CPO-induced seismic anisotropy in UHP eclogites

Ultrahigh-pressure (UHP) eclogites often show strong plastic deformation and anisotropy of seismic properties. We report in this paper the seismic velocity and anisotropy of eclogite calculated from the crystallographic preferred orientations (CPOs) of constituent minerals (garnet, omphacite, quartz and rutile) and single crystal elastic properties. We also compared the calculated results with the measured results in similar eclogites. Our results suggest that (1) Except that garnet is a seismically quasi-isotropic mineral, omphacite, quartz, coesite and rutile all have strong seismic anisotropies (AVp = 23.0% − 40.9%, Max. AVs = 18.5% − 47.1%). They are the major sources for anisotropy in eclogite. The average seismic velocities are fast in garnet and rutile, moderate in omphacite and coesite, and slow in quartz. (2) The deformed eclogites have the maximum Vp (8.33–8.75 km/s) approximately parallel to foliation and lineation, the minimum Vp (8.25–8.62 km/s) approximately normal to foliation and lineation and the Vp anisotropies of 1.0–1.7%. Their Vs are 4.93–4.97 km/s. The corresponding maximum anisotropies (0.73%–1.78%) of Vs are at 45° to both foliation and lineation and the minimum anisotropies at positions normal to lineation on the foliation plane. The Vs₁ polarization planes are approximately parallel to foliation. The mean Vp and Vs of eclogite under UHP peak metamorphism conditions (P = 3−5 GPa, T = 900−1100°C) are estimated to be 3.4%–7.2% and 6.3%–12.1% higher than those at ambient pressure and temperature conditions, respectively. (3) Omphacite component dominates the anisotropy of eclogite while garnet component reduces the anisotropy and increases the seismic velocities. Quartz component has a small effect on the anisotropy but reduces the seismic velocities of eclogite. The effect of rutile component is negligible on seismic properties of eclogite due to its trivial volume fraction. (4) The increase of volume fraction of omphacite in eclogite will reduce the seismic velocities and increase the anisotropy. Omphacitite has seismic velocities reduced by 6%–8% and anisotropies increased to 3%–4% compared to those of garnetite. Our results suggest that the seismic properties calculated with single crystal elastic properties and CPOs are equivalent to those measured in laboratory. Moreover, it provides insights into the mineral physical interpretations of eclogite seismic properties. Supported by the National Natural Science Foundation of China (Grant Nos. 40502021 and 40521001) and the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. TRT0441) and US National Science Foundation (GRant No. EAR#0003631)