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Experimental evidence supporting a global melt layer at the base of the Earth’s upper mantle

Abstract : The low-velocity layer (LVL) atop the 410-km discontinuity has been widely attributed to dehydration melting. In this study, we experimentally reproduced the wadsleyite-to-olivine phase transformation in the upwelling mantle across the 410-km discontinuity and investigated in situ the sound wave velocity during partial melting of hydrous peridotite. Our seismic velocity model indicates that the globally observed negative Vs anomaly (−4%) can be explained by a 0.7% melt fraction in peridotite at the base of the upper mantle. The produced melt is richer in FeO (~33 wt.%) and H2O (~16.5 wt.%) and its density is determined to be 3.56–3.74 g cm−3. The water content of this gravitationally stable melt in the LVL corresponds to a total water content in the mantle transition zone of 0.22 ± 0.02 wt.%. Such values agree with estimations based on magneto-telluric observations.
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D. Freitas, Geeth Manthilake, Federica Schiavi, J. Chantel, N. Bolfan-Casanova, et al.. Experimental evidence supporting a global melt layer at the base of the Earth’s upper mantle. Nature Communications, Nature Publishing Group, 2017, 8 (1), pp.2186. ⟨10.1038/s41467-017-02275-9⟩. ⟨hal-01679036⟩

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