Exploring reactions under stress in the mantle using mineral analogues

Convection in the Earth’s mantle controls Earth evolution. Different mineral assemblages occur at different depths in the mantle and variations over short distances are detectable as seismic discontinuities, thus providing key information on mantle conditions. Although controlled primarily by depth, the positions of discontinuities are affected by temperature, in ways that are quite well characterised. However, they are also affected by stress in ways that are poorly understood. This project will investigate the effects of stress on solid state reactions using analogues to mantle minerals, which can be studied more readily in experiments. You will be part of a team of researchers studying feedbacks between mantle convection and mineral reactions.

You will undertake experiments at high pressures (1.5-2 GPa in Griggs apparatus, much higher in DT-Cup) under stress to investigate the conditions under which reactions occur, and their rates. You will examine the microstructures of experimental products to understand the mechanisms enabling reaction, and you will build theoretical models to explain, unite and extrapolate experimental and microstructural observations. In the mantle olivine transforms via wadsleyite to ringwoodite (spinel) as pressure increases, with complicated reaction pathways starting at ~410 km depth. But if we take the germanate Mg₂GeO₄ analogue of Mg₂SiO₄ olivine, this transforms directly to germanate spinel at lower pressure, hence is easier to study. Vaughan et al. (1984) showed that stress has a major control on this reaction. You will expand the range of such experiments. You will use Electron Backscatter Diffraction amongst other techniques on the products to characterise the microstructures (crystallographic orientations and intracrystalline distortions) down to the level of individual interfaces to better understand the effects of stress on reaction kinetics. Vaughan et al. (1984) linked kinetics of that reaction to stress in a model that is in harmony with diverse experiments on reactions in stressed systems (Wheeler 2020). That explanation will be further developed in this project, using a combination of theoretical models (phase field modelling in particular). Subsequent experiments will use Mg-Fe solid solution germanates to understand how those elements repartition during the olivine to ringwoodite reaction, possibly influencing reaction rate. You will also study reactions under stress involving several minerals. Albite breakdown to jadeite and quartz under stress is of direct relevance to metamorphism of oceanic crust in subduction zones. This also serves as an analogue reaction for ringwoodite breakdown to bridgmanite and periclase at ~660 km in the mantle. Physical and chemical characterisation of experimental products will lead to models for reaction rate and mechanism, including using phase field modelling software, and the student will benefit from overlapping with the entire project team.

Training

John Wheeler will supervise the general project development and provide expertise in microstructural investigation and in numerical modelling. Holger Stünitz leads the Griggs apparatus laboratory in Orleans and will provide training in experimental setup and interpretation. Simon Hunt is the designer of the DT-Cup apparatus which can go to higher pressures and also be used for X-ray synchrotron work allowing direct monitoring of reaction evolution. He will provide training in experimental setup and interpretation and expertise on analogue materials. You will benefit by working as part of a team on a NERC Large Grant involving postdoctoral researchers undertaking experiments and numerical modelling thereof, as well as seismology and mantle convection modelling. Generic PhD training will be provided through the Liverpool Doctoral College https://www.liverpool.ac.uk/pgr-development/offering/catalogue/. You will gain broad experience of techniques applicable in Earth and Materials science, and of communicating with scientists with diverse backgrounds.

Candidates should hold or expect to gain a minimum of a 2:1 Bachelor Degree, and Masters Degree with Merit/Distinction, or equivalent in a discipline/s focused on Earth sciences, though we will consider applicants with Materials science or other physical science backgrounds.

For enquiries please contact Professor John Wheeler on: [Email Address Removed]

To apply for this opportunity, please visit: https://www.liverpool.ac.uk/study/postgraduate-research/how-to-apply/ and click the 'Ready to apply? Apply online.'