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Effect of phase transitions on compressional-wave velocities in the Earth’s mantle

Abstract

The velocities of seismic waves in the Earth are governed by the response of the constituent mineral assemblage to perturbations in pressure and stress. The effective bulk modulus is significantly lowered if the pressure of the seismic wave drives a volume-reducing phase transformation1,2. A comparison between the amount of time required by phase transitions to reach equilibrium and the sampling period thus becomes crucial in defining the softening and attenuation of compressional waves within such a two-phase zone3. These phenomena are difficult to assess experimentally, however, because data at conditions appropriate to the Earth’s deep interior are required. Here we present synchrotron-based experimental data that demonstrate softening of the bulk modulus within the two-phase loop of olivine–ringwoodite on a timescale of 100 s. If the amplitude of the pressure perturbation and the grain size are scaled to those expected in the Earth, the compressional-wave velocities within the discontinuities at 410, 520 and, possibly, 660 km are likely to be significantly lower than otherwise expected. The generalization of these observations to aluminium-controlled phase transitions raises the possibility of large velocity perturbations throughout the upper 1,000 km of the mantle.

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Figure 1: Calculated P-wave velocities ( VP) versus depth in the Earth using pyrolite composition and adiabatic gradient8.
Figure 2: Example of the olivine content versus time, from X-ray diffraction observations.
Figure 3: Stress versus strain for one cycle during sinusoidal loading with period of 1,000 s.
Figure 4: Sample stress–strain relationship at 1,350 K.

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Acknowledgements

We thank L. Wang and D. Lindsley for support for this project. We acknowledge support from the National Synchrotron Light Source and the Consortium for Materials Properties in Earth Sciences, and financial support from the US Department of Energy (contract number DE-AC02_98CH10886) and the US National Science Foundation (awards EAR-0711365, EAR652887 and EAR 01-35554). This is MPI publication number 466.

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Li, L., Weidner, D. Effect of phase transitions on compressional-wave velocities in the Earth’s mantle. Nature 454, 984–986 (2008). https://doi.org/10.1038/nature07230

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