[meteorite-list] Shirokovsky - Some Thoughts

[Paul H]

Peter Marmet wrote:

>as far as I know, the lastest info is still this 
>special statement of the meteoritical society:

http://www.meteoriticalsociety.org/bulletin/shirokovsky.pdf
"

One really curious thing about the Shirokovsky 
material is that its K-Ar age of the olivine is 
"about 270 million years".  This is curious because
there are metalliferous ultramafic rocks of that age,
which contain dunite, a rock composed entirely of 
olivine. 

What is also most curious is that Au, Ir, and Os of 
the olivine is typical of olivine found in Cu-Ni 
deposits. It is curious because, there are large 
ultramafic Cu-Ni ore deposits that occur in Russia
which contain olivine-rich rocks. For example,
there are "seven major ore deposits associated 
with the basal portions of such olivine rich 
intrusives" in the Noril'sk / Talnakh Ni-Cu-PGE 
(Platinum-Group Elements) ore deposits. By coincidence

or not, these olivine-rich intrusives are part of  
subsurface sills associated with extensive flood 
basalt accumulations, called the "Siberian Traps",  
is about 251 million years in age, which is quite 
close to the "about 270 million years age" of the
Shirokovsky material. The inexact age determination
made on it, could be off by 20 million years.

Within the Central Urals, there are numerous
ultramafic
complexes that contain dunites associated with rich
chromite and PGE (Platinum-Group Elements) 
mineralization and ores. Any of these could be
potential nearby sources of the olivine, which might 
have been associated with ore being transported or 
smelted. 

Given the detailed analyses of the olivine in the 
Shirokovsky material, a person should be able to 
determine whether any potential / possible source for 
the olivine described in the above Meteoritical 
Society Report exists within Russia.

mark ford wrote: 

>In all the pictures I have seen, to me 
>it doesn't quite seem right for a
>pallasite the crystals look a bit too 
>sharp and pointed', (just an observation) 
>anyone agree with me?

I agree completely with you. Go read  "[meteorite-
list] Shirokovsky Texture - A Red Flag As to Its 
Origin??" at:

http://six.pairlist.net/pipermail/meteorite-list/2003-June/091826.html

There I stated:

" I have been looking at a number of pictures of
fragments of Shirokovsky that were shown on 
various web pages and pictures of pallasites.  
After comparing them, there appears to be, in 
my opinion, a rather drastic difference in the 
texture of the olivine grains within each. In the 
pallasites, indvidual grains are equant and either 
rounded or euhedral. Also, in the pallasites, a 
person can find examples of olivine grains that 
have coalesced together. 

Examples of this texture can be seen at:

http://www.meteoriteman.com/collection/thiel.htm
http://www.meteoriteman.com/collection/dora.htm
http://www.meteoriteman.com/collection/mtvernon.htm
http://meteorites.asu.edu/pallasite.jpg

In comparison, pictures of the pieces of 
Shirokovsky that I have found have a different 
texture. the olivine crystals are far more angular 
and variable in outline than found in the pallasites,
for which I found pictures. In fact, some of the
olivine pieces in pictures of Shirokovsky are even 
flat rectagular blades quite unlike anything seen 
in true pallasites. In addition, I don't see any of 
the merged / coalesced olivines seen in other, 
Overall, the Shirokovsky has the appearance of a 
terrazzo floor made up of fragmented rocks." 

Yours,

Paul
Baton Rouge, LA



		
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