Sound velocities of OH-clinohumite and implications for seismic velocities of hydrated subducting slab

Abstract Subducting slabs play a crucial role in Earth’s water cycle. Hydrous minerals are important water carriers from the surface to the deep mantle in subducting slabs, and their physical properties are crucial to understanding the water cycle. Clinohumite [Mg9Si4O16(OH,F)2] is one of the main transformation products of serpentine under high pressure. As a member of the dense hydrous magnesium silicates (DHMSs), it is considered as a potential medium for water and other volatile components (e.g., fluorine) in subducting slabs to enter Earth’s deep interior. In this study, we measured the compressional (P) and shear (S) wave velocities of OH-clinohumite (Mg9Si4O18H2) at simultaneous pressure and temperature conditions up to 10 GPa and 873 K. Elastic properties were also obtained, yielding Ks0 = 114.4 (5) GPa, G0 = 75.2 (6) GPa, KS′ = 4.0 (1), G′ = 1.1 (1), ∂KS/∂T = −0.024 (1) GPa/K, and ∂G/∂T = −0.017 (2) GPa/K. Using these data, we calculated the density and wave velocity variations of harzburgite with different water contents under cold-slab upper-mantle conditions. We also discuss the effect of clinohumite with different XOH [OH/(F + OH) in molar] on the seismic wave velocities of cold or hot subducting slabs, and find that the wave velocity anomaly caused by clinohumite gradually decreases with increasing XOH. Seismic stratigraphic imaging across different subducting slabs reveals variations in velocity anomalies linked to clinohumite with differing hydroxyl (XOH) concentrations, allowing for the estimation of water fugacity within the subducting slab.