Kinetic Controls on the Generation and Composition of Granitic Melts

As shown by Tuttle and Bowen (1958), the generation of granitic melts in nature entails thermally prograde melting reactions. Once minimum-melting assemblages lose grain contact, continued melting requires mineral dissolution and diffusion of major components through the melt. Hence knowledge of the systematic of diffusion in the granite system is important for interpreting major element compositions of crustal melts. Our recent experimental work on dissolution and diffusion in the H2O-saturated haplogranite system at 800ºC shows strong affinities among Al, H2O and alkalis in melt. Dissolution of excess Al increases with melt H2O concentration in a predictable manner. Alkalis diffuse in response to the creation and removal of chemical gradients in Al and H2O, in proportions related to the equilibrium melt ASI. Alkali diffusion mechanisms imply a near-instantaneous chemical communication and mass transfer throughout macroscopic melt reservoirs, such that Al/Na and ASI ratios represent time-equivalent markers of spatially connected melt reservoirs. Sodium migrates toward melt domains richer in H2O whereas K moves in the opposite direction, thus melt domains higher in H2O will show lower K*.

Water-saturated melting of leucogranitic protoliths produces liquids whose normative composition and kinetics of equilibration with restite depend directly on the degree of solidus temperature overstepping. Low degrees of overstepping (0-70ºC) produce homogeneous disequilibrium melts close to the mininum, due to stoichiometric melting and lack of re-crystallization of feldspars; equilibration timescales are 104-105 years. High degrees of overstepping (100-130ºC) produce disequilibrium, heterogeneous liquids with constant Ab/Or but variable Qtz/(Ab+Or); equilibration timescales are much shorter, ten-to-hundred of years, due to recrystallization of feldspars. Equilibration between melt and peraluminous minerals takes also ten-to-hundred of years. Lower diffusivities in H2O-undersaturated melts will increase these timescales. Predictable relationships among ASI, water content, and temperature permit calculation of melt residence times for granite melts in the presence of peraluminous minerals.