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/
> -----Original Message-----
> From: Metamorphic Studies Group [mailto:GEO-
> [log in to unmask]] On Behalf Of pradeep das
> Sent: 19 November 2010 10:49
> To: [log in to unmask]
> Subject: perplex
>
> Dear All,
> I am learning pseudosections for different rock types. Now I am running
> successfully perplex (version 07), But still few points I could not
understand.
> There are several options in perplex for the calculation of different
> properties like specific enthalpy, density etc. Please let me know what is
the
> difference between compositional isopleths and modal volume diagram. As I
> understand that using compositional isopleths we may calculate the PT
> conditions. What is the modal volume diagram and how it is different from
> the compositional isopleths.
>
> I will be grateful for your suggestions.
>
> Yours,
> Pradeep Das
> Research scholar
> Geometamorphism group
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