Seismic velocities, anisotropy and shear-wave splitting of antigorite serpentinites and tectonic implications for subduction zones Shaocheng Ji, Awei Li, Qian Wang, Changxing Long, Hongcai Wang, Denis Marcotte, and Matthew Salisbury Journal of Geophysical Research: Solid Earth Volume 118, Issue 3, pages 1015–1037, 2013, DOI: 10.1002/jgrb.50110 Abstract Antigorite, the high temperature form of serpentinite, is believed to play a criticalrole in various geological processes of subduction zones. We have measured P- and S-wave velocities (V p and V s ), anisotropy and shear-wave splitting of 17 serpentinite samples containing 90% antigoriteat pressures up to 650 MPa. The new results, combined with data forlow temperature lizardite and/or chrysolite, reveal distinct effects of low and high temperature (LT and HT) serpentinization on the seismic properties of mantlerocks. At 600 MPa, V p =5.10 and 6.68 km/s, V s =2.32 and 3.67 km/s, and V p /V s =2.15 and 1.81 for pure LT and HT serpentinites, respectively. Above the crack-closure pressure (~150 MPa), thevelocity ratio of antigorite serpentinites displays little dependence onpressure or temperature. Serpentine contents within subduction zones and forearc mantle wedges where temperature is 300 °Cshould be at least twice that of previous estimates based on LT serpentinization. The presence of seismic anisotropy, high-pressure fluids or partial melt is also needed to interpret HT serpentinized mantle with V p 6.68 km/s, V s 3.67km/s and V p /V s 1.81. The intrinsic anisotropy of the serpentinites (3.8-16.9% with an average value of 10.5% for V p , and3.6-18.3% with an average value of 10.4% for V s ) is caused by dislocation creep-induced lattice-preferred orientation (LPO) of antigorite. Three distinct patterns of seismic anisotropy correspond to threetypes of antigorite fabrics (S-, L-, and LS-tectonites) formed by threecategories of strain geometry (i.e., coaxial flattening, coaxial constriction,and simple shear), respectively. Our resultsarethought to provide a new explanation for various anisotropic patterns ofsubduction systems observed worldwide. http://onlinelibrary.wiley.com/doi/10.1002/jgrb.50110/abstract
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