Shear and longitudinal velocities were measured by the ultrasonic phase comparison method as a function of pressure to 8 kbar on synthetic glasses of basalt, andesite, rhyolite, and quartz composition and on natural obsidian. Velocities of most of the glasses decrease anomalously with pressure, but increasingly more-normal behavior occurs with decrease in SiO2 content. The pressure derivatives of rigidity and bulk modulus increase linearly, from -3.39 to -0.26 and from -5.91 to +2.09, respectively, with decrease in SiO2 content from 100 to 49%. The change from negative to positive in the pressure derivatives of both moduli and observed at Poisson's ratio of about 0.25 is consitent with the Smyth model for the anomalous elastic behavior of glass. If the temperature in the upper mantle is about 1500oC, tholeiitic basalt would be molten in accordance with the partial melt explanation for the low-velocity zone; at 1300oC and below, basalt would be in the glassy state, especially if more felsic than tholeiite. -Authors
Additional publication details
Elastic moduli of rock glasses under pressure to 8 kilobars and geophysical implications.