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

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

Nov 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010pepi..183..219f&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 183, Issue 1-2, p. 219-228.

Physics

Scientific paper

P-wave velocity ( V p ) 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 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 ˜50 km depth beneath NE Japan, and such a slight low-velocity layer (5-8% slower V p ) at >˜50 km has been observed in several subducting slabs. However, the high V p 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.

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