The origin of low velocity layers in subduction zones: elasticity of hydrous phases at high pressures and temperatures

A 5-10 km thick low velocity layer (LVL) at the top of the subducting slab has been seismically detected in many subduction zone settings. The LVL may persist up to a depth of 250 km. The shear wave velocity of the LVL could be 10 % slower than the surrounding mantle. Based on thermal profile of the subducting slabs it is hard to relate the LVL with partial melting or pore fluids. We have undertaken experimental and theoretical studies to constrain the pressure and temperature dependence of elasticity of eclogitic rocks formed from hydrated oceanic crust. In particular, we focus on hydrous phases such as lawsonite [CaAl₂Si₂O₇(OH)₂.H₂O]. Our study shows that lawsonite has unusual shear modulus at high pressure and temperatures relevant at 250 km depths. Formation of hydrous phases such as lawsonite at the expense of garnet additionally lowers the seismic velocities.