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Unravelling partial melt distribution in the oceanic low velocity zone

Emmanuel Gardès 1 Mickaël Laumonier 2 Malcolm Massuyeau 3 Fabrice Gaillard 4
1 MADIR - Matériaux, Défauts et IRradiations
CIMAP - UMR 6252 - Centre de recherche sur les Ions, les MAtériaux et la Photonique
4 Magma - UMR7327
ISTO - Institut des Sciences de la Terre d'Orléans - UMR7327
Abstract : The widespread low seismic velocity zone (LVZ) in the shallow oceanic mantle has long been debated in terms of mantle melting. At LVZ depths, volatiles (CO2 and H2O) are present in minute amounts, which implies mantle incipient melting down to below 1000 °C with the production of minute amounts of volatile-rich melt, well below 1 vol.%. However, melt compositions and distributions in the incipient melting regime have only been inferred from experiments departing from actual mantle conditions. Here, we experimentally reproduce incipient melting by re-equilibrating a naturally CO2- and H2O-bearing mantle rock at mantle temperatures and pressure. By using cutting-edge microscopy characterizations, we evidence that minute amounts of volatile-rich melts fully interconnect in mantle rocks down to lithospheric temperatures, enabling thus the modification of geophysical signals from the mantle. These findings and the correspondence of the domain of local, sharp drops in shear wave velocity (Vs) with the domain of (CO2+H2O)-melting in the LVZ strongly supports that these geophysical anomalies relate to mantle melting. Geophysical surveys image in situ the very low and highly heterogeneous distribution of melt in the mantle generated by the very low and highly heterogeneous distribution of volatiles probed by surficial geochemical surveys. The global-scale geophysical signature of the LVZ appears mainly unaffected because the average background melt fraction is very low, estimated at ∼0.03-0.05 vol.% melt. However, enhanced geophysical signals arise from sporadic, localized areas where melt fraction is increased, such as the ∼0.2 vol.% melt estimated for detecting sharp Vs drops using SS precursors. In-depth deciphering of the dynamics of melt and volatiles in the LVZ calls for investigations on the seismic velocity, permeability and rheology of partially molten mantle rocks covering the diversity of mantle melt compositions, fractions and temperatures.
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Emmanuel Gardès, Mickaël Laumonier, Malcolm Massuyeau, Fabrice Gaillard. Unravelling partial melt distribution in the oceanic low velocity zone. Earth and Planetary Science Letters, Elsevier, 2020, 540, pp.116242. ⟨10.1016/j.epsl.2020.116242⟩. ⟨insu-02539354⟩

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