Teaching Phase Equilibria
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THERMOCALC AveragePT Calculations

Julie Baldwin, University of Montana

What is it?

The type of thermobarometry that THERMOCALC does relies on the calculation of multiple reactions based on an internally-consistent dataset. This has been termed 'optimal thermobarometry' or average PT (Holland & Powell, 1994)

Advantages of 'optimal thermobarometry' include its merit of consistency with other mineral equilibria methods (like pseudosections), the possibility of realistic assignments of uncertainties, and the recognition when there is little or no thermobarometric information in a mineral assemblage based on the statistical output.

How does it work?

In AveragePT, the thermodynamics of an independent set of reactions between the end members of the mineral assemblage (assumed to have been in equilibrium) is combined statistically (by a 'least squares' method) to give a PT result.

What does it actually do?

Average PT plots

Goodness of Fit



Old Way:

New Way:


Let's look at an example with garnet-biotite Fe-Mg exchange as well as GASP. The assemblage is garnet-biotite-sillimanite-plagioclase-muscovite-quartz from the Eastern Alps (Habler & Thoni, 2001). We can see the shape of the phase relations using a PT pseudosection.

AvPT example


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