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1 0 5 I H I G H L I G H T S 2 0 2 51 0 4 H I G H L I G H T S 2 0 2 5 I
PRINCIPAL PUBLICATION
Sound velocity measurements of γ-(Mg0.91Fe0.09)2SiO4 show that the ringwoodite to bridgmanite and ferropericlase phase transformation does not produce the seismically observed 660 km discontinuity, R. Huang et al., Earth Planetary Sci. Lett. 663, 119416 (2025); https:/doi.org/10.1016/j.epsl.2025.119416
REFERENCES
[1] W. Wang et al., Earth Planetary Sci. Lett. 554, 116626 (2021). [2] A.M. Dziewonski & D.L. Anderson, Phys. Earth Planet. Inter. 25, 297-356 (1981). [3] B.L. Kennett et al., Geophys. J. Int. 122, 108-124 (1995). [4] J. Brown & T. Shankland, Geophys. J. Int. 66, 579-596 (1981).
Fig. 87: Comparison of (a) P-wave (VP) and (b) S-wave (VS) velocities, and (c) density of pyrolite and harzburgite compositions with two seismological models, PREM [2] and AK135 [3]. The red and blue solid curves represent the calculated results for
pyrolite and harzburgite, respectively, along the geotherm of Brown and Shankland [4], with the shaded areas indicating an estimated uncertainty of ~1%. The velocities and densities of relevant high-pressure phases are also shown for comparison. wd: wadsleyite; rw: ringwoodite; gt: majorite garnet; ak: akimotoite; cpv: Ca perovskite; brg: bridgmanite; fp: ferropericlase.
Seismic velocities were determined by combining travel-time measurements with direct imaging of sample lengths. The results featured small uncertainties (< 0.1 GPa in pressure, < 10 K in temperature, and < 0.03 km.s-1 in velocity), and only minimal extrapolation (200-300 K) was required to model mantle conditions.
The results show that both P-wave and S-wave velocities for anhydrous ringwoodite are higher than previously predicted, though the S-wave increase is smaller. These refined data yield a more accurate global equation of state for ringwoodite. When integrated into pyrolite mantle models, the results reproduce a sharp discontinuity in S-wave velocity consistent with seismic observations but fail to
generate a similarly sharp P-wave discontinuity at 660 km depth (Figure 87). This indicates that the transformation of ringwoodite alone cannot account for the seismic features of the lower MTZ.
The findings suggest that a homogeneous pyrolitic mantle composition cannot explain the full suite of seismic discontinuities observed in the MTZ. Either additional chemically distinct components must be present – implying lateral heterogeneity – or the elastic properties of other mantle phases require re-evaluation under comparable conditions. This study provides a benchmark for refining seismic interpretations and underscores the role of synchrotron-based in situ methods in probing the deep Earth.
