Experimental studies of electrical conductivities and P-wave velocities of gabbro at high pressures and high temperatures

The P-wave velocities and electrical conductivities of gabbro were measured using ultrasonic transmission method and impedance spectroscopy from room temperature to 1100degreesC at 1-2 GPa, and the factors controlling the P-wave velocity and the microscopic conductance mechanisms of the rock were analyzed. The experimental results show that the P-wave velocities of gabbro drop abruptly at temperatures of 800-850degreesC and under pressures of 1-2 GPa due to the occurrence of grain boundary phases and dehydration melting; however, the electrical conductivities and electronic conduction mechanisms have not changed obviously at temperatures of 800-850degreesC. At temperatures Below 680degreesC, only one impedance arc (1) corresponding to grain interior conduction occurs at frequencies between 12 Hz and 10(5) Hz, the second arc (11) corresponding to grain boundary conduction occurs at temperatures above 680degreesC. The total conductivity of this rock is dominated by the grain interior conductivity as the occurrence of grain boundary conduction has a small effect on the total conductivity. The laboratory-measured velocities are consistent with the average P-wave velocity observations of lower crust and upper mantle. The conductivity values correspond well with the gabbroite composition of the lower crust and upper mantle; however, they are about 1-2 orders of magnitude lower than MT data from the high conductive layers. The experiments confirm that the dehydration of hydrous minerals can induce the partial melting, and the low seismic velocity zones might be correlated with the high conductive layers if partial melting occurs.