From: "Alexander Seidel" <gsac at gmx.net>
> To: "Jeff Grossman" <jgrossman at usgs.gov>;
> <meteorite-list at meteoritecentral.com>
> Sent: Thursday, October 01, 2009 12:57 PM
> Subject: Re: [meteorite-list] CAI and chondrules
>
>
>> From my layman?s perspective and point of view: isn?t it interesting to
>> note that there is still **so much** controversy over chondrule
>> formation, those little round objects which are so evident and very
>> clearly visible in many of the meteorites in our collections, while at
>> the very same time all the basic physical conditions and evolutionary
>> laws even on small timescales seem to be quite well understood? But then
>> again all the many empirical facts obviously still have to come under
>> serious scrutiny to finally have, at best, sort of a "generally accepted
>> truth" emerge: a mainstream theory of chondrule formation that will be
>> agreed upon by most scientists - one of these days.
>>
>> Fascinating, especially in this era where many "basic" things seem to be
>> understood! I?m excited to learn more about this, as time goes by... :-)
>>
>> Alex
>> Berlin/Germany
>>
>>
>> -------- Original-Nachricht --------
>>> Datum: Thu, 01 Oct 2009 14:52:08 -0400
>>> Von: Jeff Grossman <jgrossman at usgs.gov>
>>> An: meteorite-list at meteoritecentral.com
>>> Betreff: Re: [meteorite-list] CAI and chondrules
>>
>>> I didn't say "we don't know cr*p"... I said there
>>> was not a definitive answer. We know a lot about
>>> the timing, materials, and physical conditions
>>> needed to make chondrules and CAIs, and people
>>> have used these to make models for their
>>> formation. But many of these constraints are
>>> under scrutiny right now, and some pretty basic
>>> things are still controversial. What this means
>>> is... ok, we don't know cr*p. But progress is being made.
>>>
>>> Jeff
>>>
>>>
>>> At 02:39 PM 10/1/2009, Sterling K. Webb wrote:
>>> >Hi, Carl, Francesco, Jeff, List,
>>> >
>>> >Wrong, Carl. A good solid "We don't know
>>> >crap about this yet; it's all up in the air"
>>> >is the best kind of definitive answer.
>>> >
>>> >There are probably a half-dozen scenarios
>>> >for how this data came about and there's few
>>> >reasons to choose any one over the others.
>>> >
>>> >I was glad to hear Jeff confirm it.
>>> >
>>> >
>>> >Sterling K. Webb
>>> >-----------------------------------------------------------
>>> >----- Original Message ----- From: "Carl 's"
>>> ><carloselguapo1 at hotmail.com>
>>> >To: <meteorite-list at meteoritecentral.com>
>>> >Sent: Wednesday, September 30, 2009 5:49 PM
>>> >Subject: Re: [meteorite-list] CAI and chondrules
>>> >
>>> >
>>> >
>>> >Ahhh! Good questions, Francesco. I had been
>>> >wondering about CAIs and chondrules myself. None
>>> >of the books I've read explains,how?
>>> >
>>> >Thanks also for your answer, Jeff. While not
>>> >definitive, it seems to be the only answer at this time. Thank you.
>>> >
>>> >Carl
>>> >
>>> >
>>> >Jeff Grossman wrote:
>>> >>...
>>> >
>>> >Definitive answers to what caused the formation of CAIs and chondrules
>>> >are not known.
>>> >
>>> >_________________________________________________________________
>>> >Bing? brings you maps, menus, and reviews
>>> >organized in one place. Try it now.
>>> >http://www.bing.com/search?q=restaurants&form=MLOGEN&publ=WLHMTAG&crea=TEXT_MLOGEN_Core_tagline_local_1x1
>>> >______________________________________________
>>> >http://www.meteoritecentral.com
>>> >Meteorite-list mailing list
>>> >Meteorite-list at meteoritecentral.com
>>> >http://six.pairlist.net/mailman/listinfo/meteorite-list
>>>
>>> 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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Received on Fri 02 Oct 2009 06:30:51 AM PDT