Makes sense but does this not assume a closed system and that the mineralogy of the core is only a function of available water? If all you had in the core was quartz and you had an open system you really couldn't say anything could you?
The mineralogy of the core would never be "only a function of available water." The minerals in the core would be a function of many things, most importantly the elements that make up those minerals. Whether the system is open or closed, at this point in the research, is not very important. We all know that pegmatites in their late stages can cook up many varied physico-chemical environments when they reach water saturation. The point we were making is that you wouldn't have a pegmatite at all if the things started out water staurated. If the system is open when water saturation takes place then the water may escape, but that is late in the ball game.
But in fact you could probably say something, as the water's effect on the surrounding rocks would likely be detectable as mineralogic, fluid inclusion, and isotopic variations that would be localized and mappable to the vicinity of the pegmatite, and consistent with the chemistry and isotopes of the pegmatite. No system on Earth is open infinitely, or to put it another way, a system that is open at one scale is always closed at some larger scale.