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1. Microprobe analyses

2. Mineral groups

3. Solid solutions

4. Thermobarometers

5. Uncertainties

6. P-T calculations

7. Phase diagrams <



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Practical Aspects of Mineral Thermobarometry

Phase diagrams and pseudosections with THERMOCALC

The old pages on this topic are now entirely obsolete, and major revision is needed. I intend to provide here some examples of approaches to pseudosection calculation that do not attempt to duplicate THERMOCALC's own documentation, but rather reflect our own experience here in Oxford.

monitoring assemblage and mode changes in a specified bulk composition

A pseudosection is a diagram that explicitly shows the assemblage in a system of specified bulk composition as a function of two independent variables. The mineral proportions and mineral compositions are also uniquely defined at any point on the pseudosection, so that divariant fields, for example, can be contoured for mineral mode or composition. The pseudosection is the graphical expression of Duhem's Theorem, which says that a system of specified bulk composition has two (and only two) degrees of freedom. It is one of the more important predictive tools, and one of the best practical uses in petrology of a two-dimensional sheet of paper!

Constructing pseudosections

The THERMOCALC documentation provides instructions on how to get started, including practical exercises from THERMOCALC workshops, or you can try this worked example from Julie Baldwin (University of Montana) on the SERC pages.

Some construction tips

Don't be tempted to leap in and calculate any curve you might think useful, hoping to sort out later which ones (or which parts of ones) are stable. Be more systematic. Try to locate any low-variance bands, perhaps by comparison with existing comparable pseudosections, or by trying some free-energy-minimisation traverses. Then work systematically outwards from these.

Things to remember:
Tips for dealing with complex cases

If it looks as if a number of boundaries are going to converge on a very small area of P-T space ...

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This page last modified 18 December 2007