VARIATION OF ELASTICITY IN WATER-SATURATED ROCKS NEAR THE MELTING TEMPERATURE OF ICE

Abstract

Compressional and shear wave velocities of water-saturated rhyolite, granite and diabase were measured as a function of temperature near the melting point of ice. Although such porous rocks as the Carleton rhyolite have a large velocity-hysteresis, the hysteresis in low porosity rocks, such as Troy granite, is negligible. Such low porosity rocks are suitable specimens for the simulation of the effects of partial melting. The change of effective bulk modulus of the rocks occurs only in a narrow range of temperature near the melting point of ice, while the change of effective shear modulus occurs gradually over a wider range of temperature. Except for the elastic anomaly, the variation of elasticity in the rocks can be explained by Walsh's equations with an assumed aspect ratio of the inclusions equal to 0.002. While no elastic anomalies are found in polycrystals, the elastic anomaly in water-saturated rocks is found near the melting temperature of ice. The anomaly is attributed to the existence of liquid on the boundary between rock and ice due to local stresses and anomalous melting of ice under pressure. We predict that an elastic anomaly will exist in any polyphase system near the melting temperature (T_m) of the inclusions whenever aT_m/aP<0. We speculate that the origin of the low velocity zone in the upper mantle may be an elastic anomaly of a polyphase system because the melting temperature of some silicatewater system is close to the geotherm and decreases with an increase of pressure.