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

P-wave velocity (Vp) and the anisotropy of lawsonite and epidote blueschists were measured up to 1.0 GPa and 400 °C using the ultrasonic pulse transmission technique. The slowest Vp 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 Vp 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 Vp direction in epidote single crystal) and amphibole [0 0 1] axes (fastest Vp direction in amphibole single crystal) are preferentially oriented parallel to lineation to enhance Vp anisotropy of the epidote blueschist. In contrast, lawsonite [0 0 1] axes (fastest Vp 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 Vp anisotropy was observed in the lawsonite blueschist. Our experimental results, in conjunction with recent seismological observations, suggest that the Vp of the subducting oceanic crust at <50 km beneath NE and SW Japan is similar to those of blueschists (9–12% lower Vp than peridotite). In contrast, the Vp in the subducting oceanic crust markedly increases at deeper than ∼50 km depth beneath NE Japan, and such a slight low-velocity layer (5–8% slower Vp) at >∼50 km has been observed in several subducting slabs. However, the high Vp values at >∼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 ∼50 km depth in subducting oceanic crusts.