[meteorite-list] CAI and chondrules

Thu Oct 1 16:50:25 EDT 2009

        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.
>> >
>> >_________________________________________________________________
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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
>>
>>
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