Sound velocities of natural clinohumite at high pressures and implications for seismic velocities of subducted slabs in the upper mantle

Clinohumite is one of the major transformation products of serpentine at high pressures and an important H₂O and F carrier in the upper mantle at subduction zones. Sound velocity measurements on clinohumite under high P-T conditions are crucial for modeling slab structure, arc magmatism, seismicity, and deep H₂O cycling process. The compressional (P-) and shear (S-) wave velocities of a natural F-bearing clinohumite [Mg_8.17Ti_0.19Fe_0.01(SiO₄)₄O_0.38(OH)_1.17,F_0.74] were investigated using ultrasonic interferometry technique up to 14.6 GPa at room temperature in a multi-anvil apparatus. The variations of both the P- and S-wave velocities of clinohumite with pressure are the smallest when compared to the common anhydrous and other dense hydrous magnesium silicate (DHMS) phases in the upper mantle. Finite strain analysis of the acoustic velocities and density data yields the adiabatic bulk (K_S) and shear (G) moduli and their pressure derivatives: K_S0 = 121.0(5), G₀ = 76.5(2), K_S’ = 3.9(1), G' = 1.1(1). Furthermore, the density and seismic velocity profiles of subducted harzburgite assemblages with various degrees of hydration were calculated up to 14 GPa along an isotherm at 1073 K. Our results indicate that clinohumite has a larger effect on the density of peridotite than phase A and 23 Å phase under the same degree of hydration of peridotite in the upper mantle. Moreover, the V_P and V_S velocity contrasts between dry harzburgite and harzburgite with 2 wt% H₂O from clinohumite are about ∼1.6% and ∼ 1.0%.