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E330: Chemical geodynamics: Combining geochemical and geophysical constraints on mantle structure and processes (Lead Supervisor: Oliver Shorttle, Earth Sciences)

Supervisors: Oliver Shorttle (Earth Sciences) and John Maclennan (Earth Sciences)

Importance of the area of research:

Earth’s giant convective system has been redistributing heat, volatile elements such as water and carbon dioxide, and myriad other chemical species for 4.5 billion years.  This multi-component transport system couples the evolution of the deep Earth with the history of the oceans, atmosphere, and biosphere.  A record of this differentiation is accessible through the 60,000 km of mid-ocean ridge encircling the globe, which provide an invaluable window into the chemical and physical structure of the upper mantle.  A deeper probe of the Earth’s interior are the mantle plumes feeding ocean island volcanism, which may bring lower mantle material to shallow depths. Combined, mid-ocean ridges and ocean islands allow us to constrain the chemistry, temperature, and spatial distribution of mantle domains.  These observations have been used to argue for chemical stratification at the core-mantle boundary, infer the dynamics of plume ascent, and constrain the heat budget of the Earth.

Project summary:

This project will link geochemical observations to complementary geophysical datasets to refine our models of mantle composition and structure.  From mid-ocean ridges and ocean islands we now possess a wealth of geochemical data that constrain mantle lithology and the spatial distributions of chemical domains, and geophysical data that record dynamic support of the lithosphere and deeper mineralogical phase transitions. By contributing targeted new analyses, and statistical and thermodynamic modelling this project will identify whether particular mantle geochemical signatures have associated geophysical expressions, and how the spatial distribution of basalt compositions at Earth’s surface relates to its deeper chemical structure.

What the student will do:

This project has three components: (1) The student will make new petrological estimates of mantle temperature, with the opportunity to supplement existing samples through fieldwork at a relevant ocean island locality such as Iceland. These estimates will be compared with geophysical constraints on mantle temperature.  (2) The petrologically constrained temperatures and lithologies of ocean island sources will be used to model structure in the underlying mantle.  These predictions will be compared to geophysical observations of phase transition depths and seismic velocities from existing studies.  (3) Statistical interrogation of mid-ocean ridge and ocean island geochemistry will identify whether spatial patterns in basalt composition can be mapped into mantle chemical structure.  Comparison of the geochemical record to geophysical observations of mantle structure will assess whether particular mantle sources are linked to certain convective phenomena or length scales.

Please contact the lead supervisor directly for further information relating to what the successful applicant will be expected to do, training to be provided, and any specific educational background requirements.

References:

Shorttle, O., Quantifying lithological variability in the mantle. Earth and Planetary Science Letters 395, 24—40 (2014).

Hoggard, M.J., White, N., and Al-Attar, D., Global dynamic topography observations reveal limited influence of large-scale mantle flow. Nature Geosciences 9, 456—463 (2016).

Weis, D., Garcia, M.O., Rhodes, M.J., Jellinek, M., and Scoates, J.S., Role of the deep mantle in generating the compositional asymmetry of the Hawaiian mantle plume. Nature Geosciences 4, 831—838 (2011).

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