Structure and elasticity of serpentine at high-pressure

Serpentines occur in the subduction zone settings, both along the slab and within the mantle wedge, they are candidates for transporting water in to the deep earth. Their presence is manifested by serpentine mud volcanoes, high electrical conductivities, magnetic and seismic anomalies. Using theoretical methods, we predict a pressure induced structural transformations in serpentine. The transformations are related to the behavior of the silicate framework and misfit between octahedral and tetrahedral layers. As the structure is compressed, the octahedral layer and tetrahedral layers are compressed at different rates. At 7 GPa, the misfit between the layers vanishes. This causes non-linear pressure dependence of tetrahedral rotational angle. This is also manifested by the onset of anomalous pressure dependence of the elastic constants c(11), C-33, C-12, C-13. Beyond 7 GPa, the misfit between the layers grows again reaching extremum at 22 GPa. This is also manifested by discontinuity in average Si-O bond length., volume of tetrahedron and re-orientation of hydroxyl vector. The symmetry of the crystal-structure however, remains unaffected. Evidence of pressure-induced hydrogen bonding is absent in serpentine, as evident from reduction of O-H bond length upon compression. Results of compression for the low-pressure regime (P< 7 GPa) is well represented by a fourth order Birch-Murnaghan finite strain expression with K-0=79 GPa. K-0(')=12 and K-0(n)=-2, where K is the bulk modulus, prime indicates pressure derivatives, and O refers to zero pressure. Our best estimates of K-0, K-0(') and the Gruneisen parameter, gamma at 300 K and zero pressure based on our results are: 61 GPa, 17, and 0.77, respectively. At low pressures, serpentine structure is anisotropic with c(11)similar to 2.4xc(33).The pressure derivative of elastic constants (partial derivative c(ij)/partial derivative P) are such, that around 22 GPa c(11)similar to c(33). An elastic instability (c(66)<0) at somewhat higher pressures (>50 GPa) is also noted. The elastic constant tensor reveals large acoustic anisotropy (41% in V-P) and seismic wave velocities that are significantly higher than those inferred from experiments on serpentinites. (C) 2008 Elsevier B.V. All rights reserved.