NERCGW4+DTP2016 1

Hidden oceans: water in the deep Earth

Main supervisor & contact: Dr. Oliver T. Lord (Bristol) — email: oliver.lord@bristol.ac.uk, tel: 01173314762 Co-supervisors: Prof. Michael J. Walter, Dr. Simon C. Kohn (Bristol), Prof. Huw Davies (Cardiff)

The cycling of water between Earth's surface and interior through subduction and volcanism has helped to maintain the habitability of our planet throughout a significant part of its history1. While the importance of water on the surface is clear, the effects of water in the deep Earth are equally profound. The presence of water can lead to the hydrolytic weakening of minerals, reducing mantle viscosity and thereby promoting convection. Water also dramatically reduces the melting points of mantle lithologies, promoting the formation of melts at mid-ocean ridges2, subduction zones3 and possibly in the deep mantle4. Such effects were crucial in the evolution of the Earth as silicate melts are a primary agent of mass transfer and chemical differentiation. There is evidence for deep melt in the Earth today, and the presence of water is a leading contender to explain recent seismic evidence for the presence of ubiquitous melt layers both above5 and below6 the mantle transition zone. The amount and distribution of water in the deep Earth is therefore a key parameter both for ongoing mineral physics investigations into the physical properties of Earth materials and those engaged in broad scale geodynamic modeling of the Earth.

Remarkably, recent estimates suggest that the deep Earth could contain more water than the hydrosphere7, bound within the crystal structures of nominally anhydrous minerals (NAMs) such as wadsleyite, ringwoodite and bridgmanite (the dominant phase in the lower mantle and the most abundant mineral in Earth) and also within dense hydrous silicate accessory phases4 such as the recently discovered phase H. However, we still have little understanding of the liquidus phase relations of hydrous mantle rocks at transition zone and lower mantle pressures and even less knowledge of the solubility limits of water in the major lower mantle phases. These questions form the primary subject of this experimental Ph. D. project.

Some of the techniques and facilities that will be employed in this project: (top left) Multi-anvil press and diamond anvil cell used 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) Diamond, the UK's X-ray synchrotron where additional experiments will be performed.

NERCGW4+DTP2016 2

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 use both the multi-anvil presses (MAP) and laser-heated diamond anvil cell (LH-DAC) technologies available at Bristol to recreate the extreme pressures and temperatures of the deep mantle. The project will employ two strategies to constrain Earth's deep-water budget. Firstly, the student will perform suites of experiments on simplified mantle analogue compositions in both the MAP and LH-DAC and determine the liquidus phase relations from a combination of field emission gun electron microscopy (at Bristol) and X-ray diffraction measurements on the extreme conditions beamline (I15) at Diamond, the UK's X-ray synchrotron user facility. Secondly the student will perform LH-DAC experiments on a suite of bridgmanite compositions with progressively increasing water content. These samples will be laser heated at pressure on the I15 beamline with simultaneous X-ray diffraction measurements from which the variation of the unit cell volume as a function of water content will allow the maximum solubility of water to be constrained. In addition, measurements of the water concentrations and dissolution mechanisms in deep mantle phases will be made at Bristol on the quenched samples using Fourier-transform infra-red spectroscopy. An understanding of the mechanism of dissolution of water in these phases is essential for predicting the nature and magnitude of the effect of dissolved water on physical properties and for thermodynamic modeling of the systematics of water solubility.

The results of this project will form the ground work for an ongoing NERC funded effort to determine the viscosity and density of hydrous melts and will be used by the group of Prof. Huw Davies at Cardiff University School of Earth and Ocean Sciences to improve their geodynamic models of the deep Earth. At the same time, Prof. Huw Davies will help to identify key areas where better experimental data is required thus maximising the impact of the research on the wider geoscientific community.

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 M.M. Hirschmann, Annu. Rev. Earth Planet. Sci. 34, 629 (2006). 2 R. Dasgupta and M.M. Hirschmann, Nature 440, 659 (2006). 3 T. Grove, N. Chatterjee, S. Parman, and E. Médard, Earth and Planetary Science Letters 249, 74 (2006). 4 M.J. Walter, A.R. Thomson, W. Wang, O.T. Lord, J. Ross, S.C. McMahon, M.A. Baron, E. Melekhova, A.K. Kleppe, and S.C. Kohn, Chemical Geology (2015). 5 B. Tauzin, E. Debayle, and G. Wittlinger, Nature Geosci 3, 718 (2010). 6 B. Schmandt, S.D. Jacobsen, T.W. Becker, Z. Liu, and K.G. Dueker, Science 344, 1265 (2014). 7 D.G. Pearson, F.E. Brenker, F. Nestola, J. McNeill, L. Nasdala, M.T. Hutchison, S. Matveev, K. Mather, G. Silversmit, S. Schmitz, B. Vekemans, and L. Vincze, Nature 507, 221 (2014).