Evidence for low viscosity garnet-rich layers in the upper mantle |
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| Authors: | Auke Barnhoorn Martyn R Drury Herman LM van Roermund |
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| Institution: | 1. College of Earth Sciences, Jilin University, Changchun, Jilin 130061, China;2. Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Natural Resources, Jilin University, Changchun, Jilin 130061, China;3. Department of Geography and Geology, Paris-Lodron-University of Salzburg, Salzburg, Austria;4. School of Earth Sciences and Resources, China University of Geoscience Beijing, 29 Xueyuan Road, Beijing 100083, PR China;5. Department of Earth Sciences, University of Adelaide, Adelaide, SA 5005, Australia;6. Key Laboratory of Submarine Geosciences and Prospecting Techniques, Ministry of Education, College of Marine Geosciences, Ocean University of China, Qingdao, Shandong, China;7. Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao, Shandong, China;1. Geological Institute, ETH Zürich, Sonneggstrasse 5, 8092 Zürich, Switzerland;2. Scientific Center for Optical and Electron Microscopy (ScopeM), ETH Zürich, Auguste-Piccard-Hof 1, 8093 Zürich, Switzerland;3. Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands |
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| Abstract: | The rheological properties of upper mantle rocks play an important role in controlling the dynamics of the lithosphere and mantle convection. Experimental studies and microstructures in naturally deformed mantle rocks usually imply that olivine controls the upper mantle rheology. Here we show for the first time evidence from the geometry of folded compositional layers in mantle rocks from Western Norway that garnet-rich rocks can have lower solid-state viscosities than olivine-rich rocks. Modeling of melt-free and dry rheology of garnet and olivine confirms that the reversed viscosity contrast between garnet-rich and olivine-rich layers for this folding event can be achieved over a relatively wide range of temperatures at low stress conditions when the fine-grained garnet deforms by diffusion creep while the coarse-grained olivine deforms by dislocation creep and/or diffusion creep.In general, modeling of the fold viscosity contrast shows that in the stable subcontinental lithospheric mantle or convecting mantle such a reversed viscosity contrast can be formed due to diffusion creep processes in fine-grained garnets in a dry mantle environment or at conditions where the garnet-pyroxene layer is partially molten, i.e. close to solidus–liquidus conditions in the upper mantle. Alternatively in cold plate tectonic settings, e.g. in subduction zones, some water-weakening is a feasible mechanism to create the reversed viscosity contrast between garnet and olivine. |
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