The Experiment Research On The Seismic Wave Velocity And Anisotropy Of Serpentinite

Antigorite is a main contributor to the formation of anomalously low seismic velocities and high anisotropy in subducting slabs and forearc mantle wedges. Here we report new results on P-wave velocities and anisotropy in 17 antigorite serpentinite samples, measured at pressures up to 600 MPa. The results combined with previous data allow us to distinguish the effects of low and high temperature (LT and HT) serpentinization on seismic velocities in the upper mantle. Serpentine contents in hydrous slabs and mantle wedges where temperature is>300 ℃ should be twice as large as previous estimates based on LT serpentinization. The intrinsic anisotropy of serpentinite, formed by plastic flow-induced lattice-preferred orientation (LPO) of antigorite, ranges from 3.8% to 16.9% with an average of 10.5%. An important finding is that antigorite serpentinite displays three patterns of seismic anisotropy [Vp(X)≈Vp(Y)＞＞Vp(Z), Vp(X)＞＞Vp(Y)≈Vp(Z), and Vp(X)>Vp(Y)>Vp(Z)]. These correspond respectively to three types of antigorite LPO (S-, L-, and LS-tectonites) formed by three categories of strain geometry (coaxial flattening, coaxial constriction, and simple shear). Our results provide a new explanation for various anisotropy patterns observed in subduction systems worldwide. For a steeply subducting slab, antigorite is most likely deformed by nearly coaxial flattening and thus the trench-parallel seismic velocity is much faster than the trench-normal velocity. For a shallowly subducting slab, however, antigorite is most likely deformed by simple shear. Within the horizontal plane, the trench-normal velocity can be smaller, equal to, or larger than the trench-parallel velocity, depending on the subducting dip angle.