CSEDI Collaborative Research: Understanding the nature of water transport between the transition zone and the lower mantle through the interdisciplinary studies

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Earth is the water planet, with much of its H2O chemically bound within its rocky interior. The goal of this project is to provide new insight into the way in which water circulates, carried by the slow heat convection of Earth's silicate mantle. Geophysicists now have strong evidence that there is a substantial amount of water in Earth's interior, perhaps more than the current mass of the oceans. There is strong evidence that most of this water resides within the mantle transition zone (MTZ), between 410 km and 660 km depth in the Earth. The MTZ spans the stability range of minerals that can incorporate water within their crystal structures, up to a mass-fraction of a percent or more. It appears that the transition zone acts as a refillable reservoir of mineral-bound water within Earth's interior. The water in Earth's surface oceans can exchange with the water in the mantle transition zone via mantle convection, on time scales of 10-30 million years. How does the transition zone maintain its elevated water content as mantle rock flows upward and downward through it, converting to water-phobic minerals at its boundaries, particularly at its lower boundary at ~660-km depth? The goal of this project is to study the MTZ and lower-mantle minerals at high pressure in the lab and with computer simulations to determine how the water behaves near 660-km depth, and to detect water expulsion from descending rock masses using scattered seismic waves. The award will provide graduate and undergraduate training, including summer research projects, for several students in a broadly interdisciplinary research project.Understanding the nature of large-scale water transport in Earth's deep interior is one of the key issues in the study of evolution of Earth (and other planets). Since diffusion is inefficient, the most important process to control the large-scale transport of water is melting and subsequent melt-solid separation. The goal of this study is to improve our understanding on these two issues. Melting likely occurs when water-rich materials in the transition zone are brought into upper or the lower mantle where the water solubility in minerals is low. However, when metallic iron is present, then a majority of water partitions into it. Hence metallic iron has a controlling effect on the melting behavior. One aspect of this research is to obtain a better understanding of metallic iron content in the lower mantle. When melt is formed, most of water (hydrogen) goes to the melt. Melt migrates up or down depending on its density relative to the unmelted residual rock. Melt density under deep-mantle conditions has not been constrained well, particularly considering the range of likely chemical compositions. This project will investigate melt density under deep-mantle conditions with different redox values and plausible variations in the iron-magnesium ratio. Combining these, the investigators will have an improved view of deep-Earth water circulation. With scattered seismic waves, primarily using the receiver-function technique, the team will map out locations beneath the mantle transition zone where water-enhanced partial melt is present.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.