The low velocity layer in subduction zone: Structure and elasticity of glaucophane at high pressures

We investigated the structure, equation of state, and elasticity of glaucophane [Na2Mg3Al2Si8O22(OH)2], up to 9 GPa, which encompasses its experimentally observed stability field. We find that the pressure–volume results for glaucophane are well represented by a third order Birch–Murnaghan formulation, with K 0 = 81 GPa, K 0 ′ = 4.5 and V 0 = 899.4 Å3. The full elastic constant tensor reveals significantly larger stiffness along the (1 0 0) plane. The [1 0 0] direction is the relatively softer. This could be rationalized in terms of the stacking of the stiffer tetrahedral units along [0 1 0] and [0 0 1] directions within the crystal structure. Glaucophane is a dominant mineral constituent of blueschist facies rock, and has significantly lower velocities compared to garnet bearing eclogites. In addition, glaucophane is anisotropic and could account for the observed low velocity layer in the subducting slabs at depth range within the thermodynamic stability of glaucophane. At high-pressures, beyond stability of glaucophane, hydrous phase such as lawsonite could account for the observed low velocity layers in certain subduction zones.