Structure and elasticity of phlogopite under compression: Geophysical implications

We have used first principles simulation based on density functional theory to calculate the equation of state and full elastic constant tensor. Based on the generalized gradient approximation, the full single crystal elastic constant tensor with monoclinic symmetry shows significant anisotropy with the compressional elastic constants: c₁₁=181GPa, c₂₂=185GPa, c₃₃=62GPa, the shear elastic constants c₄₄=14GPa, c₅₅=20GPa, c₆₆=68Ga, and c₄₆=-6GPa; the off diagonal elastic constants c₁₂=48GPa, c₁₃=12GPa, c₂₃=12GPa, c₁₅=-16GPa, c₂₅=-5GPa and c₃₅=-1GPa at zero pressure. The elastic anisotropy of phlogopite is larger than most of the layered hydrous phases relevant in the subduction zone conditions. The shear anisotropy, AV_S for phlogopite is ~77% at zero pressure condition and although it decreases upon compression it remains relatively high compared to other hydrous phases relevant in the subduction zone settings. We also note that the shear elastic constants for phlogopite are relatively low. Phlogopite also has a high isotropic bulk V_P/V_S ratio ~2.0. However, the V_P/V_S ratio also exhibits significant anisotropy with values as low as 1.49. Thus, phlogopite bearing metasomatized mantle could readily explain unusual V_P/V_S ratio as observed from seismological studies from the mantle wedge regions of the subduction zone.