Measurement of elastic wave velocities in granulite and amphibolite having identical H 2O-free bulk compositions up to 850°C at 1 GPa

Abstract

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°C at 1 GPa. One of the rocks is granulite and the other is amphibolite formed from the granulite by addition of H 2O. Both V P and V S 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 V P and V S 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 + H 2O → amphibolite) the 5–6% velocity drop could indeed be explained by an appropriate phase change (e.g., anhydrous peridotite + H 2O → amphibole peridotite).