Tracking silica in Earth's upper mantle using new sound velocity data for coesite to 5.8 GPa and 1073 K

The compressional and shear wave velocities for coesite have been measured simultaneously up to 5.8 GPa and 1073 K by ultrasonic interferometry for the first time. The shear wave velocity decreases with pressure along all isotherms. The resulting contrasts between coesite and stishovite reach ~34% and ~45% for P and S wave velocities, respectively, and ~64% and ~75% for their impedance at mantle conditions. The large velocity and impedance contrasts across coesite‐stishovite transition imply that to generate the velocity and impedance contrasts observed at the X‐discontinuity, only a small amount of silica would be required. The velocity jump dependences on silica, d(lnVP)/d(SiO2) = 0.38 (wt %)−1 and d(lnVS)/d(SiO2) = 0.52 (wt %)−1, are utilized to place constraints on the amount of silica in the upper mantle and provide a geophysical approach to track mantle eclogite materials and ancient subducted oceanic slabs. Key Points Decrease of shear wave velocity with pressure along all isotherms for coesite Coesite‐stishovite transition is a plausible cause for the X‐discontinuity Tracking silica in the upper mantle