Joint degassing and crystallization: Experimental study with a gradual pressure release

Results of experimental and theoretical study of granitic melt degassing at water fluid pressure of 1.5-0.5 kbar and temperature of 750 degrees C are reported. The preliminary consideration of homogeneous nucleation based on the classical theory has shown that the time of delay may be great, and, fo r the water content of similar to 3 wt % and less, the homogeneous nucleati on virtually is not feasible. As was established by experiments with the me lt, which initially contained air microbubbles, the gradual pressure releas e during 1-7 days resulted in the homogeneous spatial distribution of growi ng fluid bubbles. The degassing with a formation of vesiculation front prop agating from the sample surface was also observed. Due to the water loss, t he melt enters the field of alkali feldspar crystallization. It has been sh own that this mineral starts to crystallize only after a preliminary seedin g. The SEM technique was used for the study of bubble size distribution. It was established that the small bubbles are predominant. The crystallizatio n doubles the volume of released fluid and provides the bubble coalescence.