TY - JOUR
T1 - Elasticity of antigorite, seismic detection of serpentinites, and anisotropy in subduction zones
AU - Bezacier, Lucile
AU - Reynard, Bruno
AU - Bass, Jay D.
AU - Sanchez-Valle, Carmen
AU - Van de Moortèle, Bertrand
N1 - Funding Information:
This study was supported by ANR project SUBDEF grant no. ANR-08-BLAN-0192 to BR, CNRS–UIUC international exchange program, INSU program SEDIT, and by NSF grant EAR-0738871 to JDB. We thank Oxford Instruments for EBSD and in particular G. Trullenque, Gatan and Buehler societies for lending equipment necessary for the sample preparation. Electron microscopy in Lyon is supported by CLYM. Stéphane Guillot kindly donated the sample Cu12. Careful reviews by N. I. Christensen and an anonymous reviewer helped to improve the manuscript. We thank Robert D. van der Hilst for editorial handling.
PY - 2010/1/15
Y1 - 2010/1/15
N2 - 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 VP/VS 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. VP 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 VS 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 VP/VS ratio and shear wave 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 VP/VS ratios and extremely low velocities in the mantle wedge of subduction zones (down to about 6 and 3 km.s-1 for VP and VS, 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 (10-20 km) serpentinite bodies.
AB - 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 VP/VS 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. VP 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 VS 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 VP/VS ratio and shear wave 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 VP/VS ratios and extremely low velocities in the mantle wedge of subduction zones (down to about 6 and 3 km.s-1 for VP and VS, 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 (10-20 km) serpentinite bodies.
KW - anisotropy
KW - antigorite
KW - elasticity
KW - serpentine
KW - shear wave splitting
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U2 - 10.1016/j.epsl.2009.11.009
DO - 10.1016/j.epsl.2009.11.009
M3 - Article
AN - SCOPUS:72949109957
SN - 0012-821X
VL - 289
SP - 198
EP - 208
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
IS - 1-2
ER -