[meteorite-list] Re: Geological History of terrestrial "Olivine Bomb"?

[Paul]

Gerald Flaherty wrote:

"I picked up an Olivine Bomb from Norbert and 
Helke Kammel of "Rocks On Fire" a couple of 
years ago when I knew less than I know now of 
meteorites. It's Location is listed as Mt. 
Shadwell, Victoria, Australia. At the time the 
very word "olivine" immediately brought pallasite 
to mind. I think I'd just invested in my frist 
Imilac. This piece is tantalizing in every way, 
from is thick jet black volcanic crust, to its 
beautifully polished green interior. Is this 
terrestrial mantle tossed up in a violent 
volcanic blast? Are these common?" 

Techincally speaking, they are not volcanic bombs, which are 
thrown out of volcanoes during eruptions. Rather, they are 
exotic chunks of rocks, called xenoliths, carried upward by 
magma as it ascended through the crust. The best preserved 
xenoliths are those carried up rapidly from deep in the 
mantle by the formation of diatremes.

Mount Shadwell is the highest of a cluster of basaltic scoria 
cones. It is well known as a source of olivine and augite 
ultramafic xenoliths and clinopryoxene and orthoclase 
megacrysts contained in basalts and scoria. My understanding 
that although such xenoliths can be found in many basaltic 
lavas, the ones found at Mount Shadwell are uncommon for 
their size and preservation. The xenoliths found at Mt. 
Shadwell are inferred to have come from both the lower 
crust and upper mantle.

Some web pages:

1. Mount Shadwell - Victorian Resources Online
http://www.nre.vic.gov.au/dpi/vro/coranregn.nsf/pages/eruption_points_shadwell

2. Coexisting Andesitic and Carbonate Melts in a 
Lherzolite Xenolith from Mt. Shadwell, Victoria

http://www.es.mq.edu.au/geology/MNAGC98.html
http://www.es.mq.edu.au/geology/MNHP.html

3. Melting and Metasomatism in the Lithospheric Mantle Beneath
SE Australia: Trace Element Studies by Laser Microprobe by 
Marc Norman and Suzanne O'Reilly

http://www.es.mq.edu.au/GEMOC/annrep97/abs96/Norm396.htm

4. Roach, I. C., 2004, Mineralogy, Textures and P-T 
Relationships of a Suite of Xenoliths from the Monaro 
Volcanic Province, New South Wales, Australia. Journal of 
Petrology. vol. 45, no. 4, pp. 739-758.

http://petrology.oxfordjournals.org/cgi/content/abstract/45/4/739

5. Ellis. D. J., 1976, High pressure cognate inclusions 
in the Newer Volcanics of Victoria. Contributions to 
Mineralogy and Petrology. vol. 58, no. 2, pp. 149-180.

http://www.springerlink.com/content/r02x704tlm23w415/

6. Xenolith

http://en.wikipedia.org/wiki/Xenolith

Yours,

Paul H.


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