Hydrogen or water exists in very small amounts (tens to hundreds of parts per million) in the rocks of the Earth's interior. Even at these low concentrations, the total amount of water in the interior could equal or exceed that of the oceans. These small amounts of water have very significant effects on the physical properties of Earth materials, such as viscosity, strength, melting temperature, electrical conductivity, and diffusion rates. The variations of these physical properties induced by water may be related to the processes of plate tectonics and volcanism. Indeed, plate tectonics is thought to occur on the Earth and not on other terrestrial planets because of the small amounts of water in the Earth's mantle. Accordingly, it is important to quantify the effects of water on physical properties. This study of the interdiffusion of deuterium and hydrogen (that is, isotopes of hydrogen) in olivine and other mantle minerals will provide data that will contribute to the understanding of mantle flow and rheology, the distribution of water in the Earth's interior, and rates of homogenization of different regions of the Earth's interior. In the broadest sense, the results will contribute to understanding of plate tectonics and the mechanisms by which it operates, origins of earthquakes and volcanoes, and the overall abundance and distribution of the chemical elements in Earth and other planets.
This project is an experimental effort to determine hydrogen-deuterium interdiffusion (self diffusion) coefficients in mantle materials such as olivine at temperatures and pressures of the Earth's upper mantle. Analysis of deuterium and hydrogen will be performed using novel techniques of Secondary Ion Mass Spectrometry (SIMS). The results will be interpreted in terms of defect mechanisms that can be related to models of ionic processes. Previous hydrogen diffusion measurements in olivine and mantle materials have generally been made using a hydrogen incorporation method, that is, measurement of diffusion of hydrogen into 'dry' material. Such experiments are interpreted to yield values of self diffusion coefficients for H diffusion and metal vacancy diffusion. Our new interdiffusion measurements will complement these previous measurements and enable delineation of detailed mechanisms of hydrogen ionic mobility in materials. Furthermore, these measurements will enable comparison to electrical conductivity measurements on hydrous mantle minerals. The results of this study will shed new light on physical transport processes in a variety of geological environments and will be important for the interpretation and modeling of magnetotelluric data. The ultimate benefits will be more detailed understanding of the temperature profile and physical state of matter at depth in the Earth's interior.
This work will extend our understanding of hydrogen diffusion and point defects in mantle materials and will contribute to the knowledge base of Materials Science and Earth Science. The graduate student supported on this project will develop skills in experimental development, problem solving, microanalytical techniques, computational numerical analysis and collaboration in an interdisciplinary environment. She or he will attend national meetings, such as American Geophysical Union meetings, where diversity is a priority, enabling her/him to interact with scientists from around the globe. She or he will help mentor undergraduate students in research, participate in classroom exercises, and participate in outreach efforts. The work will be performed in shared multi-user facilities in collaboration with students and researchers from earth sciences, chemistry, materials science, physics and other fields, from Arizona State University as well as from other institutions from around the world. New advances in analytical and experimental methods and infrastructure will be developed as part of this project.
This work contributes to the understanding of how Earth materials behave at depth in the Earth. In order to understand how plate tectonics works, how materials from deep in the Earth’s interior may rise over long geological times scales to the surface, and how volcanoes ultimately are generated, it is important to understand the physical properties of the materials that make up the Earth at the pressure and temperature conditions the exist at depth. In this work we have examined the mechanism for how hydrogen diffuses through the key mantle mineral olivine. We have developed high pressure and high temperature methods for experimentally determining hydrogen-deuterium interdiffusion in olivine. These new values are compared to chemical diffusion measurements made previously by others using a different approach. Together these results help describe a model of hydrogen diffusion in olivine with transport taking place by several different point defect mechanisms. We also show that that hydrogen diffusion in olivine is very anisotropic (depends on crystallographic orientation). Together these results tell us hydrogen alone cannot account for high electrical conductivity anomalies in the asthenosphere (the region immediately beneath tectonic plates) and that deeper in the mantle anisotropy of electrical conductivity (determined using magnetotelluric methods) could be related to hydrogen content. In addition, we developed model that relates the viscosity of naturally-occurring silicate melts to electrical conductivity at high temperatures and pressures. This was accomplished by using the concept of ‘optical basicity’ to describe compositional variations, including H2O. This relation potentially enables estimation of rheological parameters (viscosity) of magma chambers at depth through electromagnetic observations at the surface if compositional parameters can be estimated. Funding from this award was used to support one graduate student Wyatt Du Frane, several undergraduate students, and to partially support the research of a postdoctoral fellow Dr. Anne Pommier.
