[meteorite-list] CAI and chondrules

From: Alexander Seidel <gsac_at_meteoritecentral.com>
Date: Thu, 01 Oct 2009 23:59:41 +0200
Message-ID: <20091001215941.37290_at_gmx.net>

Thank you, professional meteoriticists Alan and Jeff, for actively taking part in this interesting discussion.

With these things, as so often in the natural sciences [..see e. g. all the work in particle physics, or the efforts for a grand unification with gravity], one may overall be inclined to strive for "a theory of simplicity and sheer beauty". A simple formula so to say, sort of wishful thinking, but may be nature just does not work this way, here especially with chondrule formation, and things really happen(ed) in a much more differentiated way locally here and there in the early solar system, so that it just could not be laid out or compressed in a simple formula or theory.

If so, there is absolutely nothing strange with big controversies on this subject either now nor in the future years to come, as things finally and definitely may be accepted to be not fitting in just a simple scheme, and may be just THAT different(iated) as all those present theories on the chondrule formation mechanism actually are.

Waiting for things to evolve in this field...

Alex
Berlin/Germany

 
-------- Original-Nachricht --------
> Datum: Thu, 1 Oct 2009 13:50:25 -0700
> Von: "Alan Rubin" <aerubin at ucla.edu>
> An: "Alexander Seidel" <gsac at gmx.net>, "Jeff Grossman" <jgrossman at usgs.gov>, meteorite-list at meteoritecentral.com
> Betreff: 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 Thu 01 Oct 2009 05:59:41 PM PDT