ELASTIC-WAVE VELOCITIES IN ISOCHEMICAL GRANULITE AND AMPHIBOLITE - ORIGIN OF A LOW-VELOCITY LAYER AT THE SLAB MANTLE WEDGE INTERFACE

Simultaneous measurements of compressional and shear wave velocities, Vp and Vs, in granulite and amphibolite with identical chemistry were carried out from room temperature to 850-degrees-C at 1 GPa. Both Vp a nd Vs in granulite and amphibolite decreased linearly with increasing temperature. The values of Vp and Vs at 700-degrees-C are 7.75 km/s an d 4.37 km/s for granulite, and 7.40 km/s and 4.21 km/s for amphibolite , respectively. 3-4 % lowering of elastic wave velocities in hydrous a mphibolite compared with anhydrous granulites may be consistent with s eismological observations indicating the presence of a low-velocity la yer at the slab/mantle wedge interface in subduction zones.