This project examined the role of high-pressure minerals in Earth?s mantle in the global water cycle. H2O, while largely liquid, ice, and vapor on the Earth?s surface, can be incorporated into rocks and minerals within their crystal structures. The mineral brucite, Mg(OH)2, for example, contains the ingredients of water in the solid form. In the mantle transition zone, at 410-660 km depth, high-pressure forms of the mineral olivine can incorporate several weight percent of H2O into their crystal structures. Because the transition zone constitutes roughly 7% of the Earth?s mass, a large geochemical reservoir of ?water? could be contained in the mantle and act to form silicate melts during convection and plate tectonics. This study focused on understanding how water, incorporated into high-pressure minerals stable in the transition zone, influences physical properties such as density, and the speed of seismic waves. By directly synthesizing such minerals and measuring their physical properties, the measured influence of water was used to infer the water content of the mantle transition zone from geophysical observations such as seismic wave speeds and imaging melts. It was discovered that while hydration reduces seismic wave speeds at low pressures, at higher pressures of the transition zone, the difference between hydrous and anhydrous minerals diminishes, making their detection more difficult. However, because minerals in the transition zone can incorporate more water than minerals above and below the transition zone, mass transfer during convection can cause hydrous melts to form, which would be more easily observable by seismic imaging techniques. The elastic properties of silicate glasses, mimicking such hydrous melts, were evaluated at high pressures and it was found that they exhibit higher wave speeds than previously thought, suggesting that previous estimates for the degree of partial melting in some regions of the upper mantle may be overestimated. Small amounts of water, in the form of brucite, were discovered inside natural inclusions of diamonds originating from as deep as 1000 km, suggesting that the global water cycle extends further into the mantle than previously thought. In the very deepest regions of the mantle, near the core-mantle boundary, theoretical studies showed that partial melts could originate from dehydration of the mineral post-perovskite transforming into the mineral bridgmanite, which could explain the origin of elevated water contents in certain ocean-island basalts. In general, water moves between the crust mantle system through plate tectonics, subduction, and convection. High-pressure minerals and hydrous silicate melts in the Earth?s mantle appear to play an important role in the global water cycle on geologic time scales.

Last Modified: 06/07/2019
Modified by: Steven D Jacobsen