Origin of Ultra-Low Velocity Zones at the Core-Mantle Boundary

Principal investigator: Dr S Rost

Sponsor: NERC

Value: £503,410.95 (Leeds: 351,379.97)

Dates: 01/08/2011 - 30/07/2014

Summary

Using new seismological observations together with mineral physics constraints, we will test hypotheses as to the origin of small scale (10 to 100 kms) heterogeneities called ultra-low velocity zones at the core mantle boundary (CMB). The CMB is Earth’s most import internal boundary. It is a thermal boundary layer(with a temperature contrast in excess of 1500°K), a chemical boundary (being the contact zone between the molten iron of the core and the solid silicate rock of the mantle) and a boundary between two very different convective regimes in the core (convection velocities of the order of 10-4 m/s) and mantle (V~10-10 m/s). The CMB controls the convective patterns of the core, producing Earth’s magnetic field. Heat flow through the CMB affects convection in the mantle, driving plate tectonics. The D" region up to 300 km above
the CMB is home to some of the most fascinating structures in the interior of the Earth, ranging from a sharp discontinuity to anisotropy and strong scattering. Perhaps the most intriguing seismic discovery of
the last 15 years regarding the lowermost mantle is that of intermittent thin patches of extremely reduced seismic velocities near the CMB dubbed ultra-low velocity zones (ULVZs). ULVZs influence many aspects of mantle dynamics and it has been speculated they are the roots of mantle plumes, areas of core material entering the mantle, remnants of a global magma ocean, an influence on the path of the magnetic poles during polar reversals, and chemically distinct exotic material. Nonetheless, the origin of ULVZs remains unsolved and fundamental questions such as partial melt vs. chemical heterogeneity as source for ULVZs are debated.

To test hypotheses on the origin of ULVZs we will use a combined mineral-physical and seismological approach. Each of the proposed ULVZ models will lead to specific velocity changes, VP-to-VS ratios and
density changes for ULVZs. Currently our knowledge about ULVZ structure and lower mantle material properties is not sufficient to differentiate between the models. We will determine the elastic properties of
perovskite and post-perovskite, as function of composition, pressure and temperature to understand the elastic properties of ULVZs. New seismic probes to ULVZs will be employed to determine ULVZ velocities and
density, and to specify their lateral extent and thickness. Identification of regions devoid of ULVZs is crucial to understand the connection between mantle flow and ULVZs. We will obtain a map of the CMB indicating ULVZ regions, their seismic velocities and densities. Using forward modeling based on the mineral-physics results we will be able to thoroughly test different models of origin for ULVZs.