Elasticity of antigorite at high pressures: implications for subduction zone anisotropy

Anisotropy is ubiquitous in most subduction zone, however the nature of anisotropy is diverse including both trench parallel and trench normal splitting and delay time ranging from 0 to greater than 2 sec. Mineralogy of the upper mantle is dominated by olivine. However, the elastic anisotropy for olivine is of the order of 22%. This might require around 100 km think olivine layer to produce the delay time of 1-2 sec as observed in certain subduction zones. Hydration of oceanic lithosphere and mantle wedge leads to the formation of serpentine (antigorite). Using theoretical methods, we calculate the equation of state and elasticity of antigorite at high-pressures. At ambient condition, the elastic constant tensor reveals significantly large acoustic anisotropy (47 % in VP; 40 % in VS). The shear anisotropy is enhanced upon compression. Based on the elasticity and anisotropy of antigorite combined with deformation induced fabrics, we propose a model to explain the trench parallel anisotropy and associated large delay times of 1-2 sec observed in certain subduction zone settings.