P-WAVE VELOCITIES OF MAIN UPPER-MANTLE MINERALS AT HIGH-TEMPERATURE AND HIGH-PRESSURE AND ITS GEOLOGICAL IMPLICATION

At 0-5.0 GPa and room temperature to 1400 degrees C, the P-wave veloci ties of olivine, clinopyroxene and orthopyroxene, which are three comm on minerals from upper mantle, are measured cinder both simulated ocea nic and continental geothermal gradients. The experimental results ind icate that the P-wave velocities of these minerals increase with depth under both geothermal gradients. This implicates that pressure is mor e important than temperature in deep earth in controlling the P-wave v elocities of mantle minerals, but the increase of temperature has grea ter effect on P-wave velocities of main mantle minerals at greater dep th than at smaller depth. At low pressure, the measured P-wave velocit ies of mantle minerals are smaller then their true values due to fract uring, compaction process and recrystallization of mineral powder. The true P-wave velocities of mantle minerals can be obtained at lower pr essure by the extrapolation of measured velocities at a high pressure. At higher depth, all these three minerals show local decreases of vel ocities at their V-p-D curves, which may be related to phase transform ation, softening effect and melting effect of minerals. The depth rang e of the local velocity decreases of these minerals is nearly consiste nt with that of low velocity zone (LVZ) in North China and on explain the origin of LVZ in this area.