Excellence in Research: Effect of Hydration on the Thermo-elastic Properties of Mantle Minerals and the Geophysical Implications.

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Earth’s deep interior is not accessible to direct sampling. As temperature and pressure increase with depth, man-made instruments become unusable. The most direct observations arise from studying vibrations generated by earthquakes, called seismic waves. The waves travel within the Earth and are collected at the surface using seismographs. The seismic signal is analyzed to inform the structure and composition of Earth’s interior, as sonography is used in medical imaging. The velocity of seismic waves depends on the type of rocks they encounter. Seismological studies combined with experimentation allow identifying rocks in the Earth’s mantle. It was shown that at depths of 410 to 660 km (255 to 410 miles) - in the so-called transition zone - two dense minerals are present: wadsleyite and ringwoodite. These minerals can incorporate large amount of water in their structure under the form of OH molecules (hydroxyls). The transition zone may contain as much water as that contained in the oceans. This has implications for Earth’s mantle thermal convection, which drives plate tectonics. Yet, it is unclear how much water is stored in the transition zone. This is partly due to uncertainties on how hydroxyls affect seismic-wave propagation in minerals. Here, the researchers investigate how water incorporation in wadsleyite and ringwoodite affects the velocity of seismic waves. They synthetize in the laboratory minerals with various compositions and water contents. They carry out ultrasonic measurements at the extreme pressures and temperatures prevailing in the Earth. These experiments are performed at a national synchrotron facility, to ensure specimen quality and measure their size by radiography during the measurements. The study outcomes are critical to better understand the properties of the transition zone. It has implications for the understanding of thermal convection in the Earth. This project promotes multidisciplinary collaborations across Earth Sciences, Physics, Chemistry, and Mathematics. It provides support for a post-doctoral associate and training for undergraduate students at Delaware State University (DSU). DSU is a Historically Black University and a predominantly undergraduate institution. The project offers unique opportunities to students from groups underrepresented in Science. It fosters diversity and inclusion in Geosciences. It is co-funded by NSF Directorate for Geosciences and Historically Black Colleges and Universities - Excellence in Research (HBCU-EiR) Program. Experimental and theoretical studies indicate that wadsleyite and ringwoodite can incorporate up to 2-3 weight percent of hydroxyl (OH-) in their structures. Up to 1.5 weight percent of water was measured in a ringwoodite crystal trapped in a diamond which originated from the transition zone. Water incorporation strongly affects mineral physical and chemical properties – such as electrical and thermal conductivity, melting and flow – as well as elastic wave propagation. Here, the researchers synthetize polycrystalline samples of wadsleyite and ringwoodite containing controlled structural water. They use the 2000-ton uniaxial split-cylinder apparatus at Stony Brook University. The quality of the hot-pressed specimens is verified using X-ray diffraction, scanning transmission electron microscopy, bulk density measurements, and bench-top acoustic velocity measurements. Specimen elastic wave velocities is then quantified by ultrasonic measurements at high pressure and temperature, in the mineral stability fields. These measurements are carried out at the 6-B-MB beamline of the Advanced Photon Source (Argonne National Laboratory). The beamline is equipped with a cubic anvil high-pressure apparatus coupled with in situ ultrasonic interferometry, X-ray diffraction and imaging. Specimen water content is measured before and after the high-pressure experiments by infrared spectroscopy, secondary ion mass spectrometry, and using the Electron Probe Micro-Analyzer techniques.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.