In-situ Acoustic Velocity Measurement on Mantle Phase Transitions

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Phase transitions in the mantle resulting from pressure and temperature changes with increasing depth are believed to be responsible for the velocity discontinuities observed in seismic studies. In this project, elasticity of candidate mantle phases undergoing phase transitions at P-T conditions relevant to the Earth's mantle are studied using an integrated ultrasonic interferometry and synchrotron X-radiation technique in the multi-anvil apparatus. These experiments allow for simultaneous measurements of lattice parameter (hence specific volume and density) and P and S wave velocities across the phase transitions of mantle phases up to 26 GPa and 1800K. The phase transitions to be investigated include olivine to wadsleyite, wadsleyite to ringwoodite, and the dissociation of ringwoodite perovskite plus magnesiowustite which are believed to be predominantly responsible for the seismic discontinuities at 410-, 520-, and 660-km depths. Of great significance is that, by directly measuring the velocities of multi-phase aggregates with pyrolitic compositions, robust velocity contrasts across these phase transitions can be derived at the physical and chemical conditions relevant to these discontinuities in the mantle. The uncertainties originating from the complexities in the calculation of physical properties for a hypothetically mixed aggregate, such as extrapolations using equation of state for individual phases, iron partitioning change and their effect on elasticity and phase equilibrium, can be eliminated. As a result, the comparison of the current results with seismic data will place a tight constraint on the candidate mantle compositions responsible for the observed magnitude and lateral variations at these discontinuities.