含水鉱物を含む超塩基性岩の1GPa, 900℃までの弾性波速度の測定

抄録

Compressional and shear wave velocities, V_p and V_s, in three ultra basic rocks with hydrous minerals were measured simultaneously to 900°C under 1GPa at closed system. Both V_p and V_s in all three rocks decreased rapidly with the dehydration reactions of hydrous minerals. The relation between the velocity drop of V_p, V_drop and the amount of extricated water, W (kg/m³) at 900°C after the dehydration reactions under 1GPa is represented in the following equation:
V_drop=0.008W(km/s)
W depends on the amount of hydrous minerals in the rocks, and therefore, V_drop 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 V_p drop in the low velocity layer to the velocity in the lower layer of the descending lithosphere is approximately 5%. 5% of V_drop 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.