On 11/19/2010 12:18 PM, Richard Palin wrote:
> Pradeep,
>
> If the rock you are studying has biotite which has formed at peak PT
> conditions with a composition of Mg/(Mg+Fe)=0.6, for example, you may plot a
> compositional isopleth to estimate where these peak conditions exist. The
> isopleth will mark out all conditions on a pseudosection where biotite has
> XMg =0.6 and you may expect that true peak conditions will lie somewhere
> along that line. An intersection with an independent compositional isopleth
> will give you this point, Ca in garnet, for example.
>
> Alternatively, if your rock has 10.00% modal volume of biotite which was
> stable at peak P-T conditions, and has not undergone retrogression to
> significantly alter its proportion, you may plot a line marking out where
> 10.00% biotite will be stable in P-T space. This is an isomode line instead
> of an isopleth (as mentioned above, which is for compositional variables).
> You may also plot a separate line for garnet modal volume, or kyanite, or
> whatever. These should intersect as the compositional isopleths do and you
> can estimate peak P-T conditions this way.
>
> If the rock is accurately representing peak conditions, your compositional
> isopleths and your isomodes will all cluster at the true peak P-T conditions
> of equilibration.
>
> Thanks,
>
> Richard Palin
>
> D.Phil Research Student,
> Himalayan Studies Group
>
> University of Oxford,
> Department of Earth Sciences,
> South Parks Road,
> Oxford,
> OX1 3AN,
> United Kingdom
>
> http://www.earth.ox.ac.uk/~richardp/
>
>
----- yes, assuming overall equilibrium was attained at one P and T
condition, that the thermodynamic data are all perfect, that the system
was and has remained closed since this point,.....and several other
assumptions.
Cheers,
Sumit
--
***************** Sumit Chakraborty ****************************************
http://www.gmg.ruhr-uni-bochum.de/petrologie
Institut fuer Geologie, Mineralogie und Geophysik;
Ruhr-Universitaet Bochum;
D-44780 Bochum; Germany
Email: [log in to unmask]
Tel: +49-(0)234-322 -4395 / -8521 / -8155
Fax: +49-(0)234-321 4433
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