Enhanced Visibility of Subduction Slabs by the Formation of Dense Hydrous Phase A

Phase A (Mg7Si2O8(OH)6) is one of the important dense hydrous magnesium silicates in subducting slab, because it forms after the breakdown of antigorite serpentine and could be the dominant hydrous phase in the upper‐mantle deep slab for the water transportation into deep Earth. In this study, the compressional (P) and shear (S) wave velocities of phase A were measured at simultaneous pressure and temperature conditions up to 11 GPa and 1073 K. Combined with elastic properties of olivine and pyroxenes, we calculate the hydration effect on the velocities of harzburgite lithology in cold subduction zones throughout the depth ranges where phase A is thermodynamically stable. Our calculations suggest that the hydration increases both P‐ and S‐wave velocities of harzburgite; at ∼5 wt% hydration, its seismic detectability is enhanced by 1%–1.5% in velocity contrasts relative to its anhydrous counterpart. Plain Language Summary Oceanic plate reacts with sea water and forms several hydrous minerals, including antigorite. When cold oceanic plate went down into deep Earth, which has high pressure and high temperature, antigorite transforms into several high‐pressure hydrous minerals. Among those, phase A, an experimentally synthesized hydrous mineral, is believed to be the dominant hydrous phase. A technique called ultrasonic interferometry was used in the laboratory to simulate the propagation of seismic wave in the small sample. We used this technique to measure the speed of sound of phase A at conditions related to the upper mantle. Combined with data of other minerals, we can calculate the wave speed of hydrous harzburgite layer in the oceanic plate and compare the results with those seismic researches. We found that hydrous harzburgite with 5 weight percent of water has higher seismic wave speed than the dry one, by about 1%–1.5%. Key Points Direct measurement of P‐ and S‐wave velocities of phase A at mantle pressures and temperatures Formation of phase A increases both VP and VS of cold subduction slabs but decreases the VP/VS ratio Hydration of harzburgite increases the velocity contrasts between slabs and ambient mantle