Elastic Wave Velocity of Brucite and Its Implications for Water Cycling in Subduction Zones

Brucite is one of the important constituent minerals of saturated water peridotite in subduction zones, whose volume fraction reaches up to 15 vol%. Therefore, investigating the elastic wave velocities of brucite under high pressure is crucial for understanding the composition, velocity structure, and deep water cycling of hydrated peridotites in subduction zones. In this study, dense polycrystalline brucite was synthesized from Mg(OH)2 reagent under 4 GPa and 523 K for 2 hours. The elastic wave velocities and moduli of brucite were measured up to 14 GPa using ultrasonic interferometry. The results demonstrate that the elastic wave velocities and moduli of brucite increase with increasing pressure. By the result of seismic tomography and mineral assemblage models, we constrain the water content in the low-velocity anomaly regions of the mantle wedge using the Voigt-Reuss-Hill (VRH) model. Our estimations show the water content ranges from 3–10 wt% in the mantle wedge above the subducting slab at depths of 20–40 km and 1–3 wt% within the subducting slab within the subducting slab at depths of 60–80 km beneath northeastern Japan.