Ultrasonic Measurement Of The Transition Zone's Seismic Velocities

The Earth's mantle transition zone extends from approximately 410 to 660 km depth, is a vast and inaccessible layer of the Earth. There are no direct samples from the transition zone, so everything we know about this region is inferred from the speed that seismic sound waves transit this region. By constraining the acoustic properties of potential mineral assemblages using experiments, Earth Scientists can infer its temperature, chemistry, structure and water content.

Additionally, seismic data can be used in an analogous way to medical ultrasound, to image lateral variations, which reveal that the seismic properties of the transition zone change subtly around the globe, which presumably reflect variations in temperature, mineralogy or composition. The transition zone is particularly important for understanding the large-scale dynamic and chemical evolution of planet Earth because (i) many subducting slabs stall there for millions of years before continuing descent into the lower mantle and (ii) the transition zone has a huge water capacity, potentially holding 10 times more water than the surface oceans.

If we could use seismic observations to directly image the temperature and composition of the transition zone we would take a gigantic step towards understanding the behaviour of Earth's deep interior.

Whilst seismology provides us with ever increasingly detailed views of the Earth's interior, the challenge that remains is to interpret this data in terms of the mineralogy, chemistry and temperature of the mantle and core. However, there is surprisingly little experimental data on the seismic properties of mantle minerals under the appropriate pressure and temperature conditions for the transition zone.

Even for olivine, the most common upper mantle mineral making up almost 60% of mantle rocks down to depths of 410 km, data are restricted to < 1300 K, whereas upper mantle temperatures range from 1500 to 2000 K. The situation for other mantle phases is far worse. In lieu of data it has been commonplace to use thermodynamic databases, with the assumption that the these can be safely extrapolated to transition zone conditions.

However, we have discovered that these databases are not reliable because do not even match the available data.

The paucity of experimental data at high pressure and temperature conditions is simply due to experimental constraints. In this proposal we will employ a new method to routinely measure seismic velocities with high-frequency ultrasonic waves at real transition zone conditions using university-based equipment. We will use this to measure compressional and shear wave velocities of the most significant upper mantle and transition zone minerals as a function of changing pressure, temperature, composition and water content. We believe this will be transformational for our understanding of the deep Earth.