I second what Alan wrote, at the 90% level. With my remaining finger,
I'll add that the worst problem may be that these molten planetesimals
must magically keep metallic and silicate melts mixed together in order
to make chondrules, many of which have abundant metal. I think this
would be physically difficult, to say the least.
I think the ideas in this paper are philosophically quite attractive,
joining modern research on cosmochronology with dynamical models of the
disk. But despite this new way of thinking, the basic tenets are quite
retro. Many people up through the 1960s hypothesized that chondrules
were fragments of igneous rock. Then modern research on them began.
Study after study found problems with these models, many of which Alan
outlined. Although the new model is a twist on the old ones, it still
is subject to the same tests... and it cannot pass most of them.
Jeff
On 3/13/2013 2:03 AM, Alan Rubin wrote:
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: [email protected]
website: http://cosmochemists.igpp.ucla.edu/Rubin.html
----- Original Message ----- From: "Mendy Ouzillou" <[email protected]>
To: "met-list" <[email protected]>
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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