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Tracking water in the Earth's transition zone with seismology and mineral physics

Dr Andy Nowacki (SEE), Dr Andrew Walker (SEE)

Contact email: a.nowacki@leeds.ac.uk

The Earth’s water cycle includes the silicate mantle: subducting tectonic plates carry hydrogen down as they sink, in the form of hydrous minerals.  Much of the water is released in the upper mantle (~100 km depth), leading to volcanism at arcs, but it is thought that some remains present in slabs to much greater depths—within the mantle transition zone (410–660 km) and perhaps beyond.  It likely does so bound in high-pressure hydrous phases such as antigorite and ‘phase D’.  These mineral phases are highly elastically anisotropic, meaning their seismic velocities vary strongly with propagation direction, which may explain the presence of strong seismic anisotropy which has been observed around slabs in the transition zone (Nowacki et al., 2015; Chang et al., 2015).  Despite these key indicators for the presence of water in the transition zone, however, no-one has yet been able to quantify just how much might be cycled.

In this project you will test the hypothesis that anisotropy around slabs is due to the alignment of hydrous crystals, and by doing so constrain the amount of water which reaches the top of the lower mantle (>660 km).  This can only be done by combining seismology to make observations, and mineral physics to explain them.  Firstly, you will analyse signals from seismic waves travelling from deep-focus earthquakes and recorded at distant seismic stations, studying waveform characteristics, travel times and shear wave splitting (caused by anisotropy).  Secondly, you will build models of the transition zone and create synthetic seismic signals to compare with your data.  This will involve taking the results of mineral physics experiments on the deformation of water-bearing rocks and using them to provide the density and elastic parameters in the models.  However, since such experimental results are currently few in number, you may also perform your own mineral-physical calculations using density functional theory to determine how different hydrous phases in the transition zone deform, and what their seismic signature would be.

Figure 1:  Global analysis of shear wave splitting indicating that hydrous silicates may cause anisotropy in deep subducting slabs.  (a) Cross section through the mantle. S waves from deep-focus earthquakes (yellow circle) are measured at seismic stations (black triangle), with shear wave splitting caused around the event, and in the green upper mantle.  Core-traversing SKS waves (grey region) measure upper mantle splitting and are used to correct for this.  (b & c) Projecting the results into a common reference frame (explained in b) reveals that splitting is best explained by down-dip compression of the hydrous phase ‘D’, suggesting significant amounts of water are carried to the lower mantle.

Objectives

In this project, you will work with leading researchers at Leeds with backgrounds in seismology and mineral physics.  Depending on your interests and strengths, you may choose to focus on a number of areas to develop the work and make a novel contribution to the study of the Earth.

  1. Building on an existing dataset from Nowacki et al. (2015), collect a seismic dataset of recordings from deep earthquakes globally, and investigate the signals in terms of waveform characteristics, shear wave splitting and travel time anomalies, correcting for the effects of the upper mantle and crust beneath the seismometer.
  2.  Collate existing mineral physics calculations on the deformation mechanisms and elasticity of transition zone phases, including hydrous phases.  Construct seismic models of the transition zone corresponding to the best-sampled areas in the dataset from (1), using this mineral physics dataset.  Models will vary by the proportion of hydrous phases and their deformation mechanism, representing different amounts of water being carried in the slab.
  3. Compare the models with observations.  Initially, compute shear wave splitting and travel time delays using ray-theoretical approximations for the same paths as in the dataset from (1).  Thereafter, compute finite-frequency synthetics for a direct waveform comparison.
  4. Model in detail how transition zone hydrous phases deform at the atomic scale using density functional theory, allowing us to better translate deformation within the slab into seismic parameters.

Figure 2: Atomic scale model used to simulate the deformation of phase-D, one of the dense hydrous magnesium silicates to be studied as part of this project. The crystal structure, comprising sheets of silicate octahedra (silicon: blue; oxygen: red) separated by magnesium (yellow) and hydrogen (pink), is sheared along the horizontal red line. The excess energy associated with this distortion to the crystal structure allows larger scale models of polycrystal deformation to be constructed and used to predict seismic anisotropy.

Training

The student will work under the supervision of Dr Andy Nowacki and Dr Andrew Walker, within the Institute of Geophysics and Tectonics in the School of Earth and Environment.  You will be an integral member of the Deep Earth research group in Leeds—one of the largest and most prestigious groups in the world examining how the Earth’s mantle and core behave—and have the opportunity to work with and learn from fellow researchers, both senior academics and fellow students, in a broad swathe of geophysics.

You will have the chance to develop expertise in analysing data and scientific programming, including using the largest supercomputers in the country, alongside developing the core transferable skills of project management, team-working and communication.  You will master a wide selection of modelling tools applicable to materials science and seismology.  Opportunities to travel to international conferences will be part of the studentship.  You will have access to the broad spectrum of training courses provided by the Faculty that include an extensive range of workshops from numerical modelling, managing your degree, through to preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/).

Student profile

A good applicant should have a strong background in physics or quantitative Earth science and a passion for studying how the Earth works.  Interest and experience in seismology would be useful, as would some experience of programming, but not essential.

References

  • Chang, S.-J., A. Ferreira, J. Ritsema, H.J. van Heijst and J Woodhouse. (2014). “Global radially anisotropic mantle structure from multiple datasets: A review, current challenges, and outlook.” Tectonophysics 617, pp. 1–19. http://dx.doi.org/10.1016/j.tecto.2014.01.033
  • Nishi, M. (2015) “Deep water cycle: Mantle hydration”, Nature Geosciences, 8, pp. 9-10. http://dx.doi.org/10.1038/ngeo2326
  • Nowacki, A., J.-M. Kendall, J. Wookey and A. Pemberton “Mid-mantle anisotropy in subduction zones and deep water transport”, Geochemistry, Geophysics, Geosystems, 16, pp. 1-21. http://dx.doi.org/10.1002/2014GC005667

Related undergraduate subjects:

  • Applied mathematics
  • Earth science
  • Earth system science
  • Geological science
  • Geology
  • Geophysical science
  • Geophysics
  • Geoscience
  • Materials science
  • Mathematics
  • Physical science
  • Physics