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.
Earth is unique among the terrestrial planets in having maintained a large reservoir of liquid water on its surface over geologic time. Research under this award addressed the question of how much water may reside deep inside the Earth’s mantle. A deep geochemical reservoir of H2O in the mantle may exceed the scale of the oceans in terms of mass, although not in the familiar liquid form. Under high pressure and temperature conditions of the mantle, water can dissolve into minerals in the form of hydroxyl (OH), leading to crystalline defects with significant impact on material properties, and by extension, geophysical processes. This project examined broadly the role of silicate minerals in Earth’s deep water cycle, which will ultimately lead to a better understanding of the total mass and distribution of H2O in the Earth. Research focused on the mantle transition zone (410-660 km depth), where the minerals wadsleyite and ringwoodite may incorporate more than one weight percent of H2O. The amount of H2O that can be incorporated into the major mantle minerals was investigated by synthesizing them in the laboratory under hydrous conditions. The mechanisms of hydration were investigated using spectroscopy techniques, and the influence of hydration on the physical properties of Earth and planetary materials was determined by various experimental techniques. Using the Advanced Photon Source at Argonne National Laboratory, graduate students used synchrotron X-ray diffraction to determine the effect of hydration on the crystal structure and equation of state of hydrous wadsleyite and hydrous ringwoodite, finding a significant 5-10% reduction in their bulk compressibility. A new technique called GHz-ultrasonic interferometry was developed to measure the speed of seismic waves in hydrous mantle materials for direct comparison with observations from seismology. At the National Synchrotron Light Source, Brookhaven National Laboratory, graduate students developed new methods in high-pressure infrared spectroscopy to study the incorporation of H2O into minerals at conditions of the deep mantle. Outcomes of the research include a thermoelastic properties database of hydrated mantle minerals, which will be used broadly in solid-Earth geophysics and geochemistry to model the composition and hydration state of the Earth’s interior from seismological observations. Whereas the H2O storage capacity of the transition zone is relative high, it was found that silicate perovskite in the lower mantle does not incorporate much H2O under hydrous conditions. Thus, if the transition zone is hydrated, downwelling mantle in convection should melt upon flow across the base of the transition zone. This result gives seismologists a key indicator of hydration to search for in the seismic structure of the mantle. In addition to studying H2O in mantle minerals, graduate students participated more broadly in research on other volatiles in Earth and planetary materials. The optical reflectivity of methane (CH4) across its melting line at 90 K was determined to assist satellite observations of potential hydrocarbon lakes on Titan, Saturn’s largest moon. The incorporation of carbon in basaltic magmas was determined in samples synthesized under a range of oxidation states, finding that iron-carbonyl groups such as Fe(CO)5 instead of carbonate form at conditions relevant to volcanism on Mars and the moon. The speciation of hydrogen in lunar basaltic glasses was determined by infrared spectroscopy, finding that molecular water dominates over hydroxyl. Results of the research were disseminated through journal publications (see references), annual conferences, and occasional through media outlets to the general public. Education and outreach occurred at all levels. The PI led after school classes in Earth science to third grade minority students in the local school district as part of Northwestern’s Project Excite, run by the Center for Talent Development. Project Excite, a longitudinal program for students from third grade through middle school, aims to close the enrollment gap of minority students in advanced placement science and mathematics courses at the local high school. Results presented at the 2011 American Geophysical Union showed that over the past six years, nearly half of Project Excite students entered honors mathematics courses their freshman year of high school. At the high school level, four students held research internships in the mineral physics laboratory. At the undergraduate level, students participated in research through independent study and co-authored two papers published in international journals. At Northwestern, the PI served as Fellow of the Public Affairs Residential College and engaged students in regular discussion of science policy. Training of graduate students in experimental research encompassing mineral physics, geochemistry, materials science, and geophysics under this award has prepared them for future jobs in academia, industry, or the National laboratories.
