ModellingMantleMineralogy 1

Modelling Mantle Mineralogy: A Novel Experimental Approach

Main supervisor & contact: Dr. Oliver T. Lord (Bristol) — email: oliver.lord@bristol.ac.uk, tel: 01173314762

Co-supervisors: Prof. Michael J. Walter (Bristol)

Earth's lowermost mantle contains enigmatic seismically-detected structures including two vast, dense regions termed large low shear-wave velocity provinces (LLSVPs), one below the Pacific and one below Africa, as well as smaller, even denser patches of material at the core mantle boundary (CMB) called ultra-low velocity zones (ULVZs)1. One of the most prevalent hypotheses for the origin of this set of complex structures is that they represent the vestiges of the crystallization of one or more impact induced global magma ocean(s) present during the Hadean Eon2. Alternatively, these structures may represent the remnants of dense, subducted oceanic crust that has accumulated at the CMB as a result of mantle convection with residence times anywhere from ~3 Ga3 to only ~100s of Ma4. The aim of this project is to develop mineralogical models of the lowermost mantle by comparing its seismic response with the thermoelastic properties of the relevant mineral phases, namely their densities and sound velocities.

The studentship will be based in the School of Earth Sciences at the University of Bristol, home to the best-equipped high-pressure petrology laboratory in the country. The student will continue development work on a new internal restive heating method designed to recreate the extreme pressures and temperatures of the deep mantle with unprecedented accuracy. This method will then be used to study the major mantle phases MgSiO3 bridgmanite / post-bridgmanite, (Mg,Fe)O ferropericlase, CaSiO3 perovskite, SiO2 post-stishovite (p-st) / seifertite and Ca-ferrite structured NaAl-rich phase (NAL). The measurements of the thermoelastic properties of these phases will be performed using several complementary, cutting edge techniques. First, densities will be determined as a function of pressure and temperature by X-ray Diffraction on the extreme conditions beamline (I15) at Diamond, the UK's X-ray synchrotron

Some of the techniques and facilities that will be employed in this project: (top left) Internal resistive heating in the diamond anvil cell to re-create the pressures of the deep mantle in the laboratory at Bristol. (top right) The state-of-the-art laser heating system at Bristol used to re-create mantle temperatures in the diamond anvil cell. (bottom) The European Synchrotron Radiation Facility at Grenoble, France, where some of the experiments will be performed.

ModellingMantleMineralogy 2

user facility. Next, sound velocities will be determined in two ways: 1. using Inelastic X-ray Scattering at beamline ID28 of the European Synchrotron Research Facility (ESRF) in Grenoble, France, in collaboration with Prof. James Badro (Institut de Physique du Globe de Paris) and 2. using Brillouin scattering at the Bayreuth Geoinstitut, Germany, in collaboration with Prof. Dan Frost. Ultimately, this data will be used to create new mineralogical models designed to match the available radially averaged and global tomographic seismic data by varying mineral proportions and orientation in collaboration with Dr. James Wookey, a seismologist at Bristol with a background in studies of lower mantle structure.

The student will be involved in developing novel experimental methods using our micro-fabrication facilities and will have the opportunity to be involved in the broad range of research projects and international collaborations within this dynamic group. In addition to developing experimental and analytical skills, the student will have the chance to gain experience in a range of computing techniques, including data analysis and programming (MATLAB) methods important for future employment in industry. The applicant will also have opportunities to develop excellent communications skills (both written and oral) through the regular presentation of results both within the research group and at international conferences such as the AGU fall meeting in San Francisco. Applicants with degrees in Geology, Physics, Chemistry or related degrees will be ideally suited to this project.

Recent graduates from the Petrology group at the University of Bristol have gone on to careers in academia and industry and to become specialists in scientific instrumentation at research institutions. References 1 E.J. Garnero and A.K. McNamara, Science 320, 626 (2008). 2 S. Labrosse, J.W. Hernlund, and N. Coltice, Nature 450, 866 (2007). 3 E. Mulyukova, B. Steinberger, M. Dabrowski, and S.V. Sobolev, J. Geophys. Res. Solid Earth 120, 3824 (2015). 4 C. Huang, W. Leng, and Z. Wu, Earth and Planetary Science Letters 423, 173 (2015).