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" <[email protected]>
To: "Alexander Seidel" <[email protected]>; "Jeff Grossman" <[email protected]>;
<[email protected]>
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" <[email protected]>
To: "Jeff Grossman" <[email protected]>;
<[email protected]>
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 <[email protected]>
An: [email protected]
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"
><[email protected]>
>To: <[email protected]>
>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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>______________________________________________
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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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