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Omphacite is a major mineral phase of eclogite, which provides the main driving force for the slab subduction into the Earth's interior. We have measured the single-crystal elastic moduli of omphacite at high pressures for the first time up to 18 GPa at ambient temperature using Brillouin spectroscopy. A least squares fit of the velocity-pressure data to the third-order finite strain equation of state yields K_S0' = 4.5 (3), G₀' = 1.6 (1) with ρ₀ = 3.34 (1) g/cm³, K_S0 = 123 (3) GPa, and G₀ = 74 (2) GPa. In addition, the synchrotron single-crystal X-ray diffraction data have beenmore » collected up to 18 GPa and 700 K. The fitting to Holland-Powell thermal-pressure equation of state yields K_T0' = 4.6 (5) and α₀ = 2.7 (8) × 10^-5 K^-1. Based on the obtained thermoelastic parameters of omphacite, the anisotropic seismic velocities of eclogite are modeled and compared with pyrolite between 200 and 500 km. The largest contrast between the eclogite and pyrolite in terms of seismic properties is observed between ~310 and 410 km.« less