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Dugald and others,

I can't resist weighing in on this discussion.

Dugald started off with an elegant equilibrium discussion of how to 
view buffering, but ended with a kinetic argument. The first question 
that should be addressed for any "s-buffer" (and I like your 
introduction of this distinction) is the "buffer capacity" of the 
assemblage from an equilibrium thermodynamic perspective. After the 
equilibrium condition is established, one can (and should, as you 
point out) add the kinetic perspective, which can often dominate the 
result.

A useful quantity to calculate for your s-buffer assemblage in the 
KFMASH-O2 system for the assemblage  Grt+Bt+Mu+Mt+Qtz+aqueous vapor 
phase is the partial derivative (¸Xalm/¸f(O2))P,T. This will provide 
an equilibrium view of the sensitivity of the garnet Fe/Mg to changes 
in f(O2). Another useful quantity is (¸Mgrt/¸f(O2))T,P (where M is 
the amount of the phase), which will provide a measure of the amount 
of garnet (or any other phase) that will be consumed/produced as a 
function of f(O2). It is also useful to quantify the amount of oxygen 
required, in which case a useful derivative  is (¸Mgrt/¸M(O2))T,P.

Since most minerals are dominantly oxygen, I suspect one would find 
that most metamorphic assemblages are pretty effective s-buffers. 
That is, the value of f(O2) is pretty nearly totally controlled by 
the internal equilibrium.  I know of no examples where major changes 
in f(O2) have been documented in regionally metamorphosed rocks as a 
result of infiltration.


Cheers,


Frank
----------------------------------------------
Frank Spear
Professor and Chair
Department of Earth and Environmental Sciences, JRSC 1C25
Rensselaer Polytechnic Institute
110 8th Street
Troy, New York 12180-3590
Phone (office): 518-276-6103
           (lab)     : 518-276-4899
Fax:                  518-276-6680
http://ees2.geo.rpi.edu/spear/spear.html