[meteorite-list] Strange Asteroids Baffle Scientists

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Wed, 22 Aug 2007 20:11:54 -0500
Message-ID: <002501c7e522$98a530c0$c92ce146_at_ATARIENGINE>

Hi,

> so many large differentiated bodies...

    The big work on this was done some time ago by
John Wasson at UCLA. He concentrated not on the
iron and nickel, but on refractory elements: iridium,
germanium, gallium, and so forth. Their melting points
are high enough that they cannot be "lost" from the melt.
They show a huge range of abundances in different irons,
ranges of a thousand to one, or even 100,000 to one for
some elements. 16 main groups emerged, but there were
scores of "unique" signatures, each one a "group of one"
(meaning that we only had one representive of that group).
Just Google "Wasson irons" for 10^5 results.

    Once you have an iron core that's a melt (particularly a
small hot fast melt), it will convect like crazy, mixing the
core material pretty thoroughly. And, with a mantle of molten
rock to keep it nice and warm for a long time, unless the
body is VERY small, which it might have been... Here's a nice
abstract: http://www.agu.org/pubs/crossref/1990/89JB02747.shtml
and another: http://www.lpi.usra.edu/meetings/metsoc2006/pdf/5146.pdf

    No, wait! Good Old PSRD! Here's an entire paper that
explains how cooling rates are calculated (no lousy abstracts...):
http://www.psrd.hawaii.edu/April07/irons.html
Maybe "approximated" would be a better word. This article,
by the way, has links to a lot of other material on irons and
cores and all that wonderful stuff...

    Oh, and that article discusses the very question you asked,
how to fine and coarse irons in the same core.


Sterling K. Webb
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----- Original Message -----
From: "ensoramanda" <ensoramanda at ntlworld.com>
To: <Meteorite-list at meteoritecentral.com>
Sent: Wednesday, August 22, 2007 7:20 PM
Subject: Re: [meteorite-list] Strange Asteroids Baffle Scientists



Thanks Stirling,

Interesting relationships to the iron meteorites. I had naver considered
that they indicated so many large differentited bodies.

Is it not possible that one body may have an iron core that has a range
of nickel iron mixes thus producing several classes of iron meteorites
from the same source?. Surely a large core would have areas that cooled
at different rates producing fine and coarse irons with different
chemical makeups.

Graham

Sterling K. Webb wrote:

>Hi, Graham, List,
>
> They raise that possibility and discuss it (and the
>details of that discussion have fallen right out the little
>hole in the bottom of my brain where recent knowledge
>drains out).
>
> The interesting question (to me) was the possibility
>of bodies large enough (like Vesta) to differentiate and
>THEN be completely disrupted until the biggest piece
>is less than 10 kilometers.
>
> Another consideration is this: there are over 80 unique
>iron cores in the Zone; you need that many to produce
>all the differing elemental compositions of iron meteorites
>we know about. So, there must have been over 80 bodies
>big enough to differentiate.
>
> There is a recent study that says these "cores" seem
>to have come from the inner solar system, so the guess is
>that they were battered to cores there and then the cores
>were "migrated" to the Zone by dynamic interactions.
>However, it could also be that whole terrestrial planetoids
>were displaced to the Zone and battered apart there, and it's
>implied by a study like this, that finds some basaltic remains.
>
> Now if they continue and find more (and more) unrelated
>basaltic small bodies, one of the questions that arises is "how
>big does a body have to be to differentiate?" We know (obviously)
>that a body as big as Vesta does, but how about smaller bodies?
>Some (by no means all) theorists believe that bodies as small
>as 100 kilometers can differentiate.
>
> How could they do that? Well, IF they formed early enough,
>they would have enough short-lived isotopes to really get
>cooking! At the supernova-mediated start of the solar system,
>there were lots of "hot" isotopes available but they decayed very
>rapidly. So, the question really is "how fast did planetesimals
>and planetoids form?"
>
> Sort of a burning issue in cosmology, actually, the subject of
>controversy and the occasional awkward fistfight...
>
> Here's what they say about HED's:
>
> "Regarding this point, the oxygen isotope data
>provide evidence that most of the HEDs derive
>from a common well-mixed pool. However,
>more detailed studies recently indicate that some HEDs
>would be inconsistent with a unique origin. Among
>these we can mention the eucrites Northwest
>Africa 011 (Yamaguchi et al., 2002), Ibitira, Pasamonte,
>Caldera and ALHA 78132 (Wiechert et al., 2004).
>In particular, the fairly typical eucrite Ibitira has an
>17O value indistinguishable from the angrites, which
>is another suite of ancient basaltic meteorites. These
>meteorites are geochemically distinct from the HEDs
>and are clearly resolved on the basis of oxygen isotopes
>as well. The meteorite collection could actually represent
>several dozen parent bodies, considering also the
>abundance of iron meteorites which should have been
>part of the nucleus of distinct differentiated bodies
>(Burbine et al., 2002). In short, the diversity in the
>collection of basaltic meteorites requires more than
>one basaltic parent body, which is consistent with
>the abundance of differentiated parent bodies implied
>by the iron meteorites. In a recent work, Bottke et al.
>(2006), demonstrated that small differentiated parent
>bodies (and their fragments) should be common in
>the Main Belt."
>
> Obviously they lean toward the "small-body" theory.
>And, once you got a monstrous amount of data (by going
>there probably), you could match basaltic fragments with
>their cores by comparing the REE abundances in the irons
>with the iron in the basalts...
>
>
>Sterling K. Webb
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Received on Wed 22 Aug 2007 09:11:54 PM PDT