Anomalous elastic properties of coesite at high pressure and implications for the upper mantle X-discontinuity
Résumé
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 KS0 = 103.6 (4) GPa, G0 = 61.6 (2) GPa and K0‧ = 2.9 (1), G0‧ = 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., dln V (P or S) / dh ∼ 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 VP) / d (SiO2) ∼ 0.43 (wt%)-1 and d (ln VS) / d (SiO2) ∼ 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.