CAREER: Effects of Hydration on the Physical Properties of Mantle Materials from Atomic to Geophysical Scales

Northwestern University, Evanston IL

Investigators

Abstract

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.