SOUND VELOCITIES OF MGO TO VERY HIGH COMPRESSION

A new, experimentally simple method for obtaining sound velocities accurately from spectroscopic data taken at high pressures in the diamond anvil cell is presented. The transverse and longitudinal acoustic modes in MgO measured to over 200 kbar in the sideband fluorescence of Cr3+ in MgO directly yield the shear and compressional sound velocities with a resolution high enough to derive the geophysically important parameters: (partial derivative In rho/partial derivative In upsilon(s))T = 0.88(8) + 2.4(7) x In(rho/rho0) and (In rho/partial derivative In upsilon(p))T = 0.73(5) +/- 2.4(4) x ln(rho/rho0). These values and their high pressure trends are nearly equal to the seismically derived values for the lower mantle. These increases in partial derivative ln rho/partial derivative In upsilon and the Strong decrease in the thermal expansion coefficient with pressure suggest lower mantle lateral temperature variations derived from seismic velocity anomalies of 400-600 K. The pressure derivatives for the elastic moduli are obtained as follows: (d K(S)/d P), = 4.08 +/- 0.09, (d2K(S)/d P2)T = -0.0054 +/- 0.0016 kbar-1, (dG/dP), = 2.58 +/- 0.08, (d2G/dP2)T = -0.0034 +/- 0.0010 kbar-1, in excellent agreement with previous ultrasonic results to 32 kbar.