Elasticity of serpentine: first principles investigation

Serpentine is formed by reaction between peridotite and water which is released from hydrous mineral in subducting slab under pressure. Partially serpentinized peridotite may be a significant reservoir for water in the subducted cold slab and is considered to play an important role in subduction zone processes such as generation of arc magmatism. Precise determination of elastic properties of serpentine is essential for estimating the degree of serpentinization, and is important for investigating the transporting processes of water into deep Earth interior. Several studies on the degree of serpentinization have been reported so far based on limited experimental data on elasticity of serpentine (e.g. Bostock et al. 2002; Carlon and Miller 2003). Here we investigate by first principles calculation, the detailed structures and elastic properties of lizardite and antigorite which are lower and higher temperature polymorphs of serpentine up to 10 GPa at 2 GPa interval, and discuss the difference of compression mechanism and elasticity between these polymorphs. Our calculations are based on density functional theory within generalized gradient approximation for exchange correlation functional. We calculated the crystal structure and elasticity of antigorite m=16 polysome which contains 273 atoms in primitive cell (Capitani and Mellini 2006). We found the bulk modulus of antigorite and lizardite are almost same at ambient pressure, but they show significant difference at high pressure conditions. On the other hand, antigorite always has a few % larger shear modulus than lizardite presumably because of corrugated layer structure of antigorite. Combining the data from the present ab initio calculations, and seismological observations for the velocity and anisotropy structures of subduction zones, we analyzed the degree of serpentinization and possible water content in subducting slabs.