Elastic properties of heterogeneities in Earth’s mantle

Background

The Earth’s mantle shows heterogeneities on many scalelengths that are the imprint of plate tectonics and mantle convection. Since the 1980s seismic tomography has mapped the large-scale structure of the Earth showing fast velocity anomalies interpreted as subducting slabs, low velocity anomalies in the upper mantle that might be related to partial melt and a zoo of structures in the lower mantle of more or less unknown origin. But tomography is only able to resolve the large-scale structure with a resolution of a few hundred kilometres, missing most of the expected fine-scale structure of the mantle (Brandenburg and van Keken, 2007). Through special seismic imaging techniques, we are able to resolve seismic heterogeneities below the resolution of seismic tomography, but the properties of these heterogeneities are only roughly known making a link to composition and their origin difficult.

 

Subduction of oceanic crust is likely the major source of heterogeneity for the mantle. Oceanic crust is extracted from the mantle through partial melting at oceanic ridges forming the basaltic crust and the harzburgitic upper mantle residue. This heterogeneity is then recycled through subduction zones. The mechanisms and depths of the amalgamation of the subducted lithosphere are still unknown, although mechanical mixing will likely play a role, and depend on a range of unknown material properties and the details of mantle convection and melting processes in the subduction zones.  This project aims to resolve the seismic properties of heterogeneities in the mantle and come to conclusions of their origin and their role in mantle evolution.

Figure 1: Cartoon of scattering of the seismic wavefield at mantle heterogeneities. We will use the forward scattered seismic wavefield related to phases such as PKKP and P’P’ in combination with the back-scattered seismic wavefield related to SP and PP to measure velocity and density changes in the heterogeneities.

 

Previous Work

We are able to study the fine-scale structure of the Earth using the scattered seismic wavefield (Figure 1). Such studies (e.g. Frost et al., 2017; Kaneshima, 2019; Rost et al., 2008) have been able to resolve heterogeneities throughout the Earth from crust to inner core with likely different origins. We detect heterogeneities on scale lengths of 10 km in the upper and lower mantle that are interpreted as the remnants of subducted crust with structures in the crust and lithosphere on the other hand likely reflecting tectonic processing and/or melting, respectively. While heterogeneities in the crust and lithosphere are reasonably well understood, we cannot relate mantle heterogeneity to a single origin due to the unknown properties of the mantle heterogeneities. So far, we have only been able to estimate some of the elastic properties of the heterogeneities through statistical models, but the detailed elastic properties (seismic P-wave and S-wave velocity, density) of the detected heterogeneities remain unknown. Even fundamental properties, e.g. if heterogeneities are compositional or represent partially molten mantle or crustal material are therefore unknown. This project aims to close this knowledge gap allowing a better link of heterogeneity properties to their potential origins and mantle processes.

Objectives

We will measure the precise elastic properties of small-scale mantle heterogeneities using the scattered seismic wavefield (Figure 1) to better understand the subduction process, mantle recycling and ultimately the compositional evolution of Earth’s mantle. We will use a combination of different seismic probes sensitive to small-scale heterogeneities located in the mantle. Using a combination of P- and S-wave signals from forward and backscattering probes sensitive to the different elastic properties (P-wave and S-wave velocity and density) will allow a complete characterization of the heterogeneities and a better understanding of their origin. Sampling the same scattering heterogeneity with a variety of scattering probes will enable us to reduce the trade-offs and uncertainties of each individual probe.

 

 Work program

The project will initially (year 1) use a combination of known S-to-P and P-to-P deterministic scatterer locations (Figure 2) and use combined modelling of scattering amplitudes and waveforms of the forward and backscattered seismic wavefield to interrogate their structure. It will extend on this approach (year 2) using global seismic datasets and detailed waveform modelling to determine possible elastic parameters of the heterogeneities in different tectonic settings and changes of properties with depth. The seismological results will be combined (year 3) with recent experimental estimates of subducted crustal properties at mantle pressures and temperatures for a full dynamical interpretation. Sampling properties at different depths and studying the changes in heterogeneity structure compared to the ambient mantle will allow the detection and study of phase changes in the potentially subducted basaltic crust or indications of other potential origins for the heterogeneity.

Figure 2: Map of detected scatterers in subduction zones from Kaneshima [2019]. These heterogeneity locations will be used as a starting point for a search for these locations in the forward scattered seismic wavefield from other probes to link the subduction process to the mantle heterogeneities.

Impact of Research and Publications

The project is designed to test recent hypotheses on the origin and location of mantle. It will use new data and combine several processing techniques to test these hypotheses using the expertise of the supervisors. It will be structured into several work packages, allowing for flexibility within the project to follow the candidates interests, with each package aiming for publications in high impact journals. The work will be presented at national and international workshops and conferences.

International Collaboration

This project is an international collaboration between the University of Leeds in the United Kingdom and Kyushu University in Japan. It is expected that the student will visit the research group of Prof. Kaneshima in Japan for a prolonged visit during the project.

Student requirements

This project is suitable for students interested in seismic data analysis, numerical modelling of seismic wave propagation and numerical modelling of core structures and the structure of the Earth’s deep interior. Relevant undergraduate backgrounds include Geophysics, Geology, Physics, Natural Sciences and Applied Mathematics.

Further Information

Please contact Sebastian Rost (s.rost@leeds.ac.uk) for more information on this project.

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 grouping 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 institute hosts a large number of postdoctoral and post-graduate researchers creating a vibrant research environment. 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.

References and Further Reading:

Brandenburg, J. P., & van Keken, P. E. (2007). Deep storage of oceanic crust in a vigorously convecting mantle. Journal of Geophysical Research, 112(B6), 1–15.

Frost, D. A., Rost, S., Garnero, E. J., & Li, M. (2017). Seismic evidence for Earth’s crusty deep mantle. Earth and Planetary Science Letters, 470, 54–63.

Frost, D. A., Garnero, E. J., & Rost, S. (2018). Dynamical links between small- and large-scale mantle heterogeneity: Seismological evidence. Earth and Planetary Science Letters, 482, 135–146.

Kaneshima, S. (2019). Seismic scatterers in the lower mantle near subduction zones. Geophysical Journal International, 218(3), 1873–1891.

Konishi, K., Kawai, K., Geller, R. J., & Fuji, N. (2009). MORB in the lowermost mantle beneath the western Pacific: Evidence from waveform inversion. Earth and Planetary Science Letters, 278(3–4), 219–225.

Rost, S., Garnero, E., & Williams, Q. (2008). Seismic array detection of subducted oceanic crust in the lower mantle. Journal of Geophysical Research, 113(B6).

Stixrude, L., & Lithgow-Bertelloni, C. (2012). Geophysics of Chemical Heterogeneity in the Mantle. Annual Review of Earth and Planetary Sciences, 40(1), 569–595.