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 [Mg8.85Ti0.19Si3.93O16(OH1.11F0.89)] 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 partial derivative K-S/partial derivative T = -0.024(1) GPa/K, and partial derivative G/partial derivative T = -0.011(1) GPa/K). We comprehensively examined the effects of varying H2O, fluorine content and thermal states, on the velocity and density structures of the subducted harzburgite layer. Assuming a typical H2O 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 H2O 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.