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GEO-METAMORPHISM 2001

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Subject:

Re: Buffers

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Date:

Tue, 25 Sep 2001 18:22:09 +0100

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In my view, it is a major failing of many geochemical studies that people
are able to follow rigorously the dictates of the Phase Rule and the Law of
Mass Action, while ignoring any considerations of Mass Balance. The former
approach is fine if, and only if, you want to measure some thermodynamic
variable of a system at some specific point in time; as soon as you become
interested in geological processes, mass balance becomes important and
equilibrium thermodynamics becomes an approximation and is no longer the
ultimate truth.

In the context of the buffering debate, this means that buffering capacity
becomes an important variable. If a sliding equilibrium, such as one
involving the oxidation of an Fe2-rich silicate to magnetite, requires a
substantial amount of fluid infiltration to achieve a very small change in
the equilibrium oxygen fugacity, then it is acting as a de facto buffer. So
the critical variable is dfO2/dO2 where dO2 is the amount of "excess"
oxygen infiltrated above the equilibrium concentration, and the ratio
depends on the modal abundance and composition of the Fe2 silicates in each
individual rock. If you get upset dealing in O2 when the fugacity values
are vanishingly small, then you can recast this in terms of H2 and get the
same effect. I have not sat down and calculated this out, but if I were to
do so, I suspect that I would define a series of terms (which all of the
buffering pedants from all ends of the spectrum would ignore) to express
the subtle degrees of variation from a rock rich in Fe2 end member
silicates that would require vast fluid infiltration to achieve a
significant change in fO2, i.e. a buffer to all but the purest, through to
rocks with small amounts of Mg-end member silicates, whose Fe2
concentrations could still be used as a sensor of the fO2 in the system,
despite having very little buffering capacity. What I would like to
emphasise is that it is really important to take into account the
mineralogical and chemical composition of each rock unit to evaluate
whether they behave more as buffers or more as sensors, rather than just
agreeing some universal term that will hide a really useful range of
variation of behaviour. If a rock with a high buffering capacity has
nevertheless been extensively oxidised or reduced, that tells us something
important about geological processes, whereas an equivalent change in fO2
in a virtually pure quartzite is probably not very significant.

Enough spleen - over here the pub's been open for the past hour!

Bruce


At 08:10 25/09/01 -0700, you wrote:
>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/
>>------------------------------------------------------------------
>


--------------------------------------------
Professor Bruce Yardley
School of Earth Sciences
University of Leeds
Leeds LS2 9JT
UK

Tel. 0113 233 5227        Fax  0113 233 5259
---------------------------------------------

GEOFLUIDS now exists!    http://www.blackwell-science.com/gfl

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