FUSION KINETICS, WATER-PRESSURE, WATER DIFFUSION AND ELECTRICAL-CONDUCTIVITY IN MELTING ROCK, INTERRELATED

Natural granite and gabbro samples, with dimensions of the order of 1 cm, were exposed up to 48 hours to water at a pressure of 2 kb and temperatures up to 1060 °C, in an internally heated pressure vessel. Spatial and temporal nonequilibrium melt distributions were quantitatively studied in thin sections of the quenched samples. The main observations, in granite at about 1020 °C, indicated that melting fronts were initiated at grain boundaries exposed to water and advanced at initially high rates subject to rate limitation by water diffusion through the deepening melt. The melting slowed down sharply with a slight undersaturation due to exhaustion of free water in the inter-granular voids.The experimental observations on melting rates and water diffusion, as a function of water concentration and temperature, are correlated with data derived from other studies. A new model is advanced for water diffusion in granitic magma. It is demonstrated that rate processes in silicate melts may be significantly accelerated by water in differing patterns. Water diffusion in granitic melt is rather insensitive to water concentration whereas melting kinetics and fluidity are strongly enhanced by water but significantly decoupled from each other, all in contradiction to standard kinetic theories. Melting kinetics is shown to be especially critical in physical experiments on melting rock, and some observations on electrical conductivity are accordingly re-interprete. © 1978 Oxford University Press.