Kinetics of melting of fluorphlogopite-quartz pairs at 1 atmosphere

Melting experiments on fluorphlogopite+quartz pairs have been performed at 1 atmosphere, as a function of temperature and run duration (up to 2 months). Run products include, in addition to melt, the humite-group mineral chondrodite, and enstatite. The solidus was located between 1100 and 1150 degrees C and the amount of melt produced increases sharply at 1250 degrees C. From this temperature liquids are compositionally zoned between the two mineral interfaces and liquid immiscibility occurs at 1300 degrees C and above. The variations of the chemical composition of the melts in space and time indicates that melting proceeds by the individual dissolution of each of the reactants. In the case of the mica, this reaction is kinetically controlled by mass transport in the melt. In the case of quartz, surface reactions are dominant, as shown by the steady silica enrichment of the melt at the quartz interface with increasing run duration. Surface reactions in quartz are slower than atomic diffusion in the melt, and than nucleation and growth of cristobalite within the reacting quartz. These experiments also show that, when the equilibrium melting temperature is overstepped, there is not a unique liquid composition in the charge as long as both reactants are present. This state of disequilibrium persists until one of the reactants is entirely consumed. The melt zonation provides the driving force for dissolution because it allows constant removal from the interface of the species liberated at the crystal interface. For minerals whose interface reactions are rapid (such as the mica) dissolution is controlled by diffusion of species into the melt. This process may be the cause of chemical disequilibrium between melt and residue, because dissolution rates can be faster than chemical exchange between crystals and melt.