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Imaging the Earth's Core Mantle Boundary Structure

Dr Sebastian Rost (SEE), Dr Andy Nowacki (SEE), Dr Tarje Nissen-Meyer (Oxford), Dr Michael Thorne (Utah)

Contact email: s.rost@leeds.ac.uk

Background

The Earth’s core-mantle boundary (CMB) separates the iron alloy of the core from the silicate rocks of the mantle. It represents the boundary of the different convection cells of the core and the mantle and controls the heat flow out of the core, driving mantle convection and plate tectonics. Over the last three decades seismologists have discovered a wide variety of structures at the CMB that contradict the picture of a simple and sharp contact between iron and silicates (Figure 1). These discoveries span a wide range of scales from global anisotropy and discontinuities, to low velocity degree-two structures (called Large Low Shear Velocity Provinces, LLSVPs), and Ultra-Low Velocity Zones (ULVZs) with scales of a few ten to a few hundred kilometres but large reduction (on the order of more than 10%) in velocities.

This project aims to improve our imaging of Ultra-Low Velocity Zones (ULVZs) in Earth’s deep mantle and to use a multi-disciplinary approach to develop models of ULVZ origin, composition and evolution.

ULVZs are among the most enigmatic structures in the Earth’s deep interior. Since their discovery in the 1990s, seismologists have developed a variety of probes sensitive to ULVZ structure, which typically depend on detailed analysis of seismic waveforms. The seismic observations have led to a variety of different models for the origin of ULVZs including ULVZs as the remnants of a global magma ocean, ULVZs as piles of subducted material (e.g. of banded iron formations or crustal material), ULVZs as zones of partially molten material, ULVZs as zones of strong iron enrichment either primordial or through core-mantle interaction or ULVZs as zones of exotic, perhaps primordial, material (for a summary of ULVZ detections and origin hypotheses see McNamara et al. [2010]). ULVZs have been related to mantle plumes [Rost et al., 2006], large igneous provinces [McNamara et al., 2010], and zones of flux from core to mantle, and have been proposed as reservoirs for a wide variety of primordial material to explain geochemical data.

The wide variety of proposed origins for ULVZs and their apparent importance in our understanding of the dynamics and evolution of the Earth’s deep interior indicates that a better understanding of these features is essential for our understanding of the Earth System.

This project will aim to improve our imaging of ULVZs using a combination of new seismic data, novel waveform characterization techniques (e.g. Viterby Sparse Spike Deconvolution in conjunction with Markov Chain Monte Carlo Modeling) and improved 3D modelling of the seismic wavefield [Leng et al., 2016]. Using the improved images and material properties for ULVZs better ULVZ models using information from mineral-physics and geodynamics will be developed. The project will enable us to identify potential families of ULVZs that share a common tectonic location or common origin and to develop and test hypotheses for their generation.

The project will focus on the analysis of core-reflected seismic energy (PcP, ScP, ScS), which will allow the highest resolution images of seismic structure but with limited CMB coverage. We will overcome the geographical limitations for the sampling by exploiting new datasets from the global proliferation of high-quality broadband seismic stations around the globe.  Using these new datasets will allow us to sample many regions so far not probed for ULVZ structure (Figure 2).

The project will answer the following questions in a multi-disciplinary approach:

  1. Can ULVZ locations be correlated with certain tectonic regions, e.g. subduction zones or the edges of LLSVPs?
  2. Are ULVZs ubiquitous and is the apparently patchy discovery of ULVZs due to issues with seismic resolution?
  3. Can ULVZs be explained through partial melting or as solid-state variations of the mantle material?
  4. Have all ULVZs the same origin or are there different families with different seismic properties?
  5. Are ULVZs old or are they transient features of mantle structure and how are they stabilised in the convecting mantle?

Figure 1: Deep Earth structures and observations. Surface features (upper panel) typically related to deep Earth structure and observed deep Earth structures (lower panel). Seismology has resolved a multitude of structures in the deep mantle including LLSVPs and ULVZs, anisotropy, scattering and discontinuities. Accepted models for many of these structures are often missing. Figure from Garnero et al. [2016].

Figure 2: Global distribution of ULVZ based on seismic studies. Blue areas in the foreground indicate probed areas lacking evidence for ULVZ structure, while red patches in the foreground mark regions with detected ULVZs. Background colors show lowermost mantle seismic shear wave velocities from the tomographic study by Ritsema et al., 2004; scale bar at the bottom is for the background tomography model. Figure from McNamara et al. (2010).

Impact of Research and Publications

The project is designed to develop a complete understanding of ULVZs in a multi-disciplinary approach. It will test competing hypothesis about ULVZ origin and lower mantle dynamics and will develop models of ULVZ origin and evolution. It will be structured into several work packages with each package aiming for publications in high impact journals. The work will be presented at national and international workshops and conferences.

Fieldwork

This project will mainly use existing datasets held at international data centres. Opportunities for active fieldwork participation might arise as part of other fieldwork oriented projects within the School of Earth and Environment

An excellent Training and Research Environment

The Deep Earth Research Group (http://www.see.leeds.ac.uk/research/igt/deep-earth-research) at the University of Leeds consists of researchers in seismology, core dynamics, magneto-hydrodynamics and high-pressure mineral-physics. The group is one of the largest concentration of scientists interested in deep Earth structure and dynamics in the world. The research group is part of the Institute of Geophysics and Tectonics (IGT) with about 25 permanent staff working in a wide variety of solid Earth geoscience disciplines including Tectonophysics, Geodynamics, Petrology, Structural Geology, Seismology, Petrology, Mineral-Physics, Remote Sensing and Geochemistry (http://www.see.leeds.ac.uk/research/igt/). The successful candidate will have the opportunity to interact with internationally leading specialists in these areas and will have the opportunity to present the research work at national and international workshops and conferences.

Further Reading

Brown, S. P., M. S. Thorne, L. Miyagi, and S. Rost (2015), A compositional origin to ultralow-velocity zones, Geophys. Res. Lett., 42(4), 1039–1045, doi:10.1002/2014GL062097.

Garnero, E., and A. McNamara (2008), Structure and dynamics of Earth’s lower mantle, Science (80-. )., 320(5876), 626–628.

Garnero, E. J., A. K. McNamara, and S.-H. Shim (2016), Continent-sized anomalous zones with low seismic velocity at the base of Earth’s mantle, Nat. Geosci., 9(June), 481, doi:10.1038/ngeo2733.

Leng, K., T. Nissen-Meyer, and M. van Driel (2016), Efficient global wave propagation adapted to 3-D structural complexity: a pseudospectral/spectral-element approach, Geophys. J. Int., 207(3), 1700–1721, doi:10.1093/gji/ggw363.

McNamara, A., E. Garnero, and S. Rost (2010), Tracking deep mantle reservoirs with ultra low velocity zones, Earth Planet. Sci. Lett., 299, 1–9.

Rost, S., E. Garnero, Q. Williams, and M. Manga (2005), Seismological constraints on a possible plume root at the core-mantle boundary, Nature, 435(7042), 666–669.

Vanacore, E. A., S. Rost, and M. S. Thorne (2016), Ultralow-velocity zone geometries resolved by multidimensional waveform modelling, Geophys. J. Int., 206(1), 659–674, doi:10.1093/gji/ggw114.

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