MEASUREMENT OF ELASTIC-WAVE VELOCITIES IN GRANULITE AND AMPHIBOLITE HAVING IDENTICAL H2O-FREE BULK COMPOSITIONS UP TO 850-DEGREES-C AT 1 GPA

Simultaneous measurements of compressional and shear wave velocities ( V-P and V-S) in two rocks with identical chemistry but with different mineral assemblages were carried out from room temperature to 850 degr ees C at 1 GPa. One of the rocks is granulite and the other is amphibo lite formed from the granulite by addition of H2O. Both V-P and V-S in the granulite and amphibolite decreased linearly with increasing temp erature, and there were obvious differences between the elastic wave v elocities in the granulite and amphibolite. For example, the values of V-P and V-S at 700 degrees C are, respectively, 7.75 km s(-1) and 4.3 7 km s(-1) for granulite and 7.40 km s(-1) and 4.21 km s(-1) for amphi bolite. The low-velocity layer lies between the descending lithosphere and the overlying asthenosphere beneath the Japanese island arc, and the ratio of velocity drop for the low-velocity layer compared to the surroundings is approximately 5-6%. If the observed velocity drop were caused by a phase change (i.e., granulite + H2O --> amphibolite) the 5-6% velocity drop could indeed be explained by an appropriate phase c hange (e.g., anhydrous peridotite + H2O --> amphibole peridotite).