Elastic wave velocities and Raman shift of MORB glass at high pressures

The elastic wave velocities of MORB glass were measured up to 18.7 GPa at 27 degrees C using an ultrasonic technique combined with in situ high-pressure X-ray diffraction and imaging. High-pressure Raman spectroscopy using a diamond anvil cell was also employed to study the local structure of the glass sample up to 20 GPa. Both the elastic wave velocities and Raman frequency at similar to 1600 cm(-1) show significant changes in their pressure dependences at similar to 10 GPa. The pressure derivatives of the P-wave and bulk sound velocities gradually increase with pressure above similar to 10 GPa, whereas the S-wave velocity is almost constant below 10 GPa, above which it increases significantly. The plot of apparent change in the Raman frequency shift versus pressure suggests that these notable changes in elastic properties may be attributed to the local structural changes in the MORB glass. Our observations demonstrate that both the density and adiabatic bulk modulus of the MORB glass increase continuously at pressures above similar to 10 GPa. These apparently conflicting tendencies can be reasonably explained if the zero-pressure density is increased continuously above similar to 10 GPa accompanied by structural modification. Since the fundamental structure of silicate glass is analogous to that of melt, the results are important and aid in understanding the elastic properties of silicate melt at high pressures.