Lab Geophysics – the acoustic velocity of mantle minerals

Theme: Solid Earth Dynamics

Primary Supervisor:

Andrew Thomson

Earth Sciences, UCL

Secondary Supervisor:

John Brodholt

Earth Sciences, UCL

Project Description:

The structure and composition of the Earth’s deep interior cannot be directly measured, but instead must be inferred from seismic velocity data generated by Earthquakes. However, this approach relies entirely on the availability of high-quality data for the seismic properties of all potential deep Earth minerals. Moreover, these must be known as a function of pressure, temperature and composition. Despite their fundamental importance for understanding the basic properties of the deep Earth, there are very few measurements of seismic velocities of minerals at the pressure and temperature conditions of the deep Earth, and so we have to rely on large extrapolations of low pressure and temperature data or on theoretical estimates.

Recently we have developed a method for measuring seismic velocities (both Vp and Vs) of samples at high PT conditions. In these experiments, sound pulses are passed into a cylindrical sample at high PT conditions in a multi-anvil cell using a piezoelectric transducer crystal. By measuring the time delay between echoes returning from the top and bottom of the sample, the velocities can be determined very precisely. These measurements have been successfully performed at conditions up to ~ 2000 °C and 24 GPa (700 km depth).

This Ph.D. project aims to use ultrasonic interferometry in the multi anvil to determine the acoustic velocity (Vp and Vs) of several important mantle mineral phases as a function of pressure and temperature. The experiments will be performed in the high-pressure laboratories at UCL and at synchrotron beamlines in Japan, USA and Europe.

Policy Impact of Research:

The chemistry and behaviour of the inside of planet Earth is shrouded in mystery. The results from this Ph.D. will provide insight into the composition deep within our planet, and help to understand the geodynamic behaviour of Earth.

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