Elasticity of Single‐Crystal Clinohumite at High Pressures and Temperatures: Implication for the H₂O and F Circulation in the Earth's Mantle

In this study, we have determined the single‐crystal elasticity of clinohumite [Mg 8.85 Ti 0.19 Si 3.93 O 16 (OH 1.11 F 0.89 )] using Brillouin measurement up to 21 GPa at 300 K and 1 bar at 750 K, respectively. The elasticity of clinohumite was determined to be K S0 = 126.2(3) GPa, G 0 = 79.7(2) GPa with pressure derivatives K S ′ = 4.2(1), G ′ = 1.3(1), pressure derivatives ∂ K S /∂ T = −0.024(1) GPa/K, and ∂ G /∂ T = −0.011(1) GPa/K). We comprehensively examined the effects of varying H 2 O, fluorine content and thermal states, on the velocity and density structures of the subducted harzburgite layer. Assuming a typical H 2 O content of 2 wt.% within harzburgite, our modeling has shown that hydrous harzburgite with clinohumite as the decomposition product of serpentine along a hot slab geotherm even has the V P and V S 0.4–0.8(6)% greater than it dry counterpart at 250–380 km depth. Yet in the top transition zone, the addition of H 2 O and F can effectively lower the sound velocities and density. The F‐bearing hydrous harzburgite has the V P and V S 1.1(5)–1.3(3)% lower than its dry counterpart, and only 0.6(5)% and 2.3(5)% greater than the pyrolitic mantle. Along cold slab geotherm, phase A will replace clinohumite as the dominant hydrous phase in the harzburgite, the V P and V S are 4.8(5)–5.3(3)% and 5.9(5)–6.0(3)% greater than the pyrolitic mantle in the upper mantle. In the top transition zone, the difference is approximately 3% in V P and 5% in V S . Our results provide crucial experimental evidence for future assessments of the seismic signals of subducted slabs with different hydrous minerals and thermal states.