SEISMIC-WAVES VELOCITIES AND ANISOTROPY IN SERPENTINIZED PERIDOTITES FROM XIGAZE OPHIOLITE - ABUNDANCE OF SERPENTINE IN SLOW-SPREADING RIDGE

The effect of serpentinization on seismic wave velocity and anisotropy has been analysed in 6 peridotite samples of the Xigaze ophiolite, ha ving harzburgitic composition and a degree of serpentinization ranging from 3% to 70%. We found: i) P- and S- wave velocities are linearly c orrelated with serpentine content; ii) anisotropy of P- and S-waves de creases with increasing serpentinization (while in fresh peridotites a nisotropy is of the order of 6%, for 70% of serpentinization it drops to 1 % for P-waves, and to 2%, for S-waves). Comparison of published l aboratory velocity data in oceanic lithologies, like gabbros and doler ites, with serpentinized peridotite data shows the difficulty of disti nguishing between gabbros and peridotites having a percentage of serpe ntine between 10 and 40% by seismic waves velocities only. This compar ison suggests that seismic velocity data are consistent with the exist ence of partialy serpentinized peridotites (up to 40% of serpentinizat ion) in the oceanic crust formed at slow-spreading ridges in accordanc e with geological observation.