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/
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