While not doubting the veracity of Dugald's calculations, I do feel it is
worth emphasising that, 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. Dugald's calculations
indicate quite a low-T under fairly reducing conditions (and in agreement
,this is quite a common greenschist facies assemblage), while the upper
limit is almost certainly given by the degenerate thermal stability limit
of muscovite + quartz in the upper amphibolite facies. Any additional
presence of graphite at lower-T's will tend to drive the reaction as
written to the left, destroying magnetite on heating. So a useless
thermobarometer but a valuable fO2 buffer!
Bruce Yardley
At 13:11 22/09/01 -0400, you wrote:
>At 10:18 PM 16/09/2001 +0530, Mallickjarun Joshi wrote:
>> Dear All, Could anybody tell me the pressure temperature conditions for
>>the reaction which if I remember correctly is in the following manner:
>>biotite+garnet+O2=muscovite+magnetite+quartz
>
>The P-T curve for this reaction would be univariant in the system KFASH
>(i.e., for the annite and almandine endmembers) provided that oxygen
>fugacity is either specified or buffered. Using the nickel-bunsenite
>buffer, we can write an equivalent reaction involving Ni and NiO so that
>f(O2) is internally buffered and need not be specified:
>
>annite + almandine + 2 NiO = muscovite + 2 magnetite + 3 Qtz + 2 Ni
>
>Using the thermodyamic database of Holland and Powell (1998 J Metam Geol),
>the P-T curve has a negative slope and is slightly convex downwards,
>passing through 10 kbar at 260 deg C and 1 kbar at 500 deg C. Mu+Mt+Qtz is
>stable on the low-T side of this curve. Adding Mg,Mn,Ca to the system would
>displace the curve to lower P-T and expand it to a zone of finite width
>(i.e., the assemblage Bt+Grt+Mu+Mt+Qtz would be stable across a range of P
>and T).
>
>>I would also welcome any
>>information on a reaction that consumes garnet and produces
>>muscovite+magnetite+quartz in the garnet zone.
>
>If the foregoing considerations are correct, we should expect garnet to be
>produced and muscovite+magnetite+quartz to be consumed in the garnet zone.
>
>Cheers, Dugald
>
>Dugald M Carmichael Phone/V-mail: 613-533-6182
>Dept of Geological Sciences and Geological Engineering
>Queen's University FAX: 613-533-6592
>Kingston ON K7L3N6 E-mail: [log in to unmask]
>
--------------------------------------------
Professor Bruce Yardley
School of Earth Sciences
University of Leeds
Leeds LS2 9JT
UK
Tel. 0113 233 5227 Fax 0113 233 5259
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