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

Abstract

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 ρ ∼ 2.69 gm cm −3 . At ρ < 2.69 gm cm −3 , we find that the pressure‐volume results for antigorite are well represented by a third order Birch‐Murnaghan formulation, with K 0 = 52 GPa, K 0 ′ = 8.5 and V 0 = 3037 Å 3 ( ρ 0 ∼ 2.48 gm cm −3 ). 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.