Integrated Seismological and Petrological Investigations of the Mantle Transition Zone

Our knowledge of the transition zone comes primarily from two sources: High-pressure experiments that constrain the phase relations and elastic coefficients of likely mantle minerals as a function of pressure, temperature, and composition; and seismic observations of the depth and sharpness of the seismic discontinuities near 410 and 660 km depth, which mark the top and bottom of the transition zone, and the velocity structure of the transition zone. The combination of these two approaches has yielded the basic structure and composition of the mantle. Because of the sensitivity of the transition zone phase transformations to temperature and composition (including water), seismic observations of the transition zone discontinuities along with the velocity structure of the transition zone are diagnostic of the thermal and chemical states of the mantle and the associated dynamics.

In this project, we apply the recently developed finite-frequency seismic tomography techniques to the transition zone study. The new method takes into account of wavefront healing of realistic seismic waves. A more robust velocity model will allow us to correct for velocity heterogeneities more accurately, obtain more coherent seismic phases from the discontinuities and better estimates of the transition zone thickness and depth to the discontinuities. The seismic observations and their initial interpretations will help to guide high-pressure experiments designed to understand the mantle conditions of different tectonic regions.