Mantle Discontinuities

An important issue in seismology is the global and regional characteristics of velocity discontinuities in the Earth’s mantle. Some of these represent known phase transitions in mantle minerals. The existence and nature of such discontinuities is very important for our understanding of the internal dynamics of the Earth (such as mixing between the upper and lower mantle) and provides the input needed by mineral physicists to connect seismic discontinuities with mineralogical phase changes.

We have made the first global map of the Lehmann discontinuity at 220 km depth, and interpreted the observations as most probably being caused by a change in deformation mechanism. We also discovered that the mid-transition zone discontinuity at 520 km depth is ‘split’ in many regions and provided the first observations of the 660 km discontinuity using PP precursors. Previous studies had interpreted these discontinuities in terms of phase transitions in olivine. Our work, on the other hand, shows that additional phase transitions in garnet are needed to explain complex features seen in the seismic data. These results fundamentally change our ideas of how the mantle works and suggest that mantle mixing may vary regionally rather than being a globally unified process and that the Earth’s transition zone is more compositionally heterogeneous than previously thought.

Our aim is to build a global data set of many different data types that can be exploited to study mantle discontinuities and compared with new mineral physical results. During the last few years, we extended my original SS and PP precursor data set to receiver functions. More recently, we have been working on short period P’P’ precursors, which give puzzling results quite different from long period PP precursors, suggesting that the mineralogical properties of transition zone discontinuities may be frequency dependent. At the moment, we are focussing on two regional studies using receiver functions, one in Iceland to image the mantle plume and the other in Europe to study the regional subduction.

Mantle Discontinuities
Examples of how seismic data can be used to study mantle discontinuities. Shown are traces for North America and Indonesia and a global overview of places where there are robust reflections from the Lehmann discontinuity. The dashed lines in the stacks represent the 95% confidence levels and the grey areas denote robust reflections. The Lehmann discontinuity at 220 km depth is present as a robust reflector below both Indonesia and North America. The 520 km discontinuity is split below Indonesia, but shows one peak under North America. There are also strong signals from lower mantle reflectors at ± 1100 km depth; these lie in a region in which recent tomographic models contain exceptionally strong high velocity abnomalies, (probably) associated with material subducted in the Indonesian subduction zone.


Key publications