Effect of Phase Transitions on P-Wave Velocities in the Earth’s Mantle

Li Li and Donald J. Weidner, Mineral Physics Institute of Stony Brook University

The velocities of seismic waves 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 transformation. A comparison between the amount of time required by phase transitions to reach equilibrium and the sampling period thus becomes crucial in order to define the softening and attenuation of P waves within a two-phase zone. Such phenomena are difficult to assess experimentally since data at earth conditions are required. Here we show synchrotron based experimental data that demonstrate softening of the bulk modulus within the two-phase loop of olivine-ringwoodite at a time scale of 100 seconds. Scaling the amplitude of the pressure perturbation and grain size to those expected in the Earth, the P wave velocities within the discontinuities at 410, 520, and possibly 660 km are likely significantly lower than otherwise expected. The generalization of these observations to Al controlled phase transitions open the possibility of large velocity perturbations throughout the upper 1000 km of the mantle.

Stress vs. strain for one cycle during a sinusoidal loading at period of 1000 seconds. For (a) the temperature is 1650K and both olivine and ringwoodite coexist with approximately equal amounts as demonstrated by X-ray diffraction. Resulting Q is about 5. For (b) the temperature is 1500K and X-ray diffraction indicates that ringwoodite dominates the sample. The Q is indistinguishable from infinity. The area inside the hysteresis loop is the energy loss during one cycle. We conclude that the phase transition is responsible for the energy dissipation.

This research was partially supported by COMPRES, EAR 10-43050

Li, L. and D.J. Weidner (2008) Effect of phase transitions on bulk dispersion and attenuation: implications for the Earth. Nature. 454: p. 984-986.

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