Constraining the hydration state of the subducting mantle with converted seismic phases

• Applicant: Dr. Wasja Bloch
• Subject Area: Geophysics ; Mineralogy, Petrology and Geochemistry
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 449279512

At convergent plate boundaries water is removed from the Earth's surface. It enters the interior of the planet through subduction zones, where it plays a number of important roles in different parts of the Earth System. While the amount of water stored in sediments and the oceanic crust has been validated through off-shore drilling experiments, little is known about how much is subducting inside the lithospheric mantle. As measurements are scarce the questions arise: How much water is stored in the subducting mantle? How does the amount vary between subduction zones? What are the factors that control mantle hydration?The analysis of seismograms that were recorded above subducting plates shows a P-to-S conversion of the incoming wave field that is indicative of a material contrast located approximately at the same depth to which the oceanic mantle is thought to be hydrated. The properties of this conversion hint at the anisotropic character of that layer. They show that anisotropy is to one part inherited from formation of the lithosphere during sea-floor spreading, and to another part likely goes back to hydration of the mantle during bend-faulting prior to subduction. The conversion signal can be used to estimate the water content of the oceanic mantle lithosphere beneath a seismic station.I propose to analyze the conversion of the so-called anisotropic mantle lid (AML) recorded at land seismometers along all convergent plate boundaries with respect to its the thickness and elastic tensor. I plan do develop an algorithm for searching the lithology, and hence water content, that best matches the so found AML properties. The result of the subsequent application of this workflow to a global station set is the water content of the subducting mantle in the perimeter of each seismic station. This global compilation of subducting mantle water can be used to test hypotheses about the role that water supply plays in a variety of Earth processes. Factors that control mantle hydration can be identified, and the global subducted water budget and rate can be estimated with higher accuracy.

• DFG Programme: WBP Fellowship
• International Connection: Canada, USA
• Hosts: Professor Dr. Pascal Audet; Professor Dr. Michael Bostock; Professorin Dr. Sarah Brownlee