[meteorite-list] Osmium and the size of parent bodies

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sat, 21 Apr 2007 15:03:02 -0500
Message-ID: <063301c78450$120cd730$862e4842_at_ATARIENGINE>

Hi, Ed, List,

    "More" work? More than who, what, when?
Are you referring to the indications of an early
cooling of the cores of irons, demonstrated by
their 187Rhenium/187Osmium ratios? As shown
in this:
http://www.geol.umd.edu/pages/faculty/WALKER/walker_homepage/meteorites.html

    "Some iron meteorites (the so called "magmatic"
irons) likely are pieces of asteroidal cores. As such
their study can provide valuable insights to planetary
core formation and crystallization processes. The
highly siderophile elements (HSE: "iron-loving",
including Re, Os, Ir, Ru, Pt and Pd) are useful in
elucidating crystallization sequence, so these elements
have been the focus of much of our work on irons.
We recently completed a 187Re-187Os and 190Pt-186Os
isotopic and elemental study of the two largest magmatic
iron meteorite groups, IIAB and IIIAB (Cook et al., 2004).
That study revealed that the cores these meteorites sample
crystallized very early in solar system history (approximately
4.5 billion years ago)... However, complex trace element
behavior for Re, Pt and Os in these groups, particularly
group IIIAB proved difficult to explain. During the past
several years we have extended our study to the magmatic
groups IVA and IVB (each of the groups presumably
sample the cores of different asteroids)."

    More by the same researcher:
http://www.lpi.usra.edu/meetings/epd2006/pdf/4008.pdf

    Crystallization age is "when it cooled" and that
is related to the size of the body but also the time of
formation. Some irons are "younger" than most:
Kodaikanal (a IIE iron) has a crystallization age of
only 3.8 billion years, something that's really hard
to explain.

    Anyway, it's a long piece with lots of details
and a substantial bibliography on osmium and its
siderophile buddies in iron meteorites.

    Osmium plays a role in this:
http://www.psrd.hawaii.edu/Mar04/fossilMeteorites.html

Headline reads: Tiny Traces of a Big Asteroid Breakup!
Fossil meteorites and chromite grains record a hundred-fold
increase in the number of meteorites that fell 480 million years
ago compared to the meteorite influx today.
    They used chromite instead of osmium for their analysis
because there was more of it, but the pattern was the same.

    (I threw this in, E.P., because you're always interested in
Big Asteroid Breakups, right?)



Sterling K. Webb
-------------------------------------------------------------
----- Original Message -----
From: "E.P. Grondine" <epgrondine at yahoo.com>
To: <meteorite-list at meteoritecentral.com>
Sent: Saturday, April 21, 2007 3:53 AM
Subject: [meteorite-list] Osmium and the size of parent bodies


Hello list -

I wonder if any more work has been done on the osmium
ratios in irons?

good hunting all,
Ed
Received on Sat 21 Apr 2007 04:03:02 PM PDT