I agree with Dugald, but not with Dave Waters and Bruce Yardley.  The
definition of buffer that Bruce implied and Dave offered is too loose to be
informative.  ANY high variance assemblage, for which a continuous reaction
can be written, can "retard changes in activity." How can that be
informative in the absence of measures of the degree to which change is

This discussion has been recurring for decades.  If we use the term
"buffer", clarity requires that we cite the phase rule degrees of freedom
attached to the equilibrium in question.



>Dugald Carmichael wrote:
>> At 09:34 AM 24/09/2001 +0100, Bruce Yardley wrote:
>> >...for almost any rock with this assemblage, biotite
>> >and garnet will be major repositories of Fe2 and so this assemblage will
>> >itself be the buffer for the oxygen fugacity. As a result it will
>> >potentially be stable over a very large P-T range. ... So a useless
>> >thermobarometer but a valuable fO2 buffer!
>> Agreed except for a semantical point. An fO2 buffer is able to hold oxygen
>> fugacity constant at specified P and T in respect to reversible loss or
>> gain of oxygen. The assemblage Grt+Bt+Ms+Mt+Qtz does not meet this
>> condition because its variance is too large. It will _define_ fO2 (by means
>> of the equilibrium among the endmembers alm, ann, mu, mt, qtz), but it is
>> not able to buffer fO2 in the same sense as Ni+NiO or QFM buffer fO2.
>No, that definition of a buffer is too restrictive to be useful.
>Chemistry texts describe buffers as able to resist or retard changes in
>activity, holding variables such as pH in a narrow range. The term
>shouldn't be restricted to ideal systems with pure end member phase
>compositions - the only ones that can _fix_ activity or fugacity at
>specified P and T. The assemblage Grt+Bt+Ms+Mt+Qtz will buffer fO2
>effectively, i.e. retard changes and hold it in a narrow range of
>values, because of the preponderant mass of condensed phases compared to
>the amount of O2 (or H2) that can normally be added or removed in a
>volatile phase. Except at extremely high fluid:rock ratio, only trivial
>changes in the annite and almandine contents of biotite and garnet would
>be needed to restore equilibrium, at an imperceptibly changed fO2.
>Dave Waters - Lecturer in Metamorphic Petrology, Oxford University
>              Dept of Earth Sciences, Parks Rd, Oxford OX1 3PR, UK
>  Tel:    +44 1865 272000
>  Direct: +44 1865 272058      Email:   [log in to unmask]
>  Fax:    +44 1865 272072