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
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.
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