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

Recent results of studies aimed to determine the water solubilities and the liquidus temperatures in synthetic and natural granitic systems are used t o constrain the minimum water contents of granitic melts generated in the c rust as a function of pressure, temperature, melt composition, and water ac tivity. It is concluded that melts generated in the crust at high temperatu re and low water activities (such as A-type granites) contain higher water contents than previously assumed (up to 20% at 800-900 degreesC and 3-7 kba r). As a consequence, the melt fraction produced at a given amount of avail able water can be up to 20% lower. The minimum water content of melts gener ated by partial melting of crustal protoliths (especially quartzofeldspathi c rocks) can be predicted adequately from the liquidus curves in granitic s ystems. This water content can be higher if the melt composition departs fr om a minimum or eutectic quartzofeldspathic composition. Melts generated fr om quartzofeldspathic rocks at low temperature (750 degreesC and below) are necessarily water-rich (>5 wt.% H2O) and have approximately the same visco sity or even slightly lower viscosities than melts generated at higher temp eratures (viscosities in the range 10(4.3)-10(5.4) Pa s). Recent experiment al data also show that the value of the positive dP/dT slope of the liquidu s curves for fixed water contents above 800 degreesC is lower than predicte d by previous models. Thus. adiabatic decompression produces a higher amoun t of melt than previously assumed (at high temperature, the formation of me lt by adiabatic decompression can be up to 50% higher than assumed). (C) 20 01 Elsevier Science B.V. All rights reserved.