Mapping Water in Earth’s Upper Mantle Using Laboratory Measurements Supervisory Team Dr. Hauke Marquardt (depict-group.org) Prof. Tarje Nissen-Meyer (https://www.earth.ox.ac.uk/people/tarje-nissen- meyer/) Key Words Mineral physics, mantle transition zone, mantle dynamics, seismology, Earth’s deep water cycle Overview The Earth’s transition zone extends between depths Global scale three-dimensional mapping of the of 410 km and 660 km and might constitute a deep water content in the transition zone requires Earth water reservoir that remains stable for geophysical remote sensing, particularly using geological timescales, thereby playing a key role in seismology. Mineral physics experiments can the dynamic and geochemical evolution of the entire provide the legend to infer the “water” content from mantle (Bercovici et al. 2003). A hydrous transition seismic velocity maps. However, significant zone might be associated with local melting events extrapolation of previous high-pressure that are detectable above (Song et al. 2004) and experiments is required to quantify effects of below the transition zone (Schmandt et al. 2014) hydration on seismic wave speeds at deep upper and may even be the source region for continental mantle pressures. flood basalts (Wang et al. 2015), affecting the global climate. The hypothesis of the transition zone being This project will focus on resolving the impact of a deep water reservoir is motivated by high- hydrogen (“water”) on geophysical observables by pressure high-temperature experiments showing combining traditional seismic wave velocity that the nominally anhydrous minerals measurements in the laboratory with novel (Mg,Fe)2SiO4 wadsleyite and ringwoodite, experimental capabilities at large-scale synchrotron constituting up to 60 vol-% of transition zone rocks, research facilities (Diamond Light Source, UK and can incorporate significant amounts of hydrogen DESY, Germany). This will permit to quantify the (“water”) as point defects into their crystal structures effects of hydrogen on seismic wave propagation at (Kohlstedt et al. 1996). The recent discovery of a realistically high pressures, temperatures and hydrous ringwoodite inclusion in a natural diamond seismic frequencies. The new measurements will containing about 1.5 wt. % of water (Pearson et al. be combined with the seismological record to 2014) confirms the experimental predictions and enhance our understanding of the hydration state of strengthens the hypothesis of a (at least partly) the Earth’s transition zone, the amount of water hydrated transition zone. However, this single recycled back into the mantle, and, ultimately, its observation does not constrain the global amount of role in the evolution of our planet. water stored in the transition zone or its spatial distribution. Also, it does not provide information on the actual path that “subducted water” takes to Methodology reach the transition zone. Direct constraints on the chemical and mineralogical composition of Earth’s mantle are derived through a comparison of seismic wave velocity models with synthetic mineral physics- based velocity models calculated from laboratory elasticity measurements (e.g. Kurnosov et al., 2017). Seismic wave velocities are calculated from the elastic moduli (bulk and shear modulus) and densities of hydrous mantle minerals determined in diamond-anvil cells at pressures of the transition zone. The elastic moduli can be measured in the laboratory using laser-based techniques (Brillouin spectroscopy). Complementary x-ray diffraction Sketch illustrating the possible cycling of water into Earth’s transition zone and its central role for global processes (J. Buchen, BGI). measurements allow for density determination. In this project, a novel multi-sample approach will be References & Further Reading employed at Oxford that allows for significantly improved measurements of small effects (such as those caused by hydration of mantle minerals) on Bercovici, D. and S.-i. Karato (2003). Whole- seismic wave velocities and densities at high mantle convection and the transition-zone water filter. Nature, 425(6953), 39-44. pressures (e.g. Schulze et al., 2017). In addition to this new approach, the implementation of laser- heating capabilities in the diamond-anvil cell at Kohlstedt, D. L., H. Keppler and D. C. Rubie Diamond Light Source (Oxfordshire, UK) now (1996). Solubility of water in the alpha , beta and gamma phases of (Mg,Fe)2SiO4. Contributions to allows for density measurements at both the Mineralogy and Petrology, 123(4), 345-357. pressure and temperature conditions of the lower mantle. Finally, the project will take advantage of Kurnosov, A., H. Marquardt, D. J. Frost, T. B. very recent developments at DESY Synchrotron 3+ (Germany), where elastic properties can now be Ballaran & L. Ziberna (2017). Evidence for a Fe - studied for the first time at seismic frequencies in rich pyrolitic lower mantle from (Al,Fe)-bearing bridgmanite elasticity data. Nature, 543(7646), 543- the diamond-anvil cell (1 Hz). The laboratory data 546. will be used to look for signs of water in seismological observables. Pearson, D. G., F. E. Brenker, F. Nestola, J. McNeill, L. Nasdala, M. T. Hutchison, S. Matveev, K. Mather, G. Silversmit, S. Schmitz, B. Vekemans Timeline and L. Vincze (2014). Hydrous mantle transition Year 1: Doctoral training courses (10 weeks), zone indicated by ringwoodite included within application for synchrotron beamtime, literature diamond. Nature, 507(7491), 221-224. review, planning of experimental campaigns, characterisation (e.g., microscopy, EPMA, XRD) Schmandt, B., S. D. Jacobsen, T. W. Becker, Z. and laboratory training. Liu and K. G. Dueker (2014). Dehydration melting at the top of the lower mantle. Science, 344(6189), Years 2 and 3: Sample preparation (polishing, FIB), 1265-1268. preparation of diamond-anvil cells, Brillouin spectroscopy, synchrotron experiments (DLS and Schulze, K., J. Buchen, K. Marquardt and H. DESY), presentation of research at national Marquardt (2017). Multi-sample loading technique conferences. for comparative physical property measurements in the diamond-anvil cell. High Pressure Research, Year 4: Data integration, thesis completion, papers 37(2), 159-169. for international journals, presentation of research at an international conference. Song, T.-R., D. V. Helmberger and S. P. Grand (2004). Low-velocity zone atop the 410-km seismic Training & Skills discontinuity in the northwestern United States. The supervisory team in Oxford are leaders in high- Nature, 427(6974), 530-533. pressure mineral physics and its application to the interpretation of geophysical observables (H. Wang, X.-C., S. A. Wilde, Q.-L. Li and Y.-N. Yang Marquardt) and have been strongly involved in (2015). Continental flood basalts derived from the synchrotron-based research in the past. T. Nissen- hydrous mantle transition zone. Nature Meyer is an expert in computational mantle Communications, 6, 7700. seismology. Further Information As part of this project you will learn how to prepare Contact: Hauke Marquardt ([email protected]) diamond-anvil cells and conduct high-pressure (Hauke Marquardt will start as an Oxford Associate experiments using a variety of techniques (XRD, Professor in Earth Sciences in April 2018). optical spectroscopy). You will further be trained in how to plan and carry out laboratory as well as synchrotron experiments, using world-leading research facilities. You will also receive training and guidance in how to model and interpret data, how to present scientific results, and how to write scientific papers for publication. .