Unraveling the Complex Depth Distribution of the Seismic Scatterers in the Mid-Lower Mantle through Phase Transition of (Al, H)-Bearing Stishovite

Seismological observation has revealed the ubiquitous presence of small-scale seismic scatterers with significant shear velocity reduction in the mid-lower mantle. In particular, more than 70% of seismic scatters were observed at depths of 700-1300 km, while 20% of them are distributed at 1300-1900 km depth. Only less than 10% of the seismic scatterers are detected at depths exceeding 1900 km. The phase transition from stishovite with rutile-type structure to post-stishovite with CaCl₂-type structure of SiO₂ in the subducted oceanic crusts was regarded as the main reason for these seismic scatterers. However, this phase transition for pure SiO₂ is expected to occur at ~1800 km along the mantle geotherm, which is deeper than most observed seismic scatterers. Although the incorporation of Al and H into stishovite can effectively reduce the phase transition pressure, the combined effect of Al and H contents and temperature on the phase transition pressure of stishovite lacks necessary experimental constraints. In this study, we used Raman spectroscopy and single-crystal X-ray diffraction to provide comprehensive understanding on how the variation in chemical composition and temperature affect the post-stishovite transition pressure. According to our results, Al content variation ranging from 0 to 0.07 a.p.f.u with H/Al ratio of 1/3 in SiO₂ can reasonably explain the depth distribution from 800 to 1900 km of the seismic scatterers observed at the circum-Pacific region. These results deepen our understanding on the complex features of mid-lower mantle seismic scatterers and corresponding dynamic processes.