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How the 410-km Discontinuity Reflects Mantle Water Content: Constraints from High-Pressure Experiments on Wadsleyite Single-Crystal Elasticity

Buchen, J., Marquardt, H., Boffa Ballaran, T., Kawazoe, T., Speziale, S., Kurnosov, A. (2017): How the 410-km Discontinuity Reflects Mantle Water Content: Constraints from High-Pressure Experiments on Wadsleyite Single-Crystal Elasticity - Abstracts, AGU 2017 Fall Meeting (New Orleans, USA 2017).



http://gfzpublic.gfz-potsdam.de/pubman/item/escidoc:3302904
Authors

Buchen ,  Johannes

Marquardt ,  Hauke

Boffa Ballaran ,  Tiziana

Kawazoe ,  Takaaki

http://gfzpublic.gfz-potsdam.de/cone/persons/resource/speziale

Speziale ,  S.
4.3 Chemistry and Physics of Earth Materials, 4.0 Geomaterials, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum;

Kurnosov ,  Alexander

Abstract
The olivine-wadsleyite phase transition gives rise to a seismic discontinuity at 410 km depth. By incorporating hydroxyl groups in its crystal structure, wadsleyite can store large amounts of water in the shallow transition zone. The velocity contrast across the 410-km seismic discontinuity has been widely used to deduce mantle mineralogy including estimates of the water content at depth. To interpret seismic observations in terms of mantle mineralogy and deep water cycling, the elastic properties of wadsleyite need to be determined at relevant pressures and temperatures. We performed simultaneous sound wave velocity and density measurements on iron-bearing wadsleyite single crystals at high pressures and first experiments at combined high pressures and high temperatures. When compared with previous work on hydrous iron-bearing wadsleyite with identical Fe/(Mg+Fe) ratio of 0.11, our results show that hydration of iron-bearing wadsleyite reduces the sound wave velocities at low pressures. At high pressures, in contrast, P-wave and S-wave velocities of hydrous and anhydrous iron-bearing wadsleyite cross over and become seismically indistinguishable at conditions of the transition zone. As a consequence, hydrated regions in the shallow transition zone cannot be detected by seismic tomography. Motivated by our experimental results, we modeled velocity, density, and acoustic impedance contrasts across the 410-km seismic discontinuity and found velocity contrasts to vary only slightly with hydration. Instead, we show that the impedance contrast caused by the olivine-wadsleyite phase transition and hence the reflectivity of the 410-km seismic discontinuity are more sensitive to hydration. Our findings give important constraints on the interpretation of seismic observations aiming to trace Earth's deep water cycle.