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E341: Deep Earth Ammonium: Recycling the Atmosphere through the Solid Earth (Lead Supervisor: Simon Redfern, Earth Sciences)

Supervisors: Simon Redfern (Earth Sciences) and Sami Mikhail (University of St Andrews)

Importance of the area of research:

Understanding the Earth’s nitrogen cycle is central to unraveling what made (and continues to make) Earth a habitable planet capable of sustaining life over billions of years. To address the question of habitability we must understand the origin of the atmosphere. Some of the most important data required are the nitrogen re-gassing capacity of the mantle through subduction. To  determinine the partitioning behaviour of nitrogen during subduction, we must understand the compressibility and compatibility of the ammonium ion, because partitioning is fundamentally controlled by charge and ionic radius of the element/compound and the occupation site in the solid which depends on the compressibility (volume reduction) of the ammonium ion in the deep Earth.

Project summary:

Earth is a dynamic planet with active plate tectonics: what comes out of the mantle sometimes goes back in, and vice versa. The exchange of nitrogen between the surface and interior is governed by volcanism (out-gassing) and subduction (in-gassing), and this interplay ultimately controls atmospheric N2 levels. The further back one looks in time the less data are available, and there is a predictable dearth of data to constrain the partial pressure of atmospheric nitrogen in the deep past. This project seeks the data to constrain the reservoirs of nitrogen in the deep Earth and determine their role in shaping our atmosphere.

What the student will do:

The student will study NH4 behaviour in stable ammonic compounds: NH4-silicates (buddingtonite, tobelite and NH4-hollandite). Using synchrotron-based diamond anvil compression studies and large-volume cell neutron studies (using the high-P/T PE cell developed in Cambridge) the student will determine the pressure-dependent compressibility within these structures, and isolate the matrix (crystal) control on the ionic compression. Neutron diffraction experiments uniquely allow the identification of the location of the hydrogen nucleus within each structure, and hence the direct measurement of the size, distortion and configuration of the ammonium ion. The compression behaviour of ammonium will be compared to potassium and sodium. Separately, we will investigate the compressibility of the ammonium ion by Raman spectroscopy of aqueous solutions at variable pressure and temperature in Cambridge, to obtain the compression characteristics at lower pressure-temperature conditions. This will allow us to chart out characteristics of mineral behaviour across the P-T conditions of relevance to shallow subduction.

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:

Mikhail, S, Sverjensky, DA. 2014, Nitrogen speciation in upper mantle aqueous fluids: implications for the origin of Earth’s N2-rich atmosphere. Nature Geoscience, 7, 816-819

Mikhail, S, Howell, D. 2016. A petrological assessment of diamond as a recorder of the mantle nitrogen cycle. American Mineralogist, 101, 780–787

Mookherjee M, Redfern SAT, Swainson I, Harlov DE. 2004. Low-temperature behaviour of ammonium ion in buddingtonite [N(D,H)4AlSi3O+] from neutron powder diffraction. Phys Chem Minerals 31:643–649.

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