DEHYDRATION AND MELTING REACTIONS UNDER DIFFERENT THERMODYNAMIC WATERREGIMES

Dehydration and melting reactions are known to proceed in different ma nners as a function of the thermodynamic water regime. When water beha ves as a mobile component, both reactions occur entirely at constant T and P values and invariable water activity. When water is inert, the mineral assemblages produced during the dehydration reaction occupy ex tensive T-P bivariant fields, and eutectic melting proceeds over a bro ad temperature range that always begins at P = P-H2O. These relations are considered in this paper, using the KASH system as an example.