Ed - Most elements are somewhat soluble eventually, but to see this you
have to have a gradient in solubility-it has to precipitate as well or we
probably would not know anything had happened. In oxidised environments, Cr
can be extremely soluble and mobile, but even under reducing conditions
there is some mobility, especially if you get unusually high or low pH, or
very high chloride levels.
Emerald is of course coloured by Cr and several deposits, notably the
Columbian ones, are actually low grade metamorphic veins in black slates,
formed from extremely saline brines derived from the dissolution of salt
deposits under metamorphic conditions. It isn't understood but presumably
the high ligand concetrations are driving this.
Ultramafic rocks not only carry relatively high Cr levels but can give rise
to high pH environments as water gets in, which facilitates redistribution
of Cr. Primary Cr-diopside is an obvious source of Cr to fluids in
ultramafics, in addition to chromite, and these minerals change a lot as
they recrystallise in response to infiltration of water, and can release Cr
to those nearby secondary phases that can accommodate it under the
retrograde environment.
All that said, clearly for most rock types most of the time, Cr is one of
the basket of elements that should be looked at as probably immobile - just
don't insist it has to be immobile if the evidence says otherwise!
Bruce
At 14:27 22/01/2007 -0500, you wrote:
>Chromium is normally considered immobile in metamorphic environments. Has
>Cr mobility
>been reported in either natural or experimental systems?
>
>Thanks!
>
>Ed
Professor Bruce Yardley
School of Earth and Environment
University of Leeds
Leeds LS2 9JT
UK
Tel: +44 (0)113 3435227
Fax: +44 (0)113 3435259
The Geological Society is (nearly) 200 years old -
find out about the Bicentennial Conference (10-12 September 2007) at:
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