Seismic and magnetic anisotropy of serpentinized ophiolite: Implications for oceanic spreading rate dependent anisotropy
Résumé
Compressional and shear wave anisotropy, shear wave birefringence, and the anisotropy of magnetic susceptibility were measured on a series of dunites sampled from the Dinardic–Hellenic ophiolites. The densities of these materials ranged from 3330 kg/m3 to 2620 kg/m3 and indicate degrees of serpentinization from 2.3% to 87.9%, respectively. Magnetic susceptibility increases and the compressional and shear wave velocities decrease in proportion to the degree of serpentinization as has been observed by other workers. In all cases the magnetic susceptibility tensor is described by an oblate spheroid whose minor axis is closely aligned to the pole of the foliation, but the magnetic anisotropy is not related to the degree of serpentinization. The compressional wave anisotropy ε monotonically decays from 12% for nearly pure olivine dunite to less than 2% for the most serpentinized sample; this observation strongly suggests that serpentinization progressively destroys the original anisotropy by consuming the preferentially aligned olivines and replacing them with randomly oriented serpentines. Typical serpentine mesh textures seen in microscopic thin section examinations support this suggestion. This loss of anisotropy with serpentinization may partly explain the apparent relationship between seismic compressional wave anisotropy of the oceanic lithosphere. The more complex geological structure of slow spreading ridges may admit more sea water via faults for deep circulation which increases serpentinization and consequence of which is decreased seismic anisotropy.