Metasediments that originally had organic material are strongly
perturbed during metamorphism in terms of fO2 relative to a common buffer
curve. Such rocks often have diagenetic ferromagnesian silicates that are
out of equilibrium with methane, and they may be reduced during the early
stages of metamorphism. We find iron formations in the granulite facies
that have fO2 comparable to QFM, but which may have started out before
metamorphism considerably above this buffer, although it is difficult to be
sure without finding the same lithology at all grades. Some high grade
iron formations have ferromagnesian silicates with Mg# of 50-75 and
register oxidation along an s-buffer producing quartz and matnetite during
metamorphism. Peter Robinson and coworkers nicely demonstrated a large
shift to more magnesian bulk compositions of metamorphic rocks in W.
Massachusetts as a result of substantial metasomatic effects. Production
of increased quantities of Fe oxides or Fe sulfides often marks the
s-process leading to more magnesian silicates, although in many cases the
metasomatic process occurred before regional metamorphism.
>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.
>Professor and Chair
>Department of Earth and Environmental Sciences, JRSC 1C25
>Rensselaer Polytechnic Institute
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