Steven D. Jacobsen - US grants

This study is aimed towards understanding the constitution of Earth's interior through experiment. The high-pressure and high-temperature elasticity of deep-Earth materials has been identified as a major Grand Challenge by the NSF COMPRES consortium. The presence of seismic anisotropy within all of Earth's major boundary layers and inner core highlights the importance of anisotropic elasticity data from mineral physics. Through this award, a state-of-the-art gigahertz-ultrasonic interferometer will be interfaced with the next generation of large-volume, gem and chemical vapor deposition (CVD) diamond-anvil pressure cells in an effort to obtain accurate thermodynamic equations of state for some geophysically relevant Earth-forming minerals. The high P-T experiments will feature a recent breakthrough in ultrasonic methods developed by the PI, wherein purely-polarized ultrasonic shear waves are transmitted into the diamond-anvil cell using a P-to-S conversion. The employment of gas-loading techniques with large gem anvils will greatly extend the working P-T range over which ultrasonic measurements can be made. The ultrasonic experiments in this study will focus on the darker, Fe-bearing silicates, oxides and metals such as ringwoodite, magnesiowustite, and metallic iron because the study of these materials is problematic for existing optical methods. Acoustic-wave travel times in single-crystal samples will be used to determine P-wave velocity, S-wave velocity, elastic-wave anisotropy and the complete elastic tensor, from which the adiabatic bulk and shear moduli are readily determined for direct comparison with seismological observation. Finally, the elastic tensor of annealed CVD diamond will be accurately determined using the GHz-technique because knowledge of the individual elastic constants will help to characterize the new anvil material and may provide insight into the origin of the extraordinary mechanical properties of CVD diamond crystals. This work will nurture collaborations between the Geophysical Laboratory and the Bayerisches Geoinstitut in Bayreuth, Germany, involving at least one graduate student and possibly several undergraduate interns attending the Carnegie Institution Summer Internship Program, which is managed in part by the PI.

This award supports the acquisition of a digitizing oscilloscope for the new Mineral Physics Laboratory in the Department of Earth and Planetary Sciences at Northwestern University. The multi-channel broadband (20-GHz) oscilloscope is capable of capturing and analyzing high-speed acoustic waveforms emitted from an ultrasonic interferometer, recently developed to determine the elastic properties of Earth and planetary materials in diamond-anvil cells at ultra-high P-T conditions (>300 km depth). The elastic properties of minerals govern the velocity of seismic waves in the Earth's mantle. Laboratory-derived elasticity data are applied in solid-Earth geophysics to interpret the observed seismic structure of the mantle in terms of constituent mineralogy and compositional variability. Recent science in the GHz-ultrasonic laboratory has focused on quantifying the effects of hydrogen impurities (water) on the elastic properties of high-pressure mantle minerals in order to test new models of a deep-Earth water cycle. The GHz-ultrasonic interferometry laboratory currently employs one postdoctoral researcher, one graduate student, and over the past two summers has trained two undergraduate students in experimental geophysics.

This project examines the role of silicate minerals in Earth's deep water cycle from atomic to geophysical scales. Under simulated mantle conditions of 400-700 km depth, some minerals have a remarkable ability to absorb water as hydroxyl (OH), resulting in modified physical properties. Experimental studies will focus on determining the effects of hydration on the behavior of Earth materials at high pressures. Results will provide geophysical indicators of mantle hydration that facilitate detection of water in the deep mantle remotely using seismic waves. Graduate and undergraduate research will capitalize on new laboratory ultrasonic techniques developed by the PI. Students will have the opportunity to lead experiments at large-scale facilities using synchrotron-light sources at two different national laboratories. Students will interface with the broader geophysical community and public interest in aiding interpretation of enigmatic structures observed seismically in the mantle, which may be related to water. Local K-12 activities focus on closing the minority science achievement gap through Project EXCITE, a partnership between Northwestern University and Evanston School District 65. Project EXCITE is a longitudinal program, which recruits minority third-grade students for a six-year program involving regular visits to the Department of Earth and Planetary Sciences. The PI will lead presentations and hands-on demonstrations that encourage their interest in Earth science, and will ultimately lead to increased enrollment of minority students in advanced-placement and honors science courses at Evanston Township High School.

Earth is unique among the terrestrial planets in maintaining a large reservoir of liquid water on its surface. The solid silicate minerals of the mantle have the potential to store another major reservoir of H2O inside the Earth and act as part of a dynamic global water cycle. Results from experimental petrology have shown that it is possible to contain several tenths of a percent H2O by weight in the mantle down to 660-km depth, equal to ocean volumes of liquid-water equivalent. However, geochemical evidence suggests that magma source regions are relatively dry. The real extent of deep water cycling and storage is essentially unknown and awaits further constraints from mineral physics and seismology. This CAREER award addresses the broader implications of deep-mantle hydration and targets new opportunities for experimental studies on the structures and physical properties of OH-bearing mantle silicate minerals. At the atomic scale, determination of hydrogen positions and elastic properties will advance understanding of why relatively low concentrations of hydrogen influence the properties of Earth materials. At the mesoscopic scale, H-diffusion will be studied using a new sample suite of gem-quality single-crystals of hydrous mantle phases, grown by the PI and students in the 5000-ton multi-anvil press at Bayerishes Geoinstitut in Bayreuth, Germany. The effects of hydration on phase transformations will be studied with in-situ techniques. Finally, experimental data will be combined with thermoelastic modeling to interpret enigmatic S-wave velocity anomalies reported from seismic tomography, such as the one recently detected beneath the eastern US.

This Instrumentation and Facilities award through the NSF Division of Earth Sciences will support acquisition of a high-precision, single-crystal X-ray diffractometer to carry out topical geomaterials research and education in the Department of Earth and Planetary Sciences (EPS) at Northwestern University. X-ray diffraction allows definitive identification of minerals at the grain-size scale and determination of crystal structures and related physical properties of materials in the crust-mantle system. This instrument will be interfaced with the PI?s unique high-pressure ultrasonic system for basic mineral physics research on thermodynamic equations of state for Earth and planetary materials.

The instrument will be installed in the new EPS Integrated Laboratories of Northwestern (EPSILoN), a new interdisciplinary research and graduate training facility with five adjoining laboratories for mineralogy/mineral physics, stable isotope geochemistry, organic geochemistry, and sedimentary and aqueous geochemistry. Basic research in the PI?s laboratory, currently supported by the faculty early career development program (CAREER) seeks to understand the role of water cycling between the crust and mantle from atomic to geophysical scales. New knowledge of the effects of hydration on the structure and elasticity of mantle materials is applied to remote (geophysical) detection of water in the deep mantle. In addition, the instrument will serve broader departmental and regional needs for specialized mineral phase analysis, characterization, and applied laboratory instruction for undergraduate and graduate courses in Earth materials at Northwestern. Research with the proposed diffractometer has broader impacts as the PI engages in technology transfer between high-pressure geosciences and applied materials research.

NON-TECHNICAL DESCRIPTION: Society has long been fascinated by diamond as the hardest known substance. Can a material harder than diamond be created? Answering this question is not so straightforward because the very definition of hardness employs diamond as the reference, and when materials under test approach or potentially exceed the hardness of the diamond-based indenter, results are difficult to interpret. A new class of superhard materials is being developed based upon nano-polycrystalline or nano-twinned structures of diamond and related compounds containing carbon, boron and nitrogen together in the same structural configuration as diamond. These materials are synthesized at high pressures and high temperatures in large-volume presses. Physical properties of novel superhard materials are investigated using a newly-developed ultrasonic system that measures the speed of sound waves at GHz frequencies. The results precisely determine physical properties such as the shear modulus, which are fundamentally correlated to hardness. New materials rivaling or potentially exceeding natural diamond in hardness and thermal stability can make possible the fabrication of ultrahard machine parts that do not burn or wear down under stress.

TECHNICAL DETAILS: High-pressure high-temperature synthesis of single-crystals intermediate in composition between cubic-boron nitride (c-BN) and diamond demonstrate a solid-solution wherein hardness and thermal stability are optimized for a certain range of compositions. Identifying the ideal composition requires improved measurement and theoretical understanding of hardness-elasticity relationships. Whereas conventional hardness measurements on materials harder than c-BN are difficult to interpret because of plastic deformation of the indenter, a novel GHz-ultrasonic interferometer is developed to measure the elastic constants of materials in the superhard and ultrahard classes with very-high precision. Under that direction, diamond-like compounds containing carbon, nitrogen, and boron and having nano-polycrystalline or nano-twinned structure are targeted to produce materials without cleavage, with high thermal stability, and with hardness rivaling and potentially exceeding natural diamond. The research improves the fundamental understanding of the relationship between hardness and elasticity thus advancing the ability to design and predict the properties of ultrahard materials. The proposal is timed to take advantage of newly developed synthesis techniques and characterization methods. Graduate student training focuses on skills for job placement in related fields in industry, academia, and at the National laboratories. Outreach activities feature participation in Project Excite, a program providing after-school enrichment experiences in science and mathematics to minority students from local elementary schools.

The global water cycle is connected to the Earth's deep interior through plate tectonics. Hydrous minerals within oceanic crust carry the components of water (H2O) into the mantle at convergent plate boundaries, having influence on melt generation and volcanism as part of a cycle that returns H2O to the surface repeatedly over geologic time. How deeply the water cycle extends into the Earth's mantle is not known. However, evidence is mounting that certain mantle minerals such as ringwoodite, found in a layer called the transition zone (410-660 km depth), may contain a significant -if not the largest- geochemical reservoir of H2O in the planet. The presence of just a few weight percent H2O bound in minerals of the transition zone would constitute more water than is present in the oceans. Determining the scale and distribution of H2O in the mantle has implications for understanding the geochemical water cycle, deep melt generation, and determining the Earth's composition and origin of Earth's water.

This study combines mineral physics experiments on laboratory-grown, hydrated mantle materials at high pressures and high temperatures with new and forthcoming regional seismic studies emanating from NSF's Earthscope (USArray) to constrain the scale and distribution of water in the Earth's mantle transition zone. At deep mantle conditions, water is no longer found in the familiar liquid form, but rather bound as hydroxyl (OH) through charge-coupled chemical substitutions in the crystal structure of high-pressure silicates. Hydrous melts may also be present at depths where the H2O storage capacity of minerals such as bridgmanite is relatively low. Experimental techniques including GHz-ultrasonic interferometry at Northwestern University and Brillouin scattering at the Advanced Photon Source, Argonne National Laboratory, will be employed to measure the influence of hydration on the elastic properties of mantle minerals such as wadsleyite, ringwoodite, and majoritic garnet, as well as on silicate glasses. Those results will be used to build a publically-available thermoelastic database, which provides input such as elastic moduli and their pressure-temperature derivatives needed to forward model expected seismic velocities in the mantle as a function of depth, temperature, and water content. Combined with observations from regional-scale seismic studies of the mantle, the results will be used to infer the mantle hydration state beneath North America and more globally as high-resolution seismic data become available.

This award is supported by the Major Research Instrumentation and the Chemistry Instrumentation Programs. Professor Mercouri Kanatzidis from Northwestern University and colleagues Kenneth Poeppelmeier, Sossina Haile, Steven Jacobsen and Danna Freedman are acquiring a single crystal X-ray diffractometer equipped with a silver micro-source, goniometer, and an efficient detector optimized for silver-radiation. In general, an X-ray diffractometer allows accurate and precise measurements of the full three-dimensional structure of a molecule, including bond distances and angles, and provides accurate information about the spatial arrangement of a molecule relative to neighboring molecules. The studies described here impact many areas, including organic and inorganic chemistry, materials chemistry and biochemistry. This instrument is an integral part of teaching as well as research and research training of undergraduate and graduate students in chemistry and biochemistry at this institution where students receive hands-on access to the diffractometers and collect data on their own experimental samples. The new diffractometer is also used for the biannual international Summer School co-organized with the American Crystallographic Association and Northwestern University. Collaborations are in place with other research institutions such as the University of Chicago, the Ohio State University, Illinois Institute of Technology, Loyola University Chicago, Lake Forest College and Roosevelt University.

The award is aimed at enhancing research and education at all levels. It is especially used for exploring solid state chemistry of chalcogenides and analyzing solid-state-related electrochemical processes. The diffractometer is also utilized for the analyses of synthesized compounds and minerals as well as synthesized oxides and oxide-fluorides. The instrument is employed in projects with applications in sustainable energy, geology, planetary sciences, ceramics and nanomaterials.

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.