Anomalous elastic properties of coesite at high pressure and implications for the upper mantle X-discontinuity

Compressional and shear wave velocities of coesite have been measured using ultrasonic interferometry in a multi-anvil apparatus up to 12.6 GPa at room temperature for the first time. While the P wave velocity increases continuously with pressure, the S wave exhibits an anomalous softening and the velocity decreases continuously with pressure. Finite strain analysis of the data yielded K S 0 = 103.6 ( 4 )   GPa , G 0 = 61.6 ( 2 )   GPa and K 0 ′ = 2.9 ( 1 ) , G 0 ′ = 0.3 ( 1 ) for the bulk and shear moduli and their pressure derivatives, respectively. The anomalous elastic behavior of coesite results in large velocity and impedance contrasts across the coesite–stishovite transition, reaching ∼39% and ∼48% for P and S wave velocity contrasts, and ∼70% and 78% for P and S wave impedance contrasts, respectively, at pressure ∼8 GPa, with P and S wave velocity perturbations showing no apparent dependence on depths (i.e., d ln ⁡ V ( P or S ) / d h ∼ 0 ) within 8–12 GPa. These unusually large contrasts and depth independent characteristics render the transition between the two silica polymorphs one of the most plausible candidates for the cause of the seismically observed X-discontinuity. The current P and S wave velocity perturbation dependences on the SiO2 content, d ( ln ⁡ V P ) / d ( SiO 2 ) ∼ 0.43   (wt%) − 1 and d ( ln ⁡ V S ) / d ( SiO 2 ) ∼ 0.60   (wt%) − 1 , can serve as a geophysical probe to track ancient subducted eclogite materials to gain insights on the geodynamics of the mantle.