Elastic P Wave Velocity and Attenuation in Dunite up to 4.0 GPa

Elastic P wave velocity (vP) and quality factor value (QP, on behalf of attenuation) of dunite are determined using ultrasonic pulse transmission and spectral ratio methods in a multi-anvil apparatus up to 4.0 GPa at room temperature. During the experimental pressure range (0.4~4.0 GPa), vP and QP increase about 11.8% (from 7.6 km/s to 8.5 km/s) and 393.3% (from 54 to 266.4). The experimental results show that vP and QP are pressure dependent and a greater rising rate at the lower pressures than that at higher pressures: the increase rate of QP at 0.4~2.4 GPa is 358.5%, while only 7.6% at 2.4~4.0 GPa. 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 rising (0.4~2.4 GPa), QP of dunite increases because the energy loss in the pores and between the surfaces of cracks and the mineral boundaries is reduced due to the porosity decreasing and the mineral boundaries contact more tightly. While at higher pressures (2.4~4.0 GPa), most microcracks in dunite have been closed and mineral grains are compacted very tightly, as a result the internal frictional energy loss due to the relative motion at the grain boundaries and across crack surfaces became less, which results in small change of elastic wave attenuation in dunite.