P-wave velocity and anisotropy of lawsonite and epidote blueschists: Constraints on water transportation along subducting oceanic crust

P-wave velocity (V-p) and the anisotropy of lawsonite and epidote blueschists were measured up to 1.0 GPa and 400 degrees C using the ultrasonic pulse transmission technique. The slowest V-p in the direction normal to foliation is similar between lawsonite and epidote blueschists (7.0-7.2 km/s at 1.0 GPa and room temperature), while the fastest V-p in the direction parallel to lineation markedly differs between lawsonite blueschists (7.4-7.6 km/s at 1.0 GPa and room temperature) and epidote blueschist (7.9 km/s at 1.0 GPa and room temperature). Crystallographic orientation measurements for main constituent minerals revealed that both epidote [0 1 0] axes (fastest V-p direction in epidote single crystal) and amphibole [0 0 1] axes (fastest V-p direction in amphibole single crystal) are preferentially oriented parallel to lineation to enhance V-p anisotropy of the epidote blueschist. In contrast, lawsonite [0 0 1] axes (fastest V-p direction in lawsonite single crystal) are oriented subnormal to foliation, whereas amphibole [0 0 1] axes are oriented subparallel to lineation, so that relatively weak V-p anisotropy was observed in the lawsonite blueschist. Our experimental results, in conjunction with recent seismological observations, suggest that the V-p of the subducting oceanic crust at <50 km beneath NE and SW Japan is similar to those of blueschists (9-12% lower V-p than peridotite). In contrast, the V-p in the subducting oceanic crust markedly increases at deeper than similar to 50 km depth beneath NE Japan, and such a slight low-velocity layer (5-8% slower V-p) at >similar to 50 km has been observed in several subducting slabs. However, the high V-p values at >similar to 50 km depth are difficult to be explained by blueschists. This indicates that the blueschist would be at least partially transformed to hydrous mineral-bearing eclogite at similar to 50 km depth in subducting oceanic crusts. (C) 2010 Elsevier B.V. All rights reserved.