Collaborative Research: Thin Film Insights into Phase Transformations and Deep-Focus Earthquakes

Michigan State University, East Lansing MI

Investigators

Abstract

The Earth’s surface is made of a series of tectonic plates which, over geologic time, move across the surface. Where plates move towards each other, one sinks into the Earth’s mantle, in the process known as subduction. As the rock sinks, the rock experiences increasing pressure, and the minerals, including the mineral olivine, Mg2SiO4, adopt new, denser crystals structures. These crystal structure changes result in volume reductions which have been postulated to contribute to deep earthquakes in subducting plates. Unfortunately, past experiments have not been able to fully explain the large number of deep-focus subduction zone earthquakes occurring between 475-660 km in depth. The hypothesis of this project is that the large shear stresses or grain-size reductions likely encountered during subduction deepen the Mg2SiO4 crystal structure transformations to the conditions found at the depth the earthquakes occur. To evaluate this hypothesis, this project will carefully measure the crystal structures of nano-grains of Mg2SiO4 thin films under stress as a function of pressure and temperature in diamond anvil cells. If successful, this work will boost our understanding of deep-Earth processes and help launch a new field of Thin Film Mineral Physics where the novel composition, grain size, and deviatoric stress control possible in thin film samples can be used to study a variety of natural or synthetic materials in extreme environments. This project will enable expanding outreach efforts to programs focused on STEM across age levels, from elementary and middle school, to university students, and to grandparents. The mantle discontinuities at ~410, ~520, and ~660 km critically impact deep-Earth structure and dynamics. These discontinuities have been attributed to phase transformations between olivine, wadsleyite, ringwoodite, and the bridgmanite + periclase assemblage. Recently, this team detected phase transformation in thin films at high-pressure. In these experiments, the thin film Mg2SiO4 1) forsterite-to-wadsleyite and 2) (akimotoite + periclase)-to-(bridgmanite + periclase) phase transformations occurred at pressure and temperature conditions similar to those reported for bulk Mg2SiO4. In contrast, the thin film wadsleyite-to-ringwoodite transformation occurred ~500 K higher at 18 GPa (~2.5 GPa lower at 1900 K) than it does in bulk Mg2SiO4. This suggests that the either small grain sizes or large deviatoric stresses possible in thin films (and postulated to exist within subducting slabs) may impact the wadsleyite-to-ringwoodite transformation within subduction zones. Hence, the objective of this work is to 1) determine if the previously observed thin film Mg2SiO4 phase boundary shifts can be reproduced in anhydrous Mg2SiO4 thin films, 2) carefully map out the 0.1 - 30 GPa and 300 - 2300 K phase boundaries of anhydrous Mg2SiO4 thin films, and 3) establish the fabrication and testing protocols necessary to identify the mechanisms responsible for any observed phase boundary shifts. To achieve these aims, the project will produce anhydrous Mg2SiO4 thin films via Pulsed Laser Deposition, 2) optimize thin film sample-loading procedures into a Diamond Anvil Cell and use synchrotron-based X-Ray Diffraction or Raman spectroscopy to construct the world’s first high-pressure phase diagram of a thin-film sample. In addition to elucidating how grain size and deviatoric stress may impact Mg2SiO4 phase transformations, this work will highlight how thin films, capable of supporting static tensile or compressive deviatoric stresses up to ~10 GPa, can be loaded into laser-heated Diamond Anvil Cells to apply large, well-controlled, and complex deviatoric stress states on optically-accessible samples. The proposed work will have a broad impact by 1) allowing PI Nicholas to produce a new Michigan State University (MSU) demonstration station for 4-8th grade students on “Rocks and Minerals”, 2) allowing PI Nicholas, PI Li, and PI Chen to incorporate interdisciplinary Materials Science and Geophysics teaching strategies and content into their courses, and 3) exciting broader society about science via the development of a new “Amazing Crystals” course for the Grandparent University Summer Camp run by Michigan State University each June. 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.