Mid-mantle anisotropy: Elasticity of aluminous phases in subducted MORB

Abstract

[1] Aluminous phase with the calcium ferrite and calcium titanate structure constitutes around 20% by volume of the subducted mid ocean ridge basalt (MORB) at lower mantle depths. Using first principle simulations, we calculate the equation of state and elasticity of NaAlSiO4 (NaCF) and MgAl2O4 (MgCF and MgCT) up to >150 GPa, encompassing the full range over which NaCF, MgCF and MgCT has been observed experimentally. We calculate the isotropically averaged elastic wave velocities and the anisotropy from our single crystal elastic constants. The elasticity of these phases is sensitive to the chemistry. The bulk modulus decreases with MgAlNa−1Si−1 substitution with a ∂K0/∂x ∼ −15 GPa, whereas the shear modulus stiffens with a ∂G0/∂x ∼ 10 GPa. At lower mantle conditions, the temperature derivative of bulk, ∂K/∂T and shear ∂G/∂T modulus are −0.006 and −0.013 GPa K−1, respectively. The chemistry is likely to have significant influence on the elasticity of these phases. Slab penetrating the lower mantle often develops significant anisotropy. The full elastic constant tensor of these aluminous phases reveal significant anisotropy and are likely candidates to account for the large delay times observed in the Tonga-Kermadec subduction zones.