[meteorite-list] Origin of chondrules

From: Alan Rubin <aerubin_at_meteoritecentral.com>
Date: Tue, 12 Mar 2013 23:03:23 -0700
Message-ID: <74A2B69D0591446FBF9DA3B194F67651_at_igpp.ucla.edu>

I'll be happy to give my opinion on the paper. I think it is completely
wrong. Here is my reasoning:
1. Many chondrules are surrounded by secondary igneous shells, still others
by igneous rims. These shells and rims indicate that the chondrules haev
experienced more than one melting event.
2. Many FeO-rich (i.e., Type-II) porphyritic olivine chondrules contain
relict grains of different FeO contents and different O-isotopic
compositions, again indicating multiple melting. This is very hard in a
collision model.
3. One might expect molten planetesimals to have well-mixed melts. If the
chondrules are mainly from the larger planetesimal (the target) as one would
expect, the O isotopic compositions of the chondrules would probably be
mass-fractionated and lie on a slope-1/2 line on the standard three-isotope
diagram. We don't see this.
4. One might also expect that as the planestimal melted and began to
crystallize, it would become chemically fractionated, unlike the
unfractionated, solar, compositions of chondrules in primitive chondrites.
5. The occurrence of microchondrules in the fine-grained rims around some
normal-size chondrules and the apparent melting of pyroxene at the outer
surface of the chondrule to form the microchondrules indicates chondrule
melting by a mechanism capable of melting only the outer surface of the
chondrule. This is totally inconsistent with the formation by splashing by
the collision of molten planetesimals.
6. There are correlations between chondrule size, the proportion of
different chondrule types, the proportion of those with igneous rims and
secondary shells that are difficult to explain by splashing but come
naturally to a model invoking multiple melting in dusty nebular regions.
7. The non-spherical shapes of most CO chondrules indicates very rapid
cooling or else they would have collapsed into spheres. This might be okay
except for the fact that the large size of their phenocrysts require a
growth period thousands of times longer than the time it would take a molten
droplet to collapse into a sphere. This again indicates a flash heating
mechanism.
8. The fairly rare occurrence of chondrule-CAI mixtures are difficult to
explain by colliding molten planetesimals, but are sinple to explain by
melting of a mafic dustball that had and old CAI fragment inside.
9. Each chondrite group has its own distinctive narrow range of chondrule
sizes. In fact, about 90% of the chondrules in any group have diameters
within a factor of 2 of the mean size. One would expect molten
planetesimals to produce a similar size of chondrules range for each group.
Furthermore, chondrule size is correlated with lots of other chondrule
properties (proportions of textural types, numbers with rims and secondary
shells, etc.) that are hard to explain by molten planetesimals.
10. And, I just don't see how we get the different chondrule textural types
by that model. Some chondrules lack olivine, others lack pyroxene, some are
coarse grained, some are fine-grained, some have a mixture of different size
grains, some include relict grains. This seems impossible to produce by the
molten planetesimal model.
Since I only have 10 fingers, I'll stop there.


Alan Rubin
Institute of Geophysics and Planetary Physics
University of California
3845 Slichter Hall
603 Charles Young Dr. E
Los Angeles, CA 90095-1567
phone: 310-825-3202
e-mail: aerubin at ucla.edu
website: http://cosmochemists.igpp.ucla.edu/Rubin.html


----- Original Message -----
From: "Mendy Ouzillou" <ouzillou at yahoo.com>
To: "met-list" <meteorite-list at meteoritecentral.com>
Sent: Tuesday, March 12, 2013 7:06 PM
Subject: [meteorite-list] Origin of chondrules


And now for something completely different ... Meteorite talk.


I am in the process of reading through a fascinating article in latest issue
of "Meteoritics and Planetary Science" called "The Origin of Chondrules and
Chondrites: Debris from Low Velocity Impacts Between Molten Planetisimals."

This paper is very well written and readable even by a novice such as
myself. What I find interesting is the proposal for a (somewhat) new theory
that chondrules did not instantly form from clumps of heated nebular dust
but instead formed 1.5 to 2.5MY after the formation of CAIs. the paper
states that chondrules formed from splashing when two differentiated
planetisimals collided at a relatively slow speed of between 10 to 100m/s.
Without being able to review the previous papers, I have to say that to me
this makes a great deal of sense and appears to solve many of the
inconsistencies that have been raised in some of the older books that I have
read.

Note: there is a typo in the paer on page 2177. Is states "A strength of the
splashing model is that it can explain why chondrules are mostly between 1.5
and 2.5MYr younger than CAI ...". The sentence should read "older", no
"younger".

Dr. Jeff Grossman, would love to hear your thoughts on this paper.

Mendy Ouzillou
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Received on Wed 13 Mar 2013 02:03:23 AM PDT