[meteorite-list] Fwd: Re: CAI and chondrules

From: aerubin at ucla.edu <aerubin_at_meteoritecentral.com>
Date: Sat, 03 Oct 2009 13:18:57 -0700
Message-ID: <20091003131857.500767nsmygttyo8_at_mail.ucla.edu>

Jeff brought up an important point on which that I would like to
elaborate. A few chondrules may sport a peculiar feature that may
have important constraints on how those few chondrules formed. The
problem is in generalizing. I may feel that those few chondrules are
a Rosetta Stone that have preserved important features that were
erased in the vast majority of chondrules because of secondary
processing. Another researcher may feel that these peculiar features
are just anomalies that have little bearing on the general problem.
There may be no objective way around this other than finding
additional examples or fitting the observations into a coherent model.
  Let me give a specific example. Sasha Krot and I reported that a
handful of chondrules have fine-grained matrix-like rims that are
loaded with microchondrules. No one disputes this observation. I
believe that these microchondrule-bearing objects are rare surviving
representatives of a general process relating to chondrule formation.
My inference is that, in many cases, after formaton, chondrules were
surrounded by dustballs and that some highly localized flash-melting
mechanism, perhaps akin to a lightning bolt, zapped the
chondrule-dustball assemblage and caused melting of the chondrule
surface (thereby forming droplets that solidified into
microchondrules). Other researchers are skeptical that the handful of
chondrules with microchondrule-rich rims have anything to say about
the general chondrule-forming process. My idea fits into my scenario
of how chondrule formation occurred. Those who favor the model in
which chondrules were produced in large-scale events involving gas
dynamic shock waves would tend to dismiss these microchondrule-bearing
chondrules as being a sideshow. Who is right? It depends on who you
ask!












Quoting Jeff Grossman <jgrossman at usgs.gov>:

> There are all kinds of controversies involved with this subject.
> The observations themselves are rarely the problem. It's how the
> observations are interpreted that leads to the controversies!
>
> Turning observations into constraints on chondrule or CAI formation
> always involves interpretation. The first steps in forming these
> constraints from observations of the natural systems usually
> involve: (1) Arguing that the observations are of primary features
> (as opposed to features formed by metamorphism, alteration, shock,
> weathering, or other late processing) and, (2) arguing that the
> observations do not reflect some kind of anomalous or unusual
> special case, but are instead applicable to the general problem. If
> you can do that, then you have a primary constraint, namely that the
> chondrule- or CAI-formation process has to be able to result in the
> observed feature. So, following some of what Alan said, he argues
> that chondrule formation must be able to produce the observed rims.
> I think nearly all researchers would agree with this statement.
>
> Where things get really messy is the next stage of the process. For
> example, one might make a list of all the processes that could
> conceivably make these rims and try to rule out all but one. Alan
> has argued that they could only form by multiple heating events, and
> therefore declare that another constraint on chondrule formation is
> that it must be able to melt the objects multiple times. But his
> arguments to get to that constraint are complex and might be
> questioned by other workers. Another worker might look at the same
> data and conclude that the rims could form by multiple accretion
> events of material splashed from asteroidal impacts. It is these
> secondary constraints that are often highly controversial.
>
> Built on top of these constraints, as well as constraints provided
> by astronomical observations, physics, and chemistry, are models for
> the early solar system. If the constraints on which the models are
> built are really good, then the models may eventually approach
> reality. But these models are houses of cards. If the constraints
> are weak, they will fall. When you ask for a definitive answer to
> how chondrules or CAIs form, you are asking for the definitive early
> solar system model. At this time, there isn't one.
>
> Jeff
>
>
>
> At 04:16 PM 10/1/2009, Alexander Seidel wrote:
>> Sorry, I should have added: the observable facts don??t seem to be
>> the problem here, but all the *CONSTRAINTS* on them involved, right
>> Jeff?
>>
>> Alex
>>
>> -------- Original-Nachricht --------
>> Datum: Thu, 01 Oct 2009 21:57:35 +0200
>> Von: "Alexander Seidel" <gsac at gmx.net>
>> An: Jeff Grossman <jgrossman at usgs.gov>, meteorite-list at meteoritecentral.com
>> Betreff: 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
>>>
>>>
>>> ______________________________________________
>>> http://www.meteoritecentral.com
>>> Meteorite-list mailing list
>>> Meteorite-list at meteoritecentral.com
>>> http://six.pairlist.net/mailman/listinfo/meteorite-list
>> ______________________________________________
>> 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
>
>
> ______________________________________________
> http://www.meteoritecentral.com
> Meteorite-list mailing list
> Meteorite-list at meteoritecentral.com
> http://six.pairlist.net/mailman/listinfo/meteorite-list
>
Received on Sat 03 Oct 2009 04:18:57 PM PDT