Complete miscibility between silicate melts and hydrous fluids in the upper mantle: experimental evidence and geochemical implications

The phase relationships between silicate melts and hydrous fluids were stud ied by direct visual observation in an externally heated diamond-anvil cell . Complete miscibility of silicate melt and water was observed for a wide r ange of melt compositions, including nepheline, jadeite, dacite, haplograni te and Ca-bearing granite. Some evidence for complete miscibility was also observed in the system basalt-H2O. The critical temperatures in all systems decrease rapidly with pressure. At 15 kbar, the critical temperature for n epheline is around 550 degrees C, for jadeite around 800 degrees C and for granitic compositions it is close to 900 degrees C. In general, the critica l temperatures appear to increase with silica content in the system. Our re sults suggest that there is complete miscibility between silicate melts and water in most of the upper mantle, except at very shallow depths. This mea ns that a water-saturated solidus cannot be defined any more in the deeper parts of the upper mantle. Very silica-rich melt inclusions found in spinel Iherzolites associated with fluid inclusion are probably the result of the unmixing of a supercritical fluid containing comparable amounts of water a nd silicate components. The decomposition of amphibole in a subducted slab occurs at conditions where the miscibility gap between fluid and silicate m elt is not yet closed, while the decomposition of phengite and lawsonite oc curs far beyond the critical curve. Accordingly, the fluids released by the breakdown of these minerals should have very different properties. Highly mobile, hydrous fluids containing little dissolved silicate should be produ ced by amphibole breakdown, while the decomposition of lawsonite and phengi te will lead to much more silicate-rich and less mobile fluid phases. (C) 1 999 Elsevier Science B.V. All rights reserved.