Maximum and minimum water contents of granitic melts generated in the crust: a reevaluation and implications

Abstract

Recent results of studies aimed to determine the water solubilities and the liquidus temperatures in synthetic and natural granitic systems are used to constrain the minimum water contents of granitic melts generated in the crust as a function of pressure, temperature, melt composition, and water activity. It is concluded that melts generated in the crust at high temperature and low water activities (such as A-type granites) contain higher water contents than previously assumed (up to 20% at 800–900°C and 3–7 kbar). As a consequence, the melt fraction produced at a given amount of available water can be up to 20% lower. The minimum water content of melts generated by partial melting of crustal protoliths (especially quartzofeldspathic rocks) can be predicted adequately from the liquidus curves in granitic systems. This water content can be higher if the melt composition departs from a minimum or eutectic quartzofeldspathic composition. Melts generated from quartzofeldspathic rocks at low temperature (750°C and below) are necessarily water-rich (>5 wt.% H 2O) and have approximately the same viscosity or even slightly lower viscosities than melts generated at higher temperatures (viscosities in the range 10 4.3–10 5.4 Pa s). Recent experimental data also show that the value of the positive d P/d T slope of the liquidus curves for fixed water contents above 800°C is lower than predicted by previous models. Thus, adiabatic decompression produces a higher amount of melt than previously assumed (at high temperature, the formation of melt by adiabatic decompression can be up to 50% higher than assumed).