The subducted oceanic crusts are regarded as the major source of lower-mantle heterogeneity. However, the limited knowledge about the physical properties of major components in the subducted oceanic crusts prevents us from fully understanding the complex features of the related geophysical anomalies. Here, we comprehensively investigated the phase transition of (Al, H)-bearing stishovite at simultaneously high P-T conditions. These results contribute to our understanding on the complex depth distribution of the mid-lower mantle seismic scatterers.

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 SiO₂ 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 SiO₂ 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.

Reference URL: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024GL114146

Bibliographic Information

Unraveling the complex features of the seismic scatterers in the mid‐lower mantle through phase transition of (Al, H)‐bearing stishovite,
Yingxin Yu, Youyue Zhang, Luo Li, Xinyue Zhang, Denglei Wang, Zhu Mao, Ningyu Sun, Yanyao Zhang, Xinyang Li, Wancai Li, Sergio Speziale, Dongzhou Zhang, Jung-Fu Lin and Takashi Yoshino,
Geophysical Research Letters, 52(14), e2024GL114146,
doi: 10.1029/2024GL114146, 2025 (July 18)

Fundings

• Japan Society for the Promotion of Science (JSPS) KAKENHI. Grant Numbers: JP21H04996, JP24K17148
• National Natural Science Foundation of China. Grant Numbers: 42425202, 42272036, 42241117

Media

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  Complex depth distribution of mid-lower mantle seismic scatterers in the circum-Pacific region and the post-stishovite transition depths affected by the various Al and H content.

  (a)The phase boundary between stishovite and post-stishovite with the varying Al content. The contour represents the phase boundary of stishovite with different Al content and the H/Al ratio of ~1/3. Geotherm: from Katsura (2022). (b) The depth distribution histogram of the lower-mantle seismic scatterers in the circum-Pacific region (He & Zheng, 2018; Kaneshima, 2019; Li & Yuen, 2014; Niu, 2014; Niu et al., 2003; Vanacore et al., 2006; Yang & He, 2015; Yuan et al., 2021). (c) Map of the mid-lower mantle scatterers around the circum-Pacific region. The color of these spots represents the Al content in SiO₂ required to account for the observed depth of these seismic scatterers. The Al content in SiO₂ was estimated under the conditions of the H/Al ratio of ~1/3 and the normal mantle geotherm. The color bars in figures a and c are the same.

  credit : Ehime University
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