Elasticity of antigorite, seismic detection of serpentinites, and anisotropy in subduction zones
Résumé
Serpentinization of the mantle wedge is an important process that influences the seismic and mechanical properties in subduction zones. Seismic detection of serpentines relies on the knowledge of elastic properties of serpentinites, which thus far has not been possible in the absence of single-crystal elastic properties of antigorite. The elastic constants of antigorite, the dominant serpentine at high-pressure in subduction zones, were measured using Brillouin spectroscopy under ambient conditions. In addition, antigorite lattice preferred orientations (LPO) were determined using an electron back-scattering diffraction (EBSD) technique. Isotropic aggregate velocities are significantly lower than those of peridotites to allow seismic detection of serpentinites from tomography. The isotropic V(P)/V(S) ratio is 1.76 in the Voigt-Reuss-Hill average, not very different from that of 1.73 in peridotite, but may vary between 1.70 and 1.86 between the Voigt and Reuss bonds. Antigorite and deformed serpentinites have a very high seismic anisotropy and remarkably low velocities along particular directions. V(P) varies between 8.9 km s(-1) and 5.6 km s(-1) (46% anisotropy), and 8.3 km s(-1) and 5.8 km s(-1) (37%), and V(S) between 5.1 km s(-1) and 2.5 km s(-1) (66%), and 4.7 km s(-1) and 2.9 km s(-1) (50%) for the single-crystal and aggregate, respectively. The V(P)/V(S) ratio and shearwave splitting also vary with orientation between 1.2 and 3.4. and 1.3 and 2.8 for the single-crystal and aggregate, respectively. Thus deformed serpentinites can present seismic velocities similar to peridotites for wave propagation parallel to the foliation or lower than crustal rocks for wave propagation perpendicular to the foliation. These properties can be used to detect serpentinite, quantify the amount of serpentinization, and to discuss relationships between seismic anisotropy and deformation in the mantle wedge. Regions of high V(P)/V(S) ratios and extremely low velocities in the mantle wedge of subduction zones (down to about 6 and 3 km.s(-1) for V(P) and V(S), respectively) are difficult to explain without strong preferred orientation of serpentine. Local variations of anisotropy may result from kilometer-scale folding of serpentinites. Shear wave splittings up to 1-1.5 s can be explained with moderately thick (1020 km) serpentinite bodies