Howard, Dug and all,
   That is why I used the term sensor. In most rocks the reactions
dependent on oxygen fugacity sense its levels (if equilibrium is
preserved), but do not buffer it. A few iron formations may have such
abundant and simple oxide assemblages such as mt-hm (and abundant qz-fa-mt
at high grades) that they can act as buffers as long as too much water
doesn't pass through the system.
eric



>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
>retarded?
>
>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.
>
>Regards
>
>Howard
>
>
>
>>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.
>>
>>Regards,
>>
>>Dave
>>--
>>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 http://www.earth.ox.ac.uk/~davewa/
>>------------------------------------------------------------------