Trench parallel anisotropy and large delay times: Elasticity and anisotropy of antigorite at high pressures

Abstract

[1] Using theoretical methods, we calculate the full elastic constant tensor and equation of state of antigorite up to 10 GPa, a pressure range that encompasses its experimentally observed stability field. At ambient conditions, the elastic constant tensor reveals significantly large acoustic anisotropy (38% in V P ; 35% in V S ) compared to the dominant mantle phase olivine. The shear anisotropy is enhanced upon compression. Upon compression, the full elastic constant tensor reveals anomaly at r ∼ 2.69 gm cm ). Our results on elasticity and anisotropy at conditions relevant to the mantle wedge indicates that a 10-20 km layer of antigorite might account for the observed shear polarization anisotropy with fast polarization parallel to the trench and associated large delay times of 1-2 sec observed in some subduction zone settings.Citation: Mookherjee, M., and G. C. Capitani (2011), Trench parallel anisotropy and large delay times: Elasticity and anisotropy of antigorite at high pressures, Geophys. Res. Lett., 38, L09315,

Focused ion beam preparation and characterization of single-crystal samples for high-pressure experiments in the diamond-anvil cell

“…Here, Av p is defined as (v p,max -v p,min )/[(v p,max + v p,min )/2]. The result of our direct measurement of acoustic velocity anisotropy is supported by a recent computational study (Mookherjee and Capitani 2011). However, due to the small deviation of our sample platelet orientation from the (010) plane, the derived anisotropy likely represents a lower bound.…”

Equation of state and elasticity of the 3.65 Å phase: Implications for the X-discontinuity

“…These fluids dictate the onset of hydrous melting (Iwamori 1998(Iwamori , 2004(Iwamori , 2007Kawamoto 2006;Frost 2006) and may trigger deep earthquakes (Brudzinski et al 2007). Mantle hydration has also been invoked to explain various geophysical observations including the "inverted moho" in the mantle wedges (Bostock et al 2002;Bezacier et al 2010Bezacier et al , 2013Mookherjee and Capitani 2011) and the "low velocity layers" in subducting lithosphere (Abers 2005;Chantel et al 2012; Mookherjee and Bezacier 2012;Kim et al 2012Kim et al , 2013. In addition, unusually large delay times between the arrivals of the two shear waves have also been explained by presence of hydrous phases in certain subduction zone settings (Long and Silver 2008;Katayama et al 2009;Bezacier et al 2010;Jung 2011;Mookherjee and Capitani 2011).…”

Antigorite crystallographic preferred orientations in serpentinites from Japan

“…Bezacier et al, 2010;Boudier et al, 2010;Christensen, 2004;Jung, 2011;Katayama et al, 2009;Mookherjee and Capitani, 2011;Watanabe et al, 2011). Compared with Bezacier et al Table 3 Maximum and minimum P-wave velocities (in km/s), P-wave anisotropy (AVp) (%), maximum shear-wave splitting (dVs max) (in km/s) and maximum S-wave anisotropy (AVs max) (%) for different samples.…”

“…12). This anisotropy has been attributed to an antigorite serpentinite layer above the subducting oceanic plate (e.g., Jung, 2011;Katayama et al, 2009;Mookherjee and Capitani, 2011). Compared with experimentally deformed olivine aggregate (Katayama et al, 2009;Zhang and Karato, 1995) which would require a slab thickness of 200-120 km for a delay time of 2 s, experimentally deformed serpentinite suggests a thickness of only 10-20 km (Katayama et al, 2009).…”

“…Antigorite serpentinites with crystallographic preferred orientations (CPO) exhibit strong elastic anisotropy (e.g., Kern, 1993;Kern et al, 1997;Mookherjee and Capitani, 2011;Watanabe et al, 2007). The CPO is considered to cause shear-wave anisotropy, with trenchparallel fast directions observed in subduction zones (Jung, 2011;Katayama et al, 2009).…”