[meteorite-list] formation of pallasites / mesosiderites

From: almitt <almitt_at_meteoritecentral.com>
Date: Thu Apr 22 10:16:35 2004
Message-ID: <3F495ED1.ECD0663C_at_kconline.com>

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
Received on Sun 24 Aug 2003 08:56:50 PM PDT