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 ∼ 2.69 gm cm -3. At < 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 3 ( 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. Copyright © 2011 by the American Geophysical Union.