Elastic P-wave Velocity And Dynamic Characteristics Of Some Rocks At High Pressures And High Temperatures

The study on the elastic property of rocks at high pressures and high temperatures (HPT) plays an important role in researching the earth interior. Though there has been much progress in the measurement of elastic wave velocity, the experimental pressures and temperatures are still not high enough(<1.0GPa, <900℃). The knowledge about the elastic wave dynamic characteristics such as waveform, amplitude, frequency and attenuation (Qp, quality value) is very limited. Therefore, the elastic wave velocity and dynamics on crustal and mantle rocks at much higher PT should be further studied.The development of the study on the elastic wave velocity and dynamics at HPT is reviewed. The compressional wave velocity(Vp) and dynamic characteristics including amplitude, energy, frequency and attenuation in some crust and mantle rocks(amphibolite, diorite, dunite, pyroxenite, serpentinized pyroxenite and serpentinite) are studied by means of the ultrasonic pulse transmission method, the ultrasonic pulse and echo-impulse method, and the spectral ratio method at the pressure from 0.4GPa to 4.0GPa up to 1240℃ in the laboratory. Some experimental products of different samples are obtained at the constant pressure of 1.0GPa and different key temperatures. The changes of the mineral and chemical contents and the fabric in the product interior are carefully observed under an optical microscope and with an electron microprobe analyzer. The reason of changes of the elastic P-wave velocity and dynamics is discussed in detail by combination of their experimental results and the p roduct i nterior c hanges. Thee xperimental r esults p rovide n ot o nly strong theory basis for understanding the interaction of the earth deep materials and studying the structure and evolution of the lithosphere, but also scientific evidence for researching the formation of low velocity zones, the evolution of the rocks in earthquake source regions, and earthquake precursors.Vp in samples increases with the pressure increasing, and shows a greater rising rate at the low pressure about 0.4-2.0GPa than that at about 2.0-4.0GPa. At the lower pressures, the pores with low aspect ratio in rocks are easy to close, and almost all of them would close with the pressure increasing. As a result, the samples show the internal inherent elasticity of the whole constituent minerals themselves and the increasing rate of Vp turns lower at the higher pressures. As the pressure rises, the absolute values of the wave crest and trough, energy and quality value of the first arriving P-waveform passing through the dunite and the pyroxenite increase, because the energy loss between the surfaces of cracks and the mineral boundaries is reduced due to the porosity decreasing and the mineral boundaries contact more tightly at higher pressures. Though the absolute value of the wave trough and QP of the first arriving P-waveform passing through the amphibolite increase, the wave crest appreciably decreases and the energy and the amplitude maximum first increase then decrease, which is possibly due to the amphibolite crashing during the loading. The main frequency (fm) drift of the first arriving P-waveform passing through the dunite,pyroxenite and amphibolite is not evident.At the constant pressure, the Vp in different rocks varies differently as the temperature increasing: the Vp of the dunite and the pyroxenite decreases linearly with the temperature increasing up to about 1240℃; the Vp of the amphibolite and the diorite decreases linearly from room temperature to 600℃ or 800℃, then rapidly drops at higher temperatures; the Vp in the serpentinized pyroxenite shows a linear decrease from room temperature to about 500℃, then an abrupt decrease, a rapid lift and again a remarkable drop as the temperature increasing. Commonly, the thermal expansion of rocks and minerals results in the linear reduction of Vp in rocks at lower temperatures, while at higher temperatures the dehydration, partial-melting, and reaction rims give rise to a sharp drop of the Vp. However, the Vp incre...