Kinetics of melting of fluorphlogopite-quartz pairs at 1 atmosphere

International audienceMelting experiments on fluorphlogopite + quartz pairs have been performed at I atmosphere, as a functionof temperature and run duration (up to 2 months). Run products incJude, in addition to melt, the humite-groupmineral chondrodite, and enstatite. The solidus was located between 1100 and I 150°C and the amount of melt producedincreases sharply at 1250°C. From this temperature liquids are compositionally zoned between the two mineralinterfaces and liquid immiscibility occurs at 1300°C and above.The variations of the chemical composition of the melts in space and time indicates that melting proceeds by theindividual dissolution of each of the reactants. In the case of the mica, this reaction is kinetically controlled by masstransport in the melt. In the case of quartz, surface reactions are dominant, as shown by the steady silica enrichmentof the melt at the quartz interface with increasing run duration. Surface reactions in quartz are slower than atomic diffusionin 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 uniqueliquid composition in the charge as long as both reactants are present. This state of disequilibrium persists until oneof the reactants is entirely consumed. The melt zonation provides the driving force for dissolution because it allowsconstant removal from the interface of the species liberated at the crystal interface. For minerals whose interface reactionsare rapid (such as the mica) dissolution is controlled by diffusion of species into the melt. This process may bethe cause of chemical disequilibrium between melt and residue, because dissolution rates can be faster than chemicalexchange between crystals and melt