At 10:46 AM 27/09/2001 +0100, Bruce Yardley wrote:
>It is interesting that we've all been writing about this question of
>buffering (or not) by Fe2 silicates while having quite different rocks and
>conditions in mind. ...
>...
Very confusing! Perhaps we should all take heed of Howard's advice: "If we
use the term 'buffer', clarity requires that we cite the phase rule degrees
of freedom attached to the equilibrium in question.":-)
I would contend that the distinction between rigorous buffering
(r-buffering?) in systems with F=2 and sliding buffering (s-buffering?) in
systems with F>2 is well worth the attention of anybody interested in how
rocks behave, regardless of terminology. Let's agree that the equilibrium
in question involves Grt,Bt,Mu,Mt,Qtz,and an aqueous vapor phase, so that
P=6. Let the system be KFASH-O2 so that there are 6 components and 2
degrees of freedom. Accordingly, the system is an r-buffer not just for fO2
but also for the chemical potential (or activity) of every other component.
If the system acts as a source or sink for O2 at constant P and T, there
will be changes in the amounts of the phases but not their compositions;
equilibration will involve only kinetically easy dissolution/precipitation,
roughly in accord with Mu+Mt+3Qtz=Alm+Ann+O2 (the coefficients will be
slightly different because Alm and Ann contain some Fe3+ and Mu contains
some Fe2+ and Fe3+). Not only this system but any system with F=2 r-buffers
the potentials of all its components. Accordingly, any such system can act
as a source or sink for any or all of its components by means of
dissolution/precipitation alone, depending only on the permeability of its
boundary and the environmental value of the chemical potential of any
component to which the boundary is permeable.
Now let's raise the variance. If we add the component MgO without adding
a new phase, F will be 3. Now the system s-buffers fO2; equilibration in
response to loss or gain of O2 must involve intracrystalline diffusion in
all three of Grt, Bt and Ms. If diffusion of alm in Grt is kinetically
prohibited, I contend that would be sufficient to prohibit the reaction and
prevent loss or gain of O2, even if diffusion of annite in Bt is
kinetically permitted. Provided that the system is closed to every
component except O2, you can pump an arbitrary H2O+H2 mixture in one side
and it will come out the other side unchanged and without changing the
composition of Bt (unless overstepping is great enough that metastably
dissolving Grt can precipitate new phases as in Bruce's example, or that a
rim of pure almandine can precipitate on the Grt). Interesting (and I think
very common) phenomenon - a system open to a component but not able to act
as a source or sink for that component.
Cheers, Dugald
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