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.     

MICROGRAVIMETRY FOR ENGINEERING APPLICATIONS*

Microgravimetric surveys for verifying bedrock soundness at the foundation of a nuclear power plant and for delineating a zone of small cavities and tracing grout emplacement at the foundation of a large cooling tower are described and used to illustrate an analysis of the method. It is pointed out that the difference between the usual large scale gravity surveys and microgravimetric surveys for engineering applications involves significantly more than a mere scaling down of grid spacings and precision tolerances. In addition to the smaller scale, the special physical environments for such microgravimetric surveys as well as strict timing and economy considerations and the demand for conclusive answers to specific questions require careful revisions of field procedures as well as data reduction and interpretation. Where properly applied, microgravimetric surveys—usually in conjunction with selective drilling—yield reliable subsurface information with a high resolution that is still limited by the gravimeter noise. Costly extensive drilling can thus be avoided. The method offers special advantages over other subsurface exploration methods in a wide variety of applications.