[meteorite-list] formation of pallasites / mesosiderites

From: j.divelbiss_at_att.net <j.divelbiss_at_meteoritecentral.com>
Date: Thu Apr 22 10:16:35 2004
Message-ID: <20030825010920.2D18953574_at_pairlist.net>

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
> 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
>
>
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Received on Sun 24 Aug 2003 09:09:17 PM PDT