Fusion Kinetics, Water Pressure, Water Diffusion and Electrical Conductivity in Melting Rock, Interrelated

Natural granite and gabbro samples, with dimensions of the orderof 1 cm, were exposed up to 48 hours to water at a pressureof 2 kb and temperatures up to 1060 °C, in an internallyheated pressure vessel. Spatial and temporal nonequilibriummelt distributions were quantitatively studied in thin sectionsof the quenched samples. The main observations, in granite atabout 1020 °C, indicated that melting fronts were initiatedat grain boundaries exposed to water and advanced at initiallyhigh rates subject to rate limitation by water diffusion throughthe deepening melt. The melting slowed down sharply with a slightundersaturation due to exhaustion of free water in the inter-granularvoids. The experimental observations on melting rates and water diffusion,as a function of water concentration and temperature, are correlatedwith data derived from other studies. A new model is advancedfor water diffusion in granitic magma. It is demonstrated thatrate processes in silicate melts may be significantly acceleratedby water in differing patterns. Water diffusion in graniticmelt is rather insensitive to water concentration whereas meltingkinetics and fluidity are strongly enhanced by water but significantlydecoupled from each other, all in contradiction to standardkinetic theories. Melting kinetics is shown to be especiallycritical in physical experiments on melting rock, and some observationson electrical conductivity are accordingly re-interprete.