The general supposition is that all L chondrites come from one parent body,
that H chondrites come from another, etc. We don't know this to be strictly
true, but there is evidence that it is broadly true. For example, about
two-thirds of all L chondrites (of different petrologic types) were shocked
about 470 Ma ago or so, indicating that they were on the same parent body at
that time. Similarly, nearly half of all H chondrites (of different
petrologic types) have a cosmic-ray exposure age of about 7.5 Ma indicating
that they were all within about a kilometer of each other (or so) at that
time. The idea that the parent bodies were stratified with the type-6
chondrites in the center and type-5s closer to the surface, etc. is known as
the onion shell model. There is some evidence for it and some against it.
Hence, some researchers are strong advocates and others reject the model.
As is clear, the situation is not yet resolved.
Alan Rubin
----- Original Message -----
From: "Dave Gheesling" <[email protected]>
To: "'Meteorite List'" <[email protected]>
Sent: Tuesday, September 08, 2009 8:40 PM
Subject: [meteorite-list] Chondritic parent bodies
All,
Pete's question re: pallasites reminds me of one I've been meaning to
throw
out to the group for a while. I believe that, by definition, L6's come
from
one parent body and L5's, say, come from another. It's clear why breccias
might simply be an association of the two. But I've seen cross section
illustrations of hypothetical asteroids more than once which indicate a
transitional progression from L3 material at the exterior/crust through
L4,
then L5, and eventually to L6 at the center/core. Presumably this is due
to
insulative properties and the like towards the interior which allow more
heat from radioactivity to build up, etc, but this also seems to indicate
a
single parent body. I'm sure Sterling & Co. might have a field day with
this one, and I'm looking forward to any responses out there...
Thanks much,
Dave
Dave Gheesling
IMCA #5967
www.fallingrocks.com
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