[meteorite-list] Re: Meteorite-list digest, Vol 1 #1866 - 9 msgs

From: Robert & Wendi Beauford <wendirob_at_meteoritecentral.com>
Date: Thu Apr 22 10:16:36 2004
Message-ID: <002d01c36b26$8184d000$7d251fd8_at_wendirob>

Thanks David, Al, and John.
That gives me more to think about. The article pointed out by David,
http://meteoritics.org/Abst_38(3).htm Morphological analysis of olivine
grains annealed in an iron-nickel matrix: Experimental constraints on the
origin of pallasites and thermal history of their parent bodies, seems to
open up a whole world of lower temperature possibilities. It seems that it
will be difficult to explain the morphology until we see more of it's
context. The impact theory of crust or mantle into core displacement just
doesn't ring right to me. Even that, I guess could make more sense, though,
with the increased significance of surface tension (wrong word I think) on
iron suspended silicate 'bubbles' in lower gravity astroid environments. I
guess the problem for me is that these don't appear clastic, and if the
conditions are fluid, why don't they just flow apart again. The article
mentioned might alow rounding of what might originally have been clastic
particles, though, and thus the creation of the illusion of a much more
fluid event. ....Unless I completely misunderstand, that is, a posibility
that is by no means remote.
-Robert Beauford : )

> Subject: [meteorite-list] formation of pallasites / mesosiderites
>
> Just a thought...
> The current explanations for mesosiderite and pallasite formation have
never
> rung quite right to me... so I was reflecting on it the other day.
> Is it possible that pallasites / mesosiderites are low gravity dependant
in
> their formation? ie that they were formed in a smaller body or bodies in
> which the temperatures necessary for differentiation were achieved, but in
> which gravity was inadequate to overcome friction and bring about
> crust/mantel/core separation, or in the same thought, near the neutral
> gravity center of only slightly larger body/bodies in which friction or
> temperature/viscosity/density variables prevented or provided inadequate
> impetus to 'float' the silicates?
> -Robert Beauford
>
> Subject: Re: [meteorite-list] formation of pallasites / mesosiderites
>
> Hi Robert and all,
>
> I don't think there is an easy answer for the formation of these two types
of
> stony-iron meteorites and it is apparent that those that study them also
think there
> is some mysteries to be solved.
>
> What is known are the cooling rates to form such stony-irons. No doubt the
Pallasites
> formed from at least two different parent bodies and the messosiderites
formed from a
> different parent body all together (unless they come from the same parent
body and are
> found at a different depth?). Cooling rates for pallasites seem to be from
2 million
> year to about 10 million. The messosiderites seem to have cooled in .2 to
.7 million.
> Now comes the tricky part.
>
> A group from the University of Denmark (Henning Haack) explored the
thermal effects of
> asteroids and more specifically regoliths on asteroid surfaces. What they
found was
> that asteroids with a powered regolith makes a good insulator. It is
possible to have
> an asteroid cool ten times slower than an asteroid with no covering (and
be tens times
> smaller for the same cooling rate) and might answer your question do
pallasites come
> from a lower gravity asteroid.
>
> It is suggested that the pallasites probably come from a body the size of
less than
> 100 km. It is thought that they are related to the IIIAB iron meteorites.
It is
> thought that a possible filtering in of metal around the olivine crystals
could have
> occurred but would have had to happen after the main core had mostly
solidified. The
> later filtering effect is shown by the pallasite metal has a more highly
fractionated
> composition which seems to plot along the IIIAB iron's latest
crystallizing age. How
> this filtering happen is somewhat of a mystery. Is it a result of impact
submerging
> the olivine in the liquid core or forcing the liquid metal up into the
olivine
> crystals? Since there are two parent bodies of the pallasites it isn't an
isolated
> occurrence.
>
> McSween says that because of the drastic differences between densities of
iron and
> silicated materials, it is not obvious how core and mantle became
intermixed.
>
> The Mesosiderite parent body has an even more complexed history. One
suggestion is a
> collision with a iron asteroid hitting an asteroid with silicated material
and
> intermixing. There is some resemblance of the HED Asteroid and the
mesosiderites.
> Could this be one of the impacted parent bodies or perhaps a fragment from
the HED
> asteroid?
>
> A-type asteroids thought to be possible parent planets of the pallasites
(such as
> asteroid 246 Asporina) from spectra taken by astronomers and plot nicely
along the
> stony-iron chemical make up.
> Even with all that is know there are no doubt some misleading items that
may later
> prove to be wrong like the relationship of the IIIAB iron meteorites with
the
> pallasites and even the cooling rates. Because of the maltitude of
variations in
> different asteroids and asteroid make up, it is possible for material to
have formed
> in different sized asteroids at different depths but under similar
conditions and make
> answering the questions about stony-iron make up hard. With time new clues
may tell us
> more about the make up of these interesting meteorites and their parent
planet
> asteroids.
>
> A good reading source (and where I picked up a lot of my information) is
Meteorites
> and Their Parent Planets by Harry Y. McSween Jr. In chapter 7 of his
second edition
> book he addresses the possible formation of the iron and stony-iron
meteorites. All my
> best and hope I shed some light.
>
> --AL Mitterling
>
> Al and Robert,
>
> I too doubt that we have most of the answers for the stoney-irons. (It is
a
> lot easier to say that than something complex like Al and Robert did.)
>
> Al, Good job...you should have edited for Harry...he used a lot more words
to
> say the same things. Actually, his book is very fascinating for the reader
> who can hang in there and go slowly.
>
> Please don't ask me to model these two stories in CFD!
>
> John
Received on Mon 25 Aug 2003 12:32:41 PM PDT