Effects of olivine fabric, melt-rock reaction, and hydration on the seismic properties of peridotites: Insight from the Luobusha ophiolite in the Tibetan Plateau

摘要

In order to constrain the effects of olivine fabric, melt-rock reaction, and hydration on the seismic properties and anisotropy of mantle rocks, we investigated serpentinized peridotites from the Luobusha ophiolite in the Indus-Tsangpo suture of the Tibetan Plateau. A-type and almost random olivine crystal-preferred orientations (CPO) occur in harzburgite and dunite samples, respectively. The dunite resulted from interactions of harzburgite with boninitic melt at similar to 800-970 degrees C, yielding pyroxene dissolution and olivine precipitation. The olivine neoblasts formed from the melt-rock reaction show no evidence of dislocation creep and developed almost random CPO. Hence, the melt-rock reaction reduced seismic anisotropy. Our results together with those from the literature indicate that A-, B-, C-, D-, and E-type CPOs of olivine generally induce V-p anisotropy patterns with Vp (X)> V-p(Y)> V-p(Z), V-p(Y)> V-p(X)> V-p(Z), V-p(Z)> V-p(X)> V-p(Y), V-p(X)> V-p(Y) approximate to V-p(Z), and V-p(X) > V-p(Z)> Vp(Y), respectively. The effect of serpentinization was calibrated by the comparison of seismic velocities and anisotropy measured up to 600 MPa with the values calculated from the CPO data. Although the low-temperature (LT, < 300 degrees C) serpentinization (lizardite and chrysotile) decreases V-p by similar to 6-10% and V-s by similar to 12%, it does not change the anisotropy pattern because the mesh-texture characterized by serpentine veins perpendicular to the principal structural directions (X, Y, and Z) reduces the velocities in these orthogonal directions to almost equal extent. Thus, the magnitude of seismic anisotropy alone cannot be used as an indicator of the degree of LT serpentinization in the mantle rocks. Furthermore, Birch's law is found to hold when peridotites undergo serpentinization.