[meteorite-list] Origin of chondrules

From: Jeff Grossman <jngrossman_at_meteoritecentral.com>
Date: Wed, 13 Mar 2013 06:28:33 -0400
Message-ID: <514054D1.3030000_at_gmail.com>

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: 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 06:28:33 AM PDT