Seismic studies have identified various small-scale scatterers in the mid-lower mantle, with low- V S anomalies and complex depth variations between 700 and 1900 km. Understanding the formation mechanisms of these scatterers is crucial for understanding mantle dynamics and chemical evolution. Previous studies suggest that their formation is linked to the structural phase transition of SiO 2 from stishovite to post-stishovite in the subducted oceanic crusts, with variations in Al and H content potentially influencing the transition depth. However, earlier experiments on (Al, H)-bearing stishovite phase transition were conducted only at high pressures and 300 K, limiting our ability to quantify the relationship between variations in subducted oceanic crust composition and small-scale scatterers in the lower mantle. In this study, we investigate the phase transition of (Al, H)-bearing stishovite under simultaneously high pressure and temperature conditions. Our results reveal that the incorporation of 0.01 a.p.f.u Al into stishovite with H/Al ratio of ~1/3 lowers the transition pressure by 6.7(3) GPa. However, the Clapeyron slope of this transition is nearly unaffected by changes in the Al content and has a value of 12.2-12.5(3) MPa/K. According to our results, Al content variation ranging from 0 to 0.07 a.p.f.u in SiO 2 can reasonably explain the depth distribution from 800 to 1900 km of the seismic scatterers observed in the circum-Pacific region. These results provide critical experimental evidence for understanding the formation and corresponding dynamic processes of small-scale scatterers in the lower mantle.
Geophysical Research Letters