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      The electrical conductivity during incipient melting in the oceanic low velocity zone

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          Abstract

          A low viscosity layer in the upper mantle, the Asthenosphere, is a requirement for plate tectonics 1 . The seismic low velocities and the high electrical conductivities of the Asthenosphere are attributed either to sub-solidus water-related defects in olivine minerals 2- 4 or to a few volume percents of partial melt 5- 8 but these two interpretations have shortcomings: (1) The amount of H 2O stored in olivine is not expected to be higher than 50 ppm due to partitioning with other mantle phases 9 , including pargasite amphibole at moderate temperatures 10 , and partial melting at high temperatures 9 ; (2) elevated melt volume fractions are impeded by the too cold temperatures prevailing in the Asthenosphere and by the high melt mobility that can lead to gravitational segregation 11, 12 . Here we determined the electrical conductivity of CO 2-H 2O-rich melts, typically produced at the onset of mantle melting. Electrical conductivity modestly increases with moderate amounts of H 2O and CO 2 but it dramatically increases as CO 2 content exceeds 6 wt% in the melt. Incipient melts, long-expected to prevail in the asthenosphere 10, 13- 15 , can therefore trigger its high electrical conductivities. Considering depleted and enriched mantle abundances in H 2O and CO 2 and their effect on the petrology of incipient melting, we calculated conductivity profiles across the Asthenosphere for various plate ages. Several electrical discontinuities are predicted and match geophysical observations in a consistent petrological and geochemical framework. In moderately aged plates (>5Ma), incipient melts most likely trigger both the seismic low velocities and the high electrical conductivities in the upper part of the asthenosphere, whereas for young plates 4 , where seamount volcanism occurs 6 , higher degree of melting is expected.

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          Most cited references 95

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          Water in the oceanic upper mantle: implications for rheology, melt extraction and the evolution of the lithosphere

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            Mantle solidus: Experimental constraints and the effects of peridotite composition

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              A new parameterization of hydrous mantle melting

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                Author and article information

                Journal
                0410462
                6011
                Nature
                Nature
                Nature
                0028-0836
                1476-4687
                1 April 2014
                1 May 2014
                01 November 2014
                : 509
                : 7498
                : 81-85
                Affiliations
                [1 ]Université d’Orléans, ISTO, UMR 7327, 45071 Orléans, France
                [2 ]CNRS/INSU, ISTO, UMR 7327, 45071 Orléans, France
                [3 ]BRGM, ISTO, UMR 7327, BP 36009, 45060 Orléans, France
                [4 ]CEA-CNRS-ENSICAEN-Université de Caen Basse Normandie, CIMAP, UMR 6252, BP 5133, 14070 Caen, France
                [5 ]Department of Geology and CSCAMM, University of Maryland, MD 20742, USA
                [6 ]Dept. of Earth Sciences, Royal Holloway University of London Egham, Surrey, UK
                Author notes

                Authors contributions: F.G. is leading the project and wrote the first draft. All authors equally contributed to the writing. D.S. and F.G. developed the experimental setup and D.S. performed the conductivity measurements. S.H-M contributed to the discussion and provided editorial assistance with manuscript. D.S. and L.H. did figure 1, E.G. and L.H. did figure 2, D.S. did figure 3 and L.H., M.M. did figure 4.

                EMS57374
                10.1038/nature13245
                4010644
                24784219
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