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

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• 作者：Yoshikazu Fujimoto ; Yoshio Kono ; Takao Hirajima ; Kyuichi Kanagawa ; Masahiro Ishikawa ; Makoto Arima
• 关键词：P-wave velocity ; Blueschist ; Water ; Oceanic crust ; Subduction zone
• 刊名：Physics of the Earth and Planetary Interiors
• 出版年：2010
• 出版时间：November 2010
• 年：2010
• 卷：183
• 期：1-2
• 页码：219-228
• 全文大小：1730 K

文摘

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.