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Re: Esquel and the origin of pallasites.

Recently an article was published(Ulff-Moller et. al 1998) attempting to 
lay down a foundation for pallasite formation. In this article 
sulphur(S) is noticed to be not abundant in pallasites, when it was 
thought that it would be present in abundance. What this means is that 
pallasites may not have formed from a concentric 
cooling(core-mantle-crust, or the reverse), but rather has more to do 
with impact processes.

Pallasites are the most beautiful of all meteorites. Their intertwining 
metal and olivine grains give them the aesthetic appeal. However, they 
are very difficult to gain any information from. This is due to the fact 
that they are composed of two phases(a phase is a term that loosely 
means mineral). These phases are metal and olivine. Aside from basic 
mineral tests that can give what temperatures the minerals formed, and 
how long it took to cool, anything else, such as mode of formation, or 
perhaps what body it originated from, is not so explicit. This requires 
a great deal of cunning and smarts to extract information.

In the case of this study, the main goal was to determine the 
relationship of S to Au,(gold) and Ir. In order to do this a large 
surface is needed. This was because it was thought the mineral 
troilite(FeS) would be more abundant therefore readily analyzed. This 
was not the case, as it often is in science. It was predicted, but did 
not appear. So what is happening?

According to theory, pallasites are materials from the core mantle/ 
boundary. This is because we know on earth that our mantle is composed 
of olivine and spinel. So relating this to pallasites, they were/are 
seen as being right on the boundary of the two environments. So here is 
the theory in a nutshell: Pallasites were formed as part of a cooling of 
an asteroid in which the magma(molten rock) did not move far.

This new study indicates that this may not be the case. Instead if the 
above theory were true, then troilite would be more abundant along with 
other elements, as these components of the magma would have not been 
able to move anywhere, so therefore would have been incorporated into 
the solidification of the magma. Since they are not present, that means 
that possibly the magma was moved through the asteroid, incorporating 
its self into already formed olivines. This may have been done by an 
impact, or a collision of some sort. Then, after a time, the S rich 
magma migrated away. 

This is only a first attempt to explain the data from this 
investigation, and is not formal theory. I put this up for the sake of  
keeping you informed of new ideas developing in the field.   




Frank


Reference:

Ulff-Moller F., Choi B.G., Rubin, A., Tran J,. Wasson J,.(1998)
    Paucity of sulfide in a large slab of Esquel: New     perspectives 
on pallasite formation.  Meteoritcs & Planetary     Sciences Vol. 33 No. 
2 221-227

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