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

Abstract

Simultaneous measurements of compressional and shear wave velocities (VP and VS) in two rocks with identical chemistry but with different mineral assemblages were carried out from room temperature to 850°C at 1 GPa. One of the rocks is granulite and the other is amphibolite formed from the granulite by addition of H2O. Both VP andVS in the granulite and amphibolite decreased linearly with increasing temperature, and there were obvious differences between the elastic wave velocities in the granulite and amphibolite. For example, the values of VP and VS at 700°C are, respectively, 7.75 km s-1 and 4.37 km s-1 for granulite and 7.40 km s-1 and 4.21 km s-1 for amphibolite.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 change (e.g., anhydrous peridotite + H2O -> amphibole peridotite).