Effect of Partial Melt on P and S Wave Velocities in Olivine-Basalt Aggragates

The presence of relatively small amounts of melt may have great effects on the elastic properties of mantle. Most of these works were achieved either by experiments at low pressure (commonly less than 1.0GPa) and high temperature or by numerical simulation, so it is essential to study the effect of partial melt on the elastic property in mantle rocks at simultaneous high pressures and temperatures. The compressional and shear wave velocities (V_P and V_S) for two synthetic aggregate samples of olivine plus 4% and 13% basalt (S1 and S2) have been measured up to 5.0GPa and 1530K(100-150km) using simultaneous ultrasonic interferometry in conjunction with in situ X-ray diffraction and X-radiographic imaging techniques. At 3.5GPa, V_P and V_S in the two samples showed an almost linear decrease as the temperature increasing, and above 1350K began to drop remarkably. At 1473K, V_P and V_S reduced about 2.1% in the sample with 13% basalt, three times more than those in the sample with 4% basalt (about 0.7%). The large increase of V_P/V_S ratios in the two samples at the temperatures above 1350K suggests that the partial melting is involved. Variations in P and S wave velocities and V_P/V_S ratios at above 1300K might be related to partial melting of the basalt in olivine samples. These results allow us to investigate the anomalous elastic behavior in the upper mantle, and provide important clues to reveal the relationship between mantle physical state and seismic observations.