Assistant Professor ZHANG Youyue of the Geodynamics Research Center (GRC), in collaboration with Professor MAO Zhu and Student YU Yingxin from the University of Science and Technology of China (USTC), has investigated the origin of mysterious small-scale seismic scatterers located in the mid–lower mantle (~700–1,900 km depth), which are characterized by a shear velocity reduction of 2%–12%.
The authors focused on stishovite, a high-pressure polymorph of silica (SiO2) that forms during the subduction of oceanic crust. Under specific high-pressure and high-temperature conditions, stishovite can undergo a phase transition into a denser structure known as post-stishovite. This transformation is accompanied by significant changes in the mineral’s physical properties.
Using high-pressure, high-temperature in-situ DAC experiments on four different compositions of (Al, H)-bearing stishovite high-quality single crystal synthesized in large-volume multi-anvil apparatus, the authors demonstrated that Al and H incorporation into stishovite significantly lowers the transition pressure at 300 K but minimally affects the Clapeyron slope. These phase changes produce strong contrasts in density and elasticity, which are capable of scattering seismic waves—providing a plausible explanation for the seismic scatterers observed at depth.
The study suggests that phase transitions in subducted (Al, H)-bearing stishovite offer a compelling mechanism for the formation of mid–lower mantle scatterers. It also emphasizes the profound impact that minor elements in mantle minerals such as aluminum and hydrogen can have on Earth’s deep seismic structure.
This study enhances our understanding of deep Earth processes and provides a strong link between subducted crustal materials, mineral phase transitions, and the fine-scale seismic heterogeneities observed in the mantle.
Title: Unraveling the Complex Features of the Seismic Scatterers in the Mid–Lower Mantle Through Phase Transition of (Al, H)‑Bearing Stishovite
DOI: 10.1029/2024GL114146