1989 年 42 巻 2 号 p. 171-181
Compressional and shear wave velocities, Vp and Vs, in three ultra basic rocks with hydrous minerals were measured simultaneously to 900°C under 1GPa at closed system. Both Vp and Vs in all three rocks decreased rapidly with the dehydration reactions of hydrous minerals. The relation between the velocity drop of Vp, Vdrop and the amount of extricated water, W (kg/m3) at 900°C after the dehydration reactions under 1GPa is represented in the following equation:
W depends on the amount of hydrous minerals in the rocks, and therefore, Vdrop can be estimated from the amount of hydrous minerals in the rocks.
The low velocity layer intervenes between the descending lithosphere and the overlying asthenosphere beneath Japanese Island Arcs, and that the ratio of Vp drop in the low velocity layer to the velocity in the lower layer of the descending lithosphere is approximately 5%. 5% of Vdrop could be explained by 10-20% in volumetric ratio of antigoritic serpentinite in the descending slab if it is assumed that the hydrous minerals are antigorites.