Elasticity of antigorite,seismic detection of serpentinites,and anisotropy in subduction zones |
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| Authors: | Lucile Bezacier Bruno Reynard Jay D Bass Carmen Sanchez-Valle Bertrand Van de Moortèle |
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| Institution: | 1. Université de Lyon, Laboratoire de Sciences de la Terre, CNRS, Ecole Normale Supérieure de Lyon, 46 Allée d''Italie, 69364 Lyon Cedex 07, France;2. Department of Geology, University of Illinois, Urbana, IL 61801, USA;3. Institute for Mineralogy and Petrology, ETH Zurich, CH-8092 Zurich, Switzerland;1. Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, Peking University and School of Earth and Space Sciences, Peking University, Beijing 100871, China;2. Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78712, USA;3. Center for High Pressure Science and Technology Advanced Research (HPSTAR), Shanghai 201900, China;4. Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany;5. HPCAT, Geophysical Laboratory, Carnegie Institution of Washington, Argonne, IL 60439, USA;6. GeoSoilEnviroCARS, The University of Chicago, Chicago, IL 60637, USA;7. Institute for Study of the Earth''s Interior, Okayama University, Misasa, Tottori 682-0193, Japan;8. Key Laboratory of High-temperature and High-pressure Study of the Earth''s Interior, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China;1. Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14853, USA;2. Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan;3. Earth-Life Science Institute, Ehime Satellite, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan;1. Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan;2. Photon Factory, High Energy Accelerator Research Organization, Tsukuba 305-0801, Japan;3. Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan;1. School of Earth and Space Sciences, Peking University, Beijing, 100871, China;2. Center for High Pressure Science and Technology Advanced Research, Beijing, 100094, China;3. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China;1. Earth Research Institute, University of California, Santa Barbara, CA, USA;2. Department of Earth Science, University of California, Santa Barbara, CA, USA;3. Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY, USA |
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| Abstract: | 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. |
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