Estimates of the transition zone temperature in a mechanically mixed upper mantle |
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| Authors: | Jeroen Ritsema Wenbo Xu Lars Stixrude Carolina Lithgow-Bertelloni |
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| Institution: | 1. Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, USA;2. Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK;1. Department of Earth and Environmental Sciences, University of Michigan, 2534 CC Little, Ann Arbor, MI 48109, USA;2. Department of Terrestrial Magnetism, Carnegie Institution for Science, 5241 Broad Branch Road NW, Washington, DC 20015, USA;3. Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK;1. Department of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan;2. Department of Earth Science, Tohoku University, 6-3 Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan;3. Department of Earth and Space Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan;4. Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, 6-3 Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan;5. Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan;6. Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan;7. Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan;1. Institute of Theoretical and Applied Geophysics, Peking University, Beijing, China;2. Department of Earth Science, Rice University, Houston, USA;3. State Key Laboratory of Petroleum Resource and Prospecting, and Unconventional Natural Gas Institute, China University of Petroleum, Beijing, China;4. Department of Geological Sciences, University of Texas at Austin, Austin, USA;5. Earthquake Research Institute, University of Tokyo, Tokyo, Japan;6. Institute for Frontier Research on Earth and Evolution, Japan Agency for Marine–Earth Science and Technology, Yokosuka, Japan;7. Department of Physics, New Mexico State University, Las Cruces, USA;1. Institute for Study of the Earth''s Interior, Okayama University, Misasa, Tottori 682-0193, Japan;2. Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Miyagi, Sendai 980-8578, Japan;3. Department of Earth and Planetary Material Sciences, Tohoku University, Miyagi, Sendai 980-8578, Japan;4. Japan Synchrotron Radiation Research Institute, Sayo, Hyogo 678-5198, Japan |
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| Abstract: | Partial melting of mantle peridotite generates a physically and chemically layered oceanic lithosphere that is cycled back into the mantle in subduction zones. Stirring times of the mantle are too long to allow for complete re-homogenization of subducted basalt and harzburgite, given the low chemical diffusivity of the solid mantle. This suggests that the Earth's mantle is a mechanical mixture of basaltic and harzburgitic components. Using a recently developed thermodynamic formulism we determine the phase equilibria and the seismic properties of a mantle comprised of a mechanical mixture of basalt and harzburgite (MM) and a homogeneous mantle (EA) with identical pyrolitic bulk chemistry. We use the theoretical shear velocity profiles as a new thermometer of the mantle below the magma-genetic zone by modeling the difference ΔT410-660 between traveltimes of shear wave reflections off the 410-km and 660-km with the potential temperature TP. ΔT410-660 are measured from waveform stacks. They indicate that, over 1000+ km wave lengths, the temperature varies by about 200 K. Lowest and highest temperatures are resolved for the western Pacific subduction zones and the central Pacific, respectively. This variation is similar for the EA and MM and is in excellent agreement with estimates of transition zone thickness and shear velocity variations. The median value of TP for the EA is 1720 K. It is about 1625 K for the MM, a value that is in better agreement with the Normal-MORB values of 1610 ± 40 K inferred from olivine-liquid equilibria given that our sampling region encompasses the Western Pacific subduction zones and the oldest parts of the Pacific Plate. We argue therefore that a mechanical mixed mantle, with generally higher velocities and steeper velocities gradients, represents a better physical reference model than a model based on a fully equilibrated assemblage. |
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