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Seismic Velocities, Anisotropy and Elastic Properties of Xiuyan Jade and its Geological Implications
Author(s): 
Pages: 12-26
Year: Issue:  1
Journal: Geotectonica et Metallogenia

Keyword:  Xiuyan jadeantigorite serpentiniteseismic wave velocitiesanisotropyshear wave splittingsubduction zonesTibetan Plateau;
Abstract: Antigorite is a main contributor to the formation of anomalously low seismic velocities and high anisotropy in subducting oceanic slabs, forearc mantle wedges and subcontinental mantle shear zones. Here we report new results on P- and S-wave velocities and anisotropy in 8 antigorite serpentinite samples (“Xiuyan jade” from Liaoning, China), measured at pressures up to 600 MPa. The results combined with previous experimental data allow us to distinguish the effects of low and high temperature (LT and HT) serpentinization on seismic velocities in the upper mantle. Serpentine contents in hydrous subducting slabs, mantle wedges and subcontinental mantle shear zones where temperature is>300 ℃should be at least twice as large as previous estimates based on LT serpentinization. The intrinsic Vp and Vs anisotropy of serpentinites, formed by plastic flow-induced lattice-preferred orientation (LPO) of antigorite, can be as high as 21%and 24%, respectively. Our results provide a new explanation for various anisotropy patterns observed in subduction systems worldwide. For a steeply subducting slab, antigorite is most likely deformed by nearly coaxial flattening and thus the trench-parallel seismic velocity is much faster than the trench-normal velocity. For a shallowly subducting slab, however, antigorite is most likely deformed by simple shear. Within the horizontal plane, the trench-normal velocity can be smaller, equal to, or larger than the trench-parallel velocity, depending on the dip angle of subduction. The geophysical characteristics of the Tibetan Plateau such as strong heterogeneities in seismic velocities and attenuation, shear wave splitting and electric conductivity can be equally explained by the presence of serpentinized shear zones within the subcontinental mantle.
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