Hydrated Silica in the Deep Mantle

Plate tectonics processes produce a large amount of volcanic rock along mid-ocean ridges that is later subducted into the Earth’s deep mantle. This material forms minerals of SiO2 (silica), which has the potential to store water bound in the crystal as hydrogen. Deep in the mantle, this water-bearing silica forms the mineral stishovite, which may result in observable seismic signatures.

At greater depths, stishovite undergoes a structural transition to post-stishovite phases that affects the velocities of seismic waves passing through the subducting slab materials. These wave velocities and depth to the phase transitions are sensitive to the amount of water stored in the silica. Therefore, understanding how stored water in stishovite and post-stishovite affects seismic wave velocities is important to our understanding of the plate tectonics processes, the structure of the lower mantle, and the global water cycle.

In recent years, seismic studies have revealed reliable velocity profiles of the lower mantle that are indicative of subducted material. However, reliable laboratory-based velocity profiles and water contents of silica mineral phases remain unavailable and difficult to be compared with seismic studies. This project will develop instrumentation and build reliable datasets to advance the knowledge of the plate tectonics processes and global water cycle occurring in the deep mantle.

This project provides career development opportunities and research resources to participating graduate student and undergraduate researchers in geoscience research, as well as facilitating outreach activities through the UTeach Outreach Summer Program.

This study will measure sound velocities, water solubility, and the depth of the phase transition from stishovite to post-stishovite phases in subducting slabs in the lower mantle. High pressure-temperature conditions are generated in the laboratory with externally-heated or laser-heated diamond anvil cells. Stishovite crystals are compressed in diamond anvil cells to measure the velocity profiles of the crystals using a combination of laboratory laser spectroscopic techniques as well as synchrotron-based X-ray diffraction.

Hydrogen content in the crystals is measured using Fourier Transform Infrared Spectroscopy and Secondary Ion Mass Spectrometry techniques. These experimental results are used to build velocity profiles and to reveal the mechanism of water solubility in the silica phases. These results are compared with seismic wave models of the region to aid to the understanding of the geophysical and geodynamic consequences of hydrated silica in the Earth’s lower mantle.

This project provides career development opportunities and research resources to participating graduate student and undergraduate researchers in geoscience research. The project supports an Open Source Project at the University of Texas at Austin to make datasets, modelling programs, and designs of the instruments available to the public. Outreach activities in this project focus on UTeach Outreach Summer Program to educate K-12 students from southwest Texas with hands on lessons in Mineral Physics Laboratory at the University of Texas at Austin.

Further educational activities include the education of non-science major undergraduate students about the planetary habitability relevant to deep-Earth and planetary processes at the University of Texas at Austin. This project is co-funded by the Geophysics and Petrology and Geochemistry Programs in the Division of Earth Sciences.

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.