It's been so long you probably won't even see this, but in case someone else is searching for an answer, here are some interesting answers I've found: (The websites are in French, so I tried to translate the intersting bits)
Igneous rock samples can be melted in the laboratory by heating them at elevated temperatures (1000 ° C to 1300 ° C). If one operates under pressure and in the presence of volatile elements, the melting temperature of the rock and its minerals is much lower (650 ° C to 950 ° C). It is known that magmas contain varying amounts of volatile elements that lower the melting point of many silicates, thus allowing the magma to remain liquid even at temperatures below those required to melt rocks in the laboratory. Indeed, the temperatures that exist in the different levels of the earth's crust are not sufficient to melt rocks in the absence of volatile elements.
The main volatile elements found in magmas can be commonly observed in the gaseous state at the vents of active volcanoes. It is essentially water vapor, carbon dioxide, hydrogen, chlorine, fluorine (in the form of acids) and many other gases [...].
Taken from http://www.fossiliraptor.be/rochesetphenomenesignes4.htm
In the case of hydrated paragneiss, during the warming of this rock, the melting begins as soon as it reaches the first reaction ((4) Qz + Ab + Gold + H2O ⇄ Liquid), around 650 ° C. Indeed, this gneiss contains all the reagents necessary for this reaction to take place.
For a dry paragneiss, on the other hand, nothing happens on the curve corresponding to reaction 4; indeed, one of the reagents, water, is lacking. However, when the temperature exceeds 850 ° C, this rock crosses the curve corresponding to reaction 5 ((5) Bt + Ab + Qz + AlS ⇄ Liquid + Cord / Gt + KF, which can be decomposed as follows: (6) Bt + AlS ⇄ Gt / Cord + KF + H2O, then 7) Qz + Ab (Pg) + Gold (KF) + H2O ⇄ Liquid) and the two coupled reactions described above take place: the biotite decomposes to liberate water; therefore, we are in a situation where all the necessary products are present: the merger can take place.
In other words, a hydrated system will melt (cross its solidus) at a lower temperature than a system without free water: the two reactions correspond respectively to "hydrate solidus" and "dry solidus"!
Taken from: https://planet-terre.ens-lyon.fr/article/reactions-de-fusion.xml#equation4