Seismic velocities, anisotropy, and shear‐wave splitting of antigorite serpentinites and tectonic implications for subduction zones

Abstract

[1] Antigorite, the high-temperature (HT) form of serpentinite, is believed to play a critical role in various geological processes of subduction zones. We have measured P-and Swave velocities (V p and V s ), anisotropy and shear-wave splitting of 17 serpentinite samples containing >90% antigorite at pressures up to 650 MPa. The new results, combined with data for low-temperature (LT) lizardite and/or chrysolite, reveal distinct effects of LT and HT serpentinization on the seismic properties of mantle rocks. 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), the velocity ratio of antigorite serpentinites displays little dependence on pressure or temperature. Serpentine contents within subduction zones and forearc mantle wedges where temperature is >300 C should 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 < 3.67 km/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 , and 3.6-18.3% with an average value of 10.4% for V s ) is caused by dislocation creepinduced lattice-preferred orientation of antigorite. Three distinct patterns of seismic anisotropy correspond to three types of antigorite fabrics (S-, L-, and LS-tectonites) formed by three categories of strain geometry (i.e., coaxial flattening, coaxial constriction, and simple shear), respectively. Our results are thought to provide a new explanation for various anisotropic patterns of subduction systems observed worldwide.Citation: Ji, S., A. Li, Q. Wang, C. Long, H. Wang, D. Marcotte, and M. Salisbury (2013), Seismic velocities, anisotropy, and shear-wave splitting of antigorite serpentinites and tectonic implications for subduction zones,

Low‐Frequency Measurements of Seismic Moduli and Attenuation in Antigorite Serpentinite

“…Note that the temperature dependence of antigorite's bulk modulus can be directly calculated from that experimentally measured for shear modulus (see below) by taking a temperature-independent Poisson's ratio (an assumption which seems reasonable for antigorite; Christensen, 1996;Ji et al, 2013) and using conversions between elastic constants. Note that the temperature dependence of antigorite's bulk modulus can be directly calculated from that experimentally measured for shear modulus (see below) by taking a temperature-independent Poisson's ratio (an assumption which seems reasonable for antigorite; Christensen, 1996;Ji et al, 2013) and using conversions between elastic constants.…”

A new calibration of seismic velocities, anisotropy, fabrics, and elastic moduli of amphibole‐rich rocks

“…The amphibole‐rich rocks display an average A p value (10.5% at 200 MPa, 9.5% at 600 MPa) higher than that of A s (8.8% at 200 MPa, 8.5% at 600 MPa). These anisotropy values, which are comparable to those of mica‐rich schists and phyllites [ Kern and Wenk , ; Barruol et al ., ; Godfrey et al ., ] as well as serpentinites [ Kern et al ., ; Ji et al ., ], are much larger than those for other categories of igneous and metamorphic rocks such as granite, diorite, gabbro, pyroxenite, peridotite, marble, felsic gneiss, and mafic granulite [ Ji et al ., , p. 616]. Metamorphic and deformed rocks that are anisotropic to ultrasonic waves should be also anisotropic at the scale of active and passive seismic experiments (lower‐frequency waves) because metamorphic and deformation fabrics are often pervasive for tens to hundreds of kilometers [ Brocher and Christensen , ].…”

Lattice-preferred orientation of olivine found in diamond-bearing garnet peridotites in Finsch, South Africa and implications for seismic anisotropy

“…Trench-parallel seismic anisotropy has been observed in the mantle wedge above subducting slabs (Long, 2013;Long and Silver, 2008;Smith et al, 2001), as well as below subducting slabs at a deeper portion of the upper mantle (Long andSilver, 2008, 2009;Russo and Silver, 1994;Tian and Zhao, 2012). Proposed mechanisms for the source of this trench-parallel seismic anisotropy include waterinduced B-type LPO of olivine in the mantle wedge (Jung, 2009;Jung and Karato, 2001a;Karato et al, 2008;Katayama and Karato, 2006;Kneller et al, 2008;Mizukami et al, 2004), LPO of serpentine in serpentinite altered from peridotite (Ji et al, 2013;Jung, 2011;Katayama et al, 2009;Soda and Wenk, 2014;Watanabe et al, 2011), trench-parallel mantle flow due to slab roll back (Long andSilver, 2008, 2009;Russo and Silver, 1994), rapid toroidal flow around slab edge (Jadamec and Billen, 2010), pressure-induced B-type LPO of olivine due to slip transition at high pressure greater than P ¼ 3 GPa Ohuchi et al, 2011;Raterron et al, 2011), aligned faults by hydration in subducting oceanic plate (Faccenda et al, 2008) and an effective orthorhombic symmetry for the oceanic asthenosphere, which is translated to the depth beneath the subducting slab (Song and Kawakatsu, 2012). However, the origin of seismic anisotropy remains poorly understood.…”