[Explorer89]

I should learn to keep my stupid mouth shut. It has been at least 25 years since Chemistry.

Nonetheless, I thought the point of a pressure cooker was to allow higher temperatures?

[monocle]

Once the vapor pressure of a liquid equals atmospheric pressure that liquid boils and temperature remains constant. By increasing atmospheric pressure a higher boiling temperature results. Some solids, such as carbon dioxide, do not pass through a liquid phase and sublimate to the gaseous phase.

[SpaceBar]

Don't beat yourself up over it, but just to elaborate a bit, mantle derived material in a semisolid state e.g. magma behaves a lot like a really dirty chemical slag/slurpy with crystals suspended in a chemical soup or melt composed primarily of the oxides of silicon, magnesium, sodium, and potassium, with various crystaline materials going in and out of existence depending on the p,T conditions and local chemical composition. Change any one of those variables and the system will attempt to readjust. Lowering pressure can lower the solidus curve of stable phases in temperature and cause an otherwise stable mineral to melt because it is now superheated, perhaps with some other phase taking its place. By this mechanism, rocks can "evolve" from mantle peridotite to highly evolved granites. Adding water or volatile components further complicates the idealized behavior of simple thermodynamic models.