Melting of fluorphlogopite-plagioclase pairs at 1 atmosphere

International audienceMelting experiments on fluorphlogopite + plagioclase (An68) pairs have been performed at 1 atmosphere,as a function of temperature and run duration. Run products include, in addition to melt, spinel, forsterite and a newlyformed calcic plagioclase. The solidus was located between 1100 and 1150°C and the amount of melt producedincreases sharply at 1200°C. From this temperature, liquids are compositionally zoned between the two mineral interfaces,and the mica reacts out after sufficient time indicating the existence of a critical temperature of melting locatedbetween 1175 and 1200°C. These two temperature domains are also characterized by differences in plagioclasereaction textures, melt proportions and reaction rates.Textural and Sr isotopic studies of the plagioclases show that at all temperatures the first step of the melting reactionis characterized by congruent dissolution of the starting plagioclase followed by precipitation of the new plagioclase.At all temperatures the melt compositions are initially located on the intersection of the cotectic line and the startingplagioclase-fluorphlogopite tie-line. Below the critical temperature of melting these melts are metastable undercooledliquids. Above the critical temperature of melting there is an array of stable melt compositions located on the plagioclase-fluorphlogopite tie-line. In both cases the melt compositions evolve with time toward more albite-rich compositionsin conjunction with the new plagioclase precipitation. The system fluorphlogopite-plagioclase thus providesan example where liquid compositions vary with time although plagioclase-melt interface equilibrium is satisfied.This is due to the involvement of a solid solution in the reaction.Above the critical temperature of melting, the rate-controlling process for melting is chemical diffusion in the melt.Below the critical temperature of melting the rate-limiting step is the formation of the metastable undercooled melt.The sluggishness of the melting reaction in this temperature regime is explained by the difficulty of forming such amelt