Assemblage stability diagrams, or pseudosections, are diagrams that display the stable mineral assemblages for a specific bulk composition over a range of P-T conditions. They can be P-T, T-X or P-X diagrams.

Pseudosections can be constructed using Program Gibbs in two ways.

The most general approach (and most difficult) is to use the contour routines to draw contours where the moles (mode) of a phase is 0.0 (with all other modes being positive). These lines must be truncated at the absolute stability limit of the assemblage (defined by univariant reactions).

The second way is far easier, but is only implemented for the KFMASH system using the petrogenetic grid routines. In this mode, the program calculates the contour (as before) and compares the P-T conditions of the contour with the stability field for the assemblage as defined by the grid, plotting only those curves where there is an intersection. Using this mode, a complete P-T pseudosection for a bulk composition in the KFMASH system can be calculated in around an hour. Future programming will expand this capability to more realistic bulk compositions.

Examples of pseudosections calculated with Gibbs

- Shaw's average high-grade pelite (high-Al pelite) (pdf)
- Low-Al pelite (Fe/Fe+Mg = 0.58) (pdf)
- Low-Al pelite (Fe/Fe+Mg = 0.75) (pdf)
- Low-Al pelite (Fe/Fe+Mg) = 0.91 - chloritoid+biotite assemblages) (pdf)

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