[meteorite-list] portales

From: Rhett Bourland <rbourlan_at_meteoritecentral.com>
Date: Thu Apr 22 10:01:31 2004
Message-ID: <IOEBKAHMGFBDJMOFGDFNIEOODLAA.rbourlan_at_evansville.net>

Hi Harlan,
First of all, good topic!!! I hope a lot of people post on this one as I'd
love to hear what everyone has to say on it. I apologize if I get a bit
wordy but I know a lot of people who may not know very much about meteorites
come here to learn so I thought I'd talk a bit. If I'm too far off base on
any of this I apologize and please let do not hesitate to correct me.
Before I even start about how I believe it formed, I'll start off with what
I know about it. It is an H6 chondrite which means that it formed deeper in
the asteroid it came from than other H chondrites like H3's, H4's, and H5's.
For a good explanation on the formation of asteroids check out "Meteorites
and Their Parent Planets" by McSween. Almost all asteroids start off as
"onion shells" with the more metamorphosed grades buried deeper in the
parent body than the lower grades. This is due to the asteroid's ability to
more easily radiate the heat from its outer layers than the more inside
parts and thus heat is what equilibrated and altered the areas closer to the
core more than the regions near the surface. Some asteroids, after being
formed as an onion shell, will undergo collisions which, depending on their
severity, will have different effects on different parent bodies. If the
impact is severe enough then the asteroid will be shattered and pieces of it
will go flying off in many directions and not reaccreate. If the impact
isn't as strong, however, the pieces of it will come back together under
gravities powers and the various grades (3's, 4's...) will be mixed
together. The high number of H breccias (like Zag which is an H3-6 or
Noblesville which is an H4-6) would seem to indicate that the H parent body
is a rubble pile asteroid. Subsequent heating in the core of the asteroid
doesn't happen because the nuclear isotopes that caused that heating have
most likely already ran their course by this time. This would seem to be
backed up by the reflectance spectra of asteroid 6 Hebe which, depending on
what area is being imaged, has areas that match the respective grades of
H's.
I also know there are large sections of iron in this chondrite that are
unlike any other meteorite out there. These large sections of irons will
display a Widmanstatten like most iron meteorites when etched. To be able
to form the necessary bands in the pattern would require that this meteorite
was formed deep within the asteroid so that there would be plenty of
insulation (in the form of rock) to keep the heat in the inside of the
asteroid so that the kamacite and taenite would have the needed time to grow
large enough to show up when etched. Something that's interesting about the
nickel-iron in Portales is that the metal in the veins of this meteorite is
different from the metal flecks seen in all chondrites (especially the H's).
Another unusual thing is that there is less free iron in the silicate areas
of Portales than in normal H chondrites (about 4% in comparison to the
normal 15%-19%) even though fayalite values remain pretty much the same as
other H's (19.3% +/-0.4%). Check out the Met. Bul. containing information
on this meteorite at
http://www.uark.edu/campus-resources/metsoc/metbull/mb83.htm
SOOOOOOOO, to get to the point of this email, how do I think Portales was
formed? Early in the H parent body's history a pretty good sized impact
happens on the H parent body. Its powerful enough to disrupt the asteroid
to its center but not necessarily powerful enough to break up the asteroid.
When it does this, some of the free metal in this region pools together to
form the large metal veins. Keep in mind, this is early enough in its
history that the nuclear isotopes that are heating this asteroid are still
active. Also, since Portales Valley is an H6 that would mean that is
towards the core of the asteroid and has plenty insulation in the rocks
above it to keep it warm enough to sustain the kamacite and taenite growth
needed to form the Widmanstatten patterns that are seen in the large metal
areas of Portales Valley.
My 8 cents,
Rhett Bourland
www.asteroidmodels.com
www.asteroidmodels.com/personal
www.meteoritecollectors.org
-----Original Message-----
From: meteorite-list-admin_at_meteoritecentral.com
[mailto:meteorite-list-admin_at_meteoritecentral.com]On Behalf Of harlan
trammell
Sent: Wednesday, June 05, 2002 11:40 AM
To: meteorite-list_at_meteoritecentral.com
Subject: [meteorite-list] portales


how do YOU think portales got formed?



Get your FREE download of MSN Explorer at http://explorer.msn.com.
______________________________________________ Meteorite-list mailing list
Meteorite-list_at_meteoritecentral.com
http://www.pairlist.net/mailman/listinfo/meteorite-list
Received on Wed 05 Jun 2002 04:23:47 PM PDT