Tracking Silica in the Earth’s Subduction Zone and Upper Mantle

Abstract:

The X-discontinuity (~300 km) in the upper mantle has been revealed under some continental or oceanic region by a number of seismic studies, at which depth the P and S wave velocities increase by about 2%. One possible cause for this discontinuity is the coesite-stishovite phase transition. In this study, we conducted ultrasonic interferometry measurements on polycrystalline coesite and stishovite up to 12.6 GPa at ambient temperature and 14GPa 1073K, respectively. While the P wave velocities of coesite continuously increase with pressure, the S wave velocities exhibit a monotonic decrease to the peak pressure of the current experiment followed by a reversible recovery upon release of pressure. As a result, within the pressure range of 8-12 GPa (corresponding to ~250-350 km depths), the velocity contrasts between coesite and stishovite reach as high as ~38% for P wave and 48%-50% for S wave together with impedance contrasts of 71-69% and ~78% for P and S waves, respectively, the highest among all known phase transitions in mantle minerals. With such extreme contrasts, the coesite-stishovite phase transition in the MORB composition with 4-10wt% of SiO₂ is sufficient to generate velocity and impedance contrasts comparable to those reported for the X-discontinuity. The current data, together with the seismic X-discontinuity, may provide a geophysical approach to track the ancient subducted oceanic slabs, and place constraints on the amount of silica in the upper mantle.