Re: Geological History of terrestrial "Olivine Bomb"?

Paul bristolia at yahoo.com
Mon Sep 4 19:39:16 EDT 2006

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 first
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?"

Technically 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.